Re: [PERFORM] performance for high-volume log insertion
On Tue, 21 Apr 2009, Stephen Frost wrote: David, * da...@lang.hm (da...@lang.hm) wrote: the database structure is not being defined by (or specificly for) rsyslog. so at compile time we have _no_ idea how many variables of what type there are going to be. my example of ($timestamp,$msg) was intended to just be a sample (avoiding typing out some elaberate set of parameters) That's fine. I don't see any reason that the API I suggested be a compile-time option. Certainly, libpq has no idea how many, or what kind, of types are being passed to it by the caller, that's why it *has* the API that it does. You just need to work out what each prepared queries' parameters are and construct the necessary arrays. I misunderstood what you were saying (more below) rsyslog provides the following items, which can be sliced and diced with substatutions, substrings, and additional inserted text. [...] Looks like mainly text fields still, so you might want to just stick with text for now. yes, almost exclusivly text fields, and the fields that could be numbers (or dates) are in text formats when we get them anyway. rsyslog message formatting provides tools for doing the nessasary escaping (and is using it for the single insert messages today) prepared statements in text mode have similar problems (although they _are_ better in defending against sql injection attacks, so a bit safer). Uhh, if you use prepared statements with PQexecPrepared, there *is no* escaping necessary. I'm not sure what you mean by 'similar problems'. Can you elaborate on that? If you mean doing 'prepared queries' by using creating a string and then using PQexec with 'EXECUTE blah (1,2,3);' then you really need to go read the documentation I suggested. That's *not* what I'm getting at when I say 'prepared queries', I'm talking about a protocol-level well-defined format for passing arguments independently of commands. A call to PQexecPrepared looks like this: PQprepare(conn, myquery, INSERT INTO TAB1 VALUES ($1, $2);, 0, NULL); values[0] = a; values[1] = b; PQexecPrepared(conn, myquery, 2, values, NULL, NULL, 0); Note that we don't ever send an 'EXECUTE myquery (1,2,3);' type of thing to libpq. libpq will handle the execute and the parameters and whatnot as part of the PG 3.0 protocol. when you said to stick with text mode, I thought you were meaning that we would create a string with EXECUTE in it and send that. it would have similar escaping issues (although with fewer vunerabilities if they mess up) so the binary mode only makes a difference on things like timestamps and numbers? (i.e. no significant added efficiancy in processing the command itself?) I'm slightly confused by what you mean by this? Binary mode is for parameters only, commands are never 'binary'. Binary mode just means that the application gives the value to the database in the format which it expects, and so the database doesn't have to translate a textual representation of a value into the binary format the database needs. I thought that part of the 'efficiancy' and 'performance' to be gained from binary modes were avoiding the need to parse commands, if it's only the savings in converting column contents from text to specific types, it's much less important. I think the huge complication is that when RedHat compiles rsyslog to ship it in the distro, they have no idea how it is going to be used (if it will go to a database, what database engine it will interface with, or what the schema of that database would look like). Only the sysadmin(s)/dba(s) know that and they need to be able to tell rsyslog what to do to get the data where they want it to be, and in the format they want it to be in. That's really all fine. You just need to get from the user, at runtime, what they want their commands to look like. Once you have that, it should be entirely possible to dynamically construct the prepared queries, most likely without the user having to know anything about COPY or prepared statements or anything. For my part, I'd want something like: table = mytable; data = $Y, $m, $d, $H, $msg; if the user creates the data this way, you just reintroduced the escaping problem. they would have to do something like data = $Y data = $m data = $d data = $H data = $msg one key thing is that it's very probable that the user will want to manipulate the string, not just send a single variable as-is I'd avoid having the user provide actual SQL, because that becomes difficult to deal with unless you embed an SQL parser in rsyslog, and I don't really see the value in that. there's no need for rsyslog to parse the SQL, just to be able to escape it appropriately and then pass it to the database for execution If the user wants to do something fancy with the data in the database, I would encourage them to put an 'ON INSERT' trigger on 'mytable' to do whatever they want with the data that's coming in. This gives you the freedom
Re: [PERFORM] performance for high-volume log insertion
On Mon, 20 Apr 2009, Greg Smith wrote: On Mon, 20 Apr 2009, da...@lang.hm wrote: any idea what sort of difference binary mode would result in? The win from switching from INSERT to COPY can be pretty big, further optimizing to BINARY you'd really need to profile to justify. I haven't found any significant difference in binary mode compared to overhead of the commit itself in most cases. The only thing I consistently run into is that timestamps can bog things down considerably in text mode, but you have to be pretty efficient in your app to do any better generating those those in the PostgreSQL binary format yourself. If you had a lot of difficult to parse data types like that, binary might be a plus, but it doesn't sound like that will be the case for what you're doing. But you don't have to believe me, it's easy to generate a test case here yourself. Copy some typical data into the database, export it both ways: COPY t to 'f'; COPY t to 'f' WITH BINARY; And then compare copying them both in again with \timing. That should let you definitively answer whether it's really worth the trouble. while I fully understand the 'benchmark your situation' need, this isn't that simple. in this case we are trying to decide what API/interface to use in a infrastructure tool that will be distributed in common distros (it's now the default syslog package of debian and fedora), there are so many variables in hardware, software, and load that trying to benchmark it becomes effectivly impossible. based on Stephan's explination of where binary could help, I think the easy answer is to decide not to bother with it (the postgres text to X converters get far more optimization attention than anything rsyslog could deploy) David Lang -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
Re: [PERFORM] performance for high-volume log insertion
On Mon, 20 Apr 2009, Stephen Frost wrote: Greg, * Greg Smith (gsm...@gregsmith.com) wrote: The win from switching from INSERT to COPY can be pretty big, further optimizing to BINARY you'd really need to profile to justify. Have you done any testing to compare COPY vs. INSERT using prepared statements? I'd be curious to know how those compare and against multi-value INSERTS, prepared and unprepared. and this is the rest of the question that I was trying (unsucessfully) to ask. is this as simple as creating a database and doing an explain on each of these? or do I need to actually measure the time (at which point the specific hardware and tuning settings become an issue again) David Lang -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
Re: [PERFORM] performance for high-volume log insertion
David, * da...@lang.hm (da...@lang.hm) wrote: I thought that part of the 'efficiancy' and 'performance' to be gained from binary modes were avoiding the need to parse commands, if it's only the savings in converting column contents from text to specific types, it's much less important. No, binary mode is about the column contents. Prepared queries is about avoiding having to parse commands (which is why the command and the data elements are seperately done). table = mytable; data = $Y, $m, $d, $H, $msg; if the user creates the data this way, you just reintroduced the escaping problem. they would have to do something like data = $Y data = $m data = $d data = $H data = $msg Yes, there is a bit of escaping that the admins will have to deal with in the config file. There's no way around that though, regardless of what you do. If you let them put SQL in, then they may have to escape stuff there too. In the end, an escape problem in the config file is something which you should really catch when you read the file in, and even if you don't catch it there, it's less problematic, not really a performance problem, and much less of a security risk, than having the escaping done on data from an untrusted source. one key thing is that it's very probable that the user will want to manipulate the string, not just send a single variable as-is You could probably let them do some manipulation, add extra non-escape-code fields, maybe tack something on the beginning and end, basically, anything that can be done in the application prior to it hitting the database should be fine. I'd avoid having the user provide actual SQL, because that becomes difficult to deal with unless you embed an SQL parser in rsyslog, and I don't really see the value in that. there's no need for rsyslog to parse the SQL, just to be able to escape it appropriately and then pass it to the database for execution If the user is providing SQL, then you need to be able to parse that SQL if you're going to do prepared queries. It might not require you to be able to fully parse SQL the way the back-end does, but anything you do that's not a full SQL parser is going to end up having limitations that won't be easy to find or document. For example, you could ask users to provide the prepared statement the way the database wants it, and then list the data elements seperately from it somehow, eg: myquery = INSERT INTO blah (col1, col2, col3) VALUES ($1, $2, $3); myvals[1] = $Y myvals[2] = $M myvals[3] = $msg The user could then do: myquery = INSERT INTO blah (col1, col2) SELECT substring($1), $2; myvals[1] = $M myvals[2] = $msg Both of these will work just fine as prepared queries. You can then parse that string by just looking for the $'s, but of course, if the user wants to put an *actual* dollar sign in, then you have to support that somehow ($$?). Then you have to deal with whatever other quoting requirements you have in your config file (how do you deal with double quotes? What about single quotes? etc, etc). You could possibly even put your escape codes into myquery and just try to figure out how to do the substitutions with the $NUMs and build your prepared query string. It gets uglier and uglier if you ask me though. In the end, I'd still worry about users coming up with new and different ways to break your sql 'parser'. one huge advantage of putting the sql into the configuration is the ability to work around other users of the database. See, I just don't see that. for example, what if the database has additional columns that you don't want to touch (say an item# sequence), if the SQL is in the config this is easy to work around, if it's seperate (or created by the module), this is very hard to do. You can do that with a trigger trivially.. That could also be supported through other mechanisms (for instance, let the user provide a list of columns to fill with DEFAULT in the prepared query). I guess you could give examples of the SQL in the documentation for how to create the prepared statement etc in the databases, but how is that much better than having it in the config file? for many users it's easier to do middlein -fancy stuff in the SQL than loading things into the database (can you pre-load prepared statements in the database? or are they a per-connection thing?) Prepared statements, at least under PG, are a per-connection thing. Triggers aren't the same, those are attached to tables and get called whenever a particular action is done on those tables (defined in the trigger definition). The trigger is then called with the row which is being inserted, etc, and can do whatever it wants with that row (put it in a different table, ignore it, etc). so back to the main questions of the advantages prepared statements avoid needing to escape things, but at the complication of a more complex API. there's still the question of the performance difference. I have
Re: [PERFORM] performance for high-volume log insertion
* da...@lang.hm (da...@lang.hm) wrote: while I fully understand the 'benchmark your situation' need, this isn't that simple. It really is. You know your application, you know it's primary use cases, and probably have some data to play with. You're certainly in a much better situation to at least *try* and benchmark it than we are. in this case we are trying to decide what API/interface to use in a infrastructure tool that will be distributed in common distros (it's now the default syslog package of debian and fedora), there are so many variables in hardware, software, and load that trying to benchmark it becomes effectivly impossible. You don't need to know how it will perform in every situation. The main question you have is if using prepared queries is faster or not, so pick a common structure, create a table, get some data, and test. I can say that prepared queries will be more likely to give you a performance boost with wider tables (more columns). based on Stephan's explination of where binary could help, I think the easy answer is to decide not to bother with it (the postgres text to X converters get far more optimization attention than anything rsyslog could deploy) While that's true, there's no substitute for not having to do a conversion at all. After all, it's alot cheaper to do a bit of byte-swapping on an integer value that's already an integer in memory than to sprintf and atoi it. Thanks, Stephen signature.asc Description: Digital signature
Re: [PERFORM] performance for high-volume log insertion
David, * da...@lang.hm (da...@lang.hm) wrote: is this as simple as creating a database and doing an explain on each of these? or do I need to actually measure the time (at which point the specific hardware and tuning settings become an issue again) No, you need to measure the time. An explain isn't going to tell you much. However, I think the point here is that if you see a 10% performance improvment on some given hardware for a particular test, then chances are pretty good most people will see a performance benefit. Some more, some less, but it's unlikely anyone will have worse performance for it. There are some edge cases where a prepared statement can reduce performance, but that's almost always on SELECT queries, I can't think of a reason off-hand why it'd ever be slower for INSERTs unless you're already doing things you shouldn't be if you care about performance (like doing a join against some other table with each insert..). Additionally, there's really no way for us to know what an acceptable performance improvment is for you to justify the added code maintenance and whatnot for your project. If you're really just looking for the low-hanging fruit, then batch your inserts into transactions and go from there. Thanks, Stephen signature.asc Description: Digital signature
Re: [PERFORM] performance for high-volume log insertion
On Tue, 21 Apr 2009, Stephen Frost wrote: * da...@lang.hm (da...@lang.hm) wrote: while I fully understand the 'benchmark your situation' need, this isn't that simple. It really is. You know your application, you know it's primary use cases, and probably have some data to play with. You're certainly in a much better situation to at least *try* and benchmark it than we are. rsyslog is a syslog server. it replaces (or for debian and fedora, has replaced) your standard syslog daemon. it recieves log messages from every app on your system (and possibly others), filters, maniulates them, and then stores them somewhere. among the places that it can store the logs are database servers (native support for MySQL, PostgreSQL, and Oracle. plus libdbi for others) other apps then search and report on the data after it is stored. what apps?, I don't know either. pick your favorite reporting tool and you'll be a step ahead of me (I don't know a really good reporting tool) as for sample data, you have syslog messages, just like I do. so you have the same access to data that I have. how would you want to query them? how would people far less experianced that you want to query them? I can speculate that some people would do two columns (time, everything else), others will do three (time, server, everything else), and others will go further (I know some who would like to extract IP addresses embedded in a message into their own column). some people will index on the time and host, others will want to do full-text searches of everything. I can talk about the particular use case I have at work, but that would be far from normal (full text searches on 10s of TB of data, plus reports, etc) but we don't (currently) use postgres to do that, and I'm not sure how I would configure postgres for that sort of load. so I don't think that my personal situation is a good fit. I looked at bizgres a few years ago, but I didn't know enough about what I was trying to do or how much data I was trying to deal with to go forward with it at that time. do I do the benchmark on the type of hardware that I use for the system above (after spending how much time experimenting to find corret tuning)? or on a stock, untuned postgres running on a desktop-type system (we all know how horrible the defaults are), how am I supposed to know if the differences that I will see in my 'benchmarks' are the result of the differences between the commands, and not that I missed some critical knob to turn? benchmarking is absolutly the right answer for some cases, especially when someone is asking exactly how something will work for them. but in this case I don't have the specific use case. I am trying to find out where the throretical advantages are for these things that 'everybody knows you should do with a database' to understand the probability that they will make a difference in this case. in this case we are trying to decide what API/interface to use in a infrastructure tool that will be distributed in common distros (it's now the default syslog package of debian and fedora), there are so many variables in hardware, software, and load that trying to benchmark it becomes effectivly impossible. You don't need to know how it will perform in every situation. The main question you have is if using prepared queries is faster or not, so pick a common structure, create a table, get some data, and test. I can say that prepared queries will be more likely to give you a performance boost with wider tables (more columns). this is very helpful, I can't say what the schema would look like, but I would guess that it will tend towards the narrow side (or at least not update very many columns explicitly) based on Stephan's explination of where binary could help, I think the easy answer is to decide not to bother with it (the postgres text to X converters get far more optimization attention than anything rsyslog could deploy) While that's true, there's no substitute for not having to do a conversion at all. After all, it's alot cheaper to do a bit of byte-swapping on an integer value that's already an integer in memory than to sprintf and atoi it. but it's not a integer in memory, it's text that arrived over the network or through a socket as a log message from another application. David Lang -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
Re: [PERFORM] performance for high-volume log insertion
On Tue, 21 Apr 2009, Stephen Frost wrote: David, * da...@lang.hm (da...@lang.hm) wrote: I thought that part of the 'efficiancy' and 'performance' to be gained from binary modes were avoiding the need to parse commands, if it's only the savings in converting column contents from text to specific types, it's much less important. No, binary mode is about the column contents. Prepared queries is about avoiding having to parse commands (which is why the command and the data elements are seperately done). thanks for the clarification. I'd avoid having the user provide actual SQL, because that becomes difficult to deal with unless you embed an SQL parser in rsyslog, and I don't really see the value in that. there's no need for rsyslog to parse the SQL, just to be able to escape it appropriately and then pass it to the database for execution If the user is providing SQL, then you need to be able to parse that SQL if you're going to do prepared queries. It might not require you to be able to fully parse SQL the way the back-end does, but anything you do that's not a full SQL parser is going to end up having limitations that won't be easy to find or document. the current situation is that rsyslog never parses the SQL (other than as text for a template, just like if you were going to write the log message to disk) if we stick with the string based API we never need to the user gives us one string 'prepare...' that we send to the database. the user then gives us another string 'execute...' that we send to the database. at no point do we ever need to parse the SQL, or even really know that it is SQL (the one exception is an escapeing routine that replace ' with '' in the strings comeing from the outside world), it's just strings assembled using the same string assembly logic that is used for writing files to disk, crafting the payload of network packets to other servers, etc. I do agree that there is a reduction in security risk. but since rsyslog is rather draconian about forcing the escaping, I'm not sure this is enough to tip the scales. one huge advantage of putting the sql into the configuration is the ability to work around other users of the database. See, I just don't see that. moving a bit away from the traditional syslog use case for a moment. with the ability to accept messages from many different types of sources (some unreliable like UDP syslog, others very reliably with full application-level acknowledgements), the ability to filter messages to different destination, and the ability to configure it to craft arbatrary SQL statements, rsyslog can be useful as an 'impedance match' between different applications. you can coherse just about any app to write some sort of message to a file/pipe, and rsyslog can take that and get it into a database elsewhere. yes, you or I could write a quick program that would reformat the message and submit it (in perl/python/etc, but extending that to handle outages, high-volume bursts of traffic, etc starts to be hard. this is very much _not_ a common use case, but it's a useful side-use of rsyslog today. I guess you could give examples of the SQL in the documentation for how to create the prepared statement etc in the databases, but how is that much better than having it in the config file? for many users it's easier to do middlein -fancy stuff in the SQL than loading things into the database (can you pre-load prepared statements in the database? or are they a per-connection thing?) Prepared statements, at least under PG, are a per-connection thing. Triggers aren't the same, those are attached to tables and get called whenever a particular action is done on those tables (defined in the trigger definition). The trigger is then called with the row which is being inserted, etc, and can do whatever it wants with that row (put it in a different table, ignore it, etc). that sounds like a lot of work at the database level to avoid some complexity on the app side (and it seems that the need to fire a trigger probably cost more than the prepared statement ever hoped to gain.) so back to the main questions of the advantages prepared statements avoid needing to escape things, but at the complication of a more complex API. there's still the question of the performance difference. I have been thinking that the overhead of doing the work itself would overwelm the performance benifits of prepared statements. What work is it that you're referring to here? doing the inserts themselves (putting the data in the tables, updating indexes, issuing a fsync) Based on what you've said about your application so far, I would expect that the run-time cost to prepare the statement (which you do only once) to be a bit of a cost, but not much, and that the actual inserts would be almost free from the application side, and much easier for the database to parse/use. the inserts are far from free ;-) but I agree that with
Re: [PERFORM] performance for high-volume log insertion
da...@lang.hm wrote: On Tue, 21 Apr 2009, Stephen Frost wrote: * da...@lang.hm (da...@lang.hm) wrote: while I fully understand the 'benchmark your situation' need, this isn't that simple. It really is. You know your application, you know it's primary use cases, and probably have some data to play with. You're certainly in a much better situation to at least *try* and benchmark it than we are. rsyslog is a syslog server. it replaces (or for debian and fedora, has replaced) your standard syslog daemon. it recieves log messages from every app on your system (and possibly others), filters, maniulates them, and then stores them somewhere. among the places that it can store the logs are database servers (native support for MySQL, PostgreSQL, and Oracle. plus libdbi for others) Well, from a performance standpoint the obvious things to do are: 1. Keep a connection open, do NOT reconnect for each log-statement 2. Batch log statements together where possible 3. Use prepared statements 4. Partition the tables by day/week/month/year (configurable I suppose) The first two are vital, the third takes you a step further. The fourth is a long-term admin thing. And possibly 5. Have two connections, one for fatal/error etc and one for info/debug level log statements (configurable split?). Then you can use the synchronous_commit setting on the less important ones. Might buy you some performance on a busy system. http://www.postgresql.org/docs/8.3/interactive/runtime-config-wal.html#RUNTIME-CONFIG-WAL-SETTINGS other apps then search and report on the data after it is stored. what apps?, I don't know either. pick your favorite reporting tool and you'll be a step ahead of me (I don't know a really good reporting tool) as for sample data, you have syslog messages, just like I do. so you have the same access to data that I have. how would you want to query them? how would people far less experianced that you want to query them? I can speculate that some people would do two columns (time, everything else), others will do three (time, server, everything else), and others will go further (I know some who would like to extract IP addresses embedded in a message into their own column). some people will index on the time and host, others will want to do full-text searches of everything. Well, assuming it looks much like traditional syslog, I would do something like: (timestamp, host, facility, priority, message). It's easy enough to stitch back together if people want that. PostgreSQL's full-text indexing is quite well suited to logfiles I'd have thought, since it knows about filenames, urls etc already. If you want to get fancy, add a msg_type column and one subsidiary table for each msg_type. So - you might have smtp_connect_from (hostname, ip_addr). A set of perl regexps can match and extract the fields for these extra tables, or you could do it with triggers inside the database. I think it makes sense to do it in the application. Easier for users to contribute new patterns/extractions. Meanwhile, the core table is untouched so you don't *need* to know about these extra tables. If you have subsidiary tables, you'll want to partition those too and perhaps stick them in their own schema (logs200901, logs200902 etc). -- Richard Huxton Archonet Ltd -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
Re: [PERFORM] GiST index performance
On Mon, 20 Apr 2009, Teodor Sigaev wrote: Looks like contrib/cube has the same error. I don't see a similar code pattern elsewhere though. Oleg, Teodor, do you concur that this is a correct patch? Is it safe to back-patch (I think it should be)? Yeah, good catch, and it doesn't touch any already-on-disk data. Although release notes should mention advice about REINDEX seg and cube opclasses. Unfortunately, it seems there is another bug in the picksplit function. My patch fixes a bug that reveals this new bug. The whole picksplit algorithm is fundamentally broken, and needs to be rewritten completely, which is what I am doing. If you apply my patch, then index sizes will go up by a factor of ten or so, because the picksplit function tends to split the set of 367 ranges into one set of 366 and another set of 1, leading to a horribly unbalanced tree. Before the patch, the different branches of the tree were unselective, so new entries would just get stuffed in anywhere, leading to a much more balanced tree. I shall have a proper fix to this problem later today. Matthew -- It's one of those irregular verbs - I have an independent mind, You are an eccentric, He is round the twist. -- Bernard Woolly, Yes Prime Minister -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
Re: [PERFORM] performance for high-volume log insertion
Hi, I just finished reading this thread. We are currently working on setting up a central log system using rsyslog and PostgreSQL. It works well once we patched the memory leak. We also looked at what could be done to improve the efficiency of the DB interface. On the rsyslog side, moving to prepared queries allows you to remove the escaping that needs to be done currently before attempting to insert the data into the SQL backend as well as removing the parsing and planning time from the insert. This is a big win for high insert rates, which is what we are talking about. The escaping process is also a big CPU user in rsyslog which then hands the escaped string to the backend which then has to undo everything that had been done and parse/plan the resulting query. This can use a surprising amount of additional CPU. Even if you cannot support a general prepared query interface, by specifying what the query should look like you can handle much of the low-hanging fruit query-wise. We are currently using a date based trigger to use a new partition each day and keep 2 months of logs currently. This can be usefully managed on the backend database, but if rsyslog supported changing the insert to the new table on a time basis, the CPU used by the trigger to support this on the backend could be reclaimed. This would be a win for any DB backend. As you move to the new partition, issuing a truncate to clear the table would simplify the DB interfaces. Another performance enhancement already mentioned, would be to allow certain extra fields in the DB to be automatically populated as a function of the log messages. For example, logging the mail queue id for messages from mail systems would make it much easier to locate particular mail transactions in large amounts of data. To sum up, eliminating the escaping in rsyslog through the use of prepared queries would reduce the CPU load on the DB backend. Batching the inserts will also net you a big performance increase. Some DB-based applications allow for the specification of several types of queries, one for single inserts and then a second to support multiple inserts (copy). Rsyslog already supports the queuing pieces to allow you to batch inserts. Just some ideas. Regards, Ken On Tue, Apr 21, 2009 at 09:56:23AM +0100, Richard Huxton wrote: da...@lang.hm wrote: On Tue, 21 Apr 2009, Stephen Frost wrote: * da...@lang.hm (da...@lang.hm) wrote: while I fully understand the 'benchmark your situation' need, this isn't that simple. It really is. You know your application, you know it's primary use cases, and probably have some data to play with. You're certainly in a much better situation to at least *try* and benchmark it than we are. rsyslog is a syslog server. it replaces (or for debian and fedora, has replaced) your standard syslog daemon. it recieves log messages from every app on your system (and possibly others), filters, maniulates them, and then stores them somewhere. among the places that it can store the logs are database servers (native support for MySQL, PostgreSQL, and Oracle. plus libdbi for others) Well, from a performance standpoint the obvious things to do are: 1. Keep a connection open, do NOT reconnect for each log-statement 2. Batch log statements together where possible 3. Use prepared statements 4. Partition the tables by day/week/month/year (configurable I suppose) The first two are vital, the third takes you a step further. The fourth is a long-term admin thing. And possibly 5. Have two connections, one for fatal/error etc and one for info/debug level log statements (configurable split?). Then you can use the synchronous_commit setting on the less important ones. Might buy you some performance on a busy system. http://www.postgresql.org/docs/8.3/interactive/runtime-config-wal.html#RUNTIME-CONFIG-WAL-SETTINGS other apps then search and report on the data after it is stored. what apps?, I don't know either. pick your favorite reporting tool and you'll be a step ahead of me (I don't know a really good reporting tool) as for sample data, you have syslog messages, just like I do. so you have the same access to data that I have. how would you want to query them? how would people far less experianced that you want to query them? I can speculate that some people would do two columns (time, everything else), others will do three (time, server, everything else), and others will go further (I know some who would like to extract IP addresses embedded in a message into their own column). some people will index on the time and host, others will want to do full-text searches of everything. Well, assuming it looks much like traditional syslog, I would do something like: (timestamp, host, facility, priority, message). It's easy enough to stitch back together if people want that. PostgreSQL's full-text indexing is quite well suited to logfiles I'd have thought, since it knows about filenames,
[PERFORM] WHERE condition not being pushed down to union parts
I have a database with two tables that relate similar data, and a view which projects and combines the data from these two tables in order to access them both in a consistent manner. With enough information, the application can specifically choose to query from one table or the other, but in the more general case the data could come from either table, so I need to query the view. When I join against the view (or an equivalent subselect), however, it looks like the joining condition is not pushed down into the individual components of the union that defines the view. This leads to a significant performance degradation when using the view; I ask the list for help in resolving this problem. The remainder of this email digs into this problem in detail. (If you were interested in background on this database, it implements a backing store for a higher level RDF database, specifically for the RDFLib project. I would be happy to talk more about this application, or the corresponding database design issues, with anyone who might be interested, in whatever forum would be appropriate.) I begin with the poorly performing query, which follows this paragraph. This query joins one of the tables to the view, and using 'explain' on this query gives the query plan listed below the query. Note that in this query plan, the join filter happens after (above) the collection of matching rows from each of the parts of the UNION. query select * from relations as component_0_statements cross join URI_or_literal_object as component_1_statements where component_0_statements.predicate = -2875059751320018987 and component_0_statements.object = -2827607394936393903 and component_1_statements.subject = component_0_statements.subject and component_1_statements.predicate = -2875059751320018987 /query query-plan QUERY PLAN --- Nested Loop (cost=96.31..36201.57 rows=1 width=128) Join Filter: (component_0_statements.subject = literalproperties.subject) - Index Scan using relations_poscindex on relations component_0_statements (cost=0.00..9.96 rows=1 width=40) Index Cond: ((predicate = (-2875059751320018987)::bigint) AND (object = (-2827607394936393903)::bigint)) - Append (cost=96.31..36044.62 rows=11759 width=88) - Bitmap Heap Scan on literalproperties (cost=96.31..16190.72 rows=5052 width=49) Recheck Cond: (literalproperties.predicate = (-2875059751320018987)::bigint) - Bitmap Index Scan on literalproperties_predicateindex (cost=0.00..95.04 rows=5052 width=0) Index Cond: (literalproperties.predicate = (-2875059751320018987)::bigint) - Bitmap Heap Scan on relations (cost=128.99..19736.31 rows=6707 width=40) Recheck Cond: (relations.predicate = (-2875059751320018987)::bigint) - Bitmap Index Scan on relations_predicateindex (cost=0.00..127.32 rows=6707 width=0) Index Cond: (relations.predicate = (-2875059751320018987)::bigint) (13 rows) /query-plan As it turns out, all of the results are in fact from the 'relations' table, so we get the same results if we query that table instead of the more general view. The corresponding query follows this paragraph, and its query plan immediately follows it. Note that in this query plan, the join condition is pushed down to the leaf node as an Index Condition, which seems to be the main source of the dramatic performance difference. query select * from relations as component_0_statements cross join relations as component_1_statements where component_0_statements.predicate = -2875059751320018987 and component_0_statements.object = -2827607394936393903 and component_1_statements.subject = component_0_statements.subject and component_1_statements.predicate = -2875059751320018987 /query query-plan QUERY PLAN --- Nested Loop (cost=0.00..26.11 rows=1 width=80) - Index Scan using relations_poscindex on relations component_0_statements (cost=0.00..9.96 rows=1 width=40) Index Cond: ((predicate = (-2875059751320018987)::bigint) AND (object = (-2827607394936393903)::bigint)) - Index Scan using relations_subjectindex on relations component_1_statements (cost=0.00..16.13 rows=1 width=40) Index Cond: (component_1_statements.subject = component_0_statements.subject) Filter: (component_1_statements.predicate = (-2875059751320018987)::bigint) (6 rows) /query-plan My research led me to a post by Tom Lane describing the conditions in which the WHERE conditions cannot be pushed down to the UNION parts: http://archives.postgresql.org/pgsql-performance/2007-11/msg00041.php. I refactored the UNION definition slightly to attempt to bring all the column types
Re: [PERFORM] WHERE condition not being pushed down to union parts
John L. Clark j...@po.cwru.edu writes: I have a database with two tables that relate similar data, and a view which projects and combines the data from these two tables in order to access them both in a consistent manner. With enough information, the application can specifically choose to query from one table or the other, but in the more general case the data could come from either table, so I need to query the view. When I join against the view (or an equivalent subselect), however, it looks like the joining condition is not pushed down into the individual components of the union that defines the view. You never mentioned what PG version you are using, but I'm betting it's 8.1.x. This should work the way you are expecting in 8.2 and up. regards, tom lane -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
Re: [PERFORM] WHERE condition not being pushed down to union parts
On Tue, Apr 21, 2009 at 10:35 AM, Tom Lane t...@sss.pgh.pa.us wrote: You never mentioned what PG version you are using, but I'm betting it's 8.1.x. This should work the way you are expecting in 8.2 and up. Naturally, I would forget (at least) one critical piece of information: $ pg_config --version PostgreSQL 8.3.7 Other ideas? Take care, John L. Clark -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
Re: [PERFORM] WHERE condition not being pushed down to union parts
John L. Clark j...@po.cwru.edu writes: On Tue, Apr 21, 2009 at 10:35 AM, Tom Lane t...@sss.pgh.pa.us wrote: You never mentioned what PG version you are using, but I'm betting it's 8.1.x. This should work the way you are expecting in 8.2 and up. Naturally, I would forget (at least) one critical piece of information: $ pg_config --version PostgreSQL 8.3.7 In that case you're going to need to provide a reproducible test case, 'cause it worksforme. regards, tom lane -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
Re: [PERFORM] performance for high-volume log insertion
Kenneth, could you join the discussion on the rsyslog mailing list? rsyslog-users rsys...@lists.adiscon.com I'm surprised to hear you say that rsyslog can already do batch inserts and am interested in how you did that. what sort of insert rate did you mange to get? David Lang On Tue, 21 Apr 2009, Kenneth Marshall wrote: Date: Tue, 21 Apr 2009 08:33:30 -0500 From: Kenneth Marshall k...@rice.edu To: Richard Huxton d...@archonet.com Cc: da...@lang.hm, Stephen Frost sfr...@snowman.net, Greg Smith gsm...@gregsmith.com, pgsql-performance@postgresql.org Subject: Re: [PERFORM] performance for high-volume log insertion Hi, I just finished reading this thread. We are currently working on setting up a central log system using rsyslog and PostgreSQL. It works well once we patched the memory leak. We also looked at what could be done to improve the efficiency of the DB interface. On the rsyslog side, moving to prepared queries allows you to remove the escaping that needs to be done currently before attempting to insert the data into the SQL backend as well as removing the parsing and planning time from the insert. This is a big win for high insert rates, which is what we are talking about. The escaping process is also a big CPU user in rsyslog which then hands the escaped string to the backend which then has to undo everything that had been done and parse/plan the resulting query. This can use a surprising amount of additional CPU. Even if you cannot support a general prepared query interface, by specifying what the query should look like you can handle much of the low-hanging fruit query-wise. We are currently using a date based trigger to use a new partition each day and keep 2 months of logs currently. This can be usefully managed on the backend database, but if rsyslog supported changing the insert to the new table on a time basis, the CPU used by the trigger to support this on the backend could be reclaimed. This would be a win for any DB backend. As you move to the new partition, issuing a truncate to clear the table would simplify the DB interfaces. Another performance enhancement already mentioned, would be to allow certain extra fields in the DB to be automatically populated as a function of the log messages. For example, logging the mail queue id for messages from mail systems would make it much easier to locate particular mail transactions in large amounts of data. To sum up, eliminating the escaping in rsyslog through the use of prepared queries would reduce the CPU load on the DB backend. Batching the inserts will also net you a big performance increase. Some DB-based applications allow for the specification of several types of queries, one for single inserts and then a second to support multiple inserts (copy). Rsyslog already supports the queuing pieces to allow you to batch inserts. Just some ideas. Regards, Ken On Tue, Apr 21, 2009 at 09:56:23AM +0100, Richard Huxton wrote: da...@lang.hm wrote: On Tue, 21 Apr 2009, Stephen Frost wrote: * da...@lang.hm (da...@lang.hm) wrote: while I fully understand the 'benchmark your situation' need, this isn't that simple. It really is. You know your application, you know it's primary use cases, and probably have some data to play with. You're certainly in a much better situation to at least *try* and benchmark it than we are. rsyslog is a syslog server. it replaces (or for debian and fedora, has replaced) your standard syslog daemon. it recieves log messages from every app on your system (and possibly others), filters, maniulates them, and then stores them somewhere. among the places that it can store the logs are database servers (native support for MySQL, PostgreSQL, and Oracle. plus libdbi for others) Well, from a performance standpoint the obvious things to do are: 1. Keep a connection open, do NOT reconnect for each log-statement 2. Batch log statements together where possible 3. Use prepared statements 4. Partition the tables by day/week/month/year (configurable I suppose) The first two are vital, the third takes you a step further. The fourth is a long-term admin thing. And possibly 5. Have two connections, one for fatal/error etc and one for info/debug level log statements (configurable split?). Then you can use the synchronous_commit setting on the less important ones. Might buy you some performance on a busy system. http://www.postgresql.org/docs/8.3/interactive/runtime-config-wal.html#RUNTIME-CONFIG-WAL-SETTINGS other apps then search and report on the data after it is stored. what apps?, I don't know either. pick your favorite reporting tool and you'll be a step ahead of me (I don't know a really good reporting tool) as for sample data, you have syslog messages, just like I do. so you have the same access to data that I have. how would you want to query them? how would people far less experianced that you want to query them? I can speculate that some people would do two columns (time, everything else), others will
Re: [PERFORM] performance for high-volume log insertion
On Tue, Apr 21, 2009 at 08:37:54AM -0700, da...@lang.hm wrote: Kenneth, could you join the discussion on the rsyslog mailing list? rsyslog-users rsys...@lists.adiscon.com I'm surprised to hear you say that rsyslog can already do batch inserts and am interested in how you did that. what sort of insert rate did you mange to get? David Lang David, I would be happy to join the discussion. I did not mean to say that rsyslog currently supported batch inserts, just that the pieces that provide stand-by queuing could be used to manage batching inserts. Cheers, Ken On Tue, 21 Apr 2009, Kenneth Marshall wrote: Date: Tue, 21 Apr 2009 08:33:30 -0500 From: Kenneth Marshall k...@rice.edu To: Richard Huxton d...@archonet.com Cc: da...@lang.hm, Stephen Frost sfr...@snowman.net, Greg Smith gsm...@gregsmith.com, pgsql-performance@postgresql.org Subject: Re: [PERFORM] performance for high-volume log insertion Hi, I just finished reading this thread. We are currently working on setting up a central log system using rsyslog and PostgreSQL. It works well once we patched the memory leak. We also looked at what could be done to improve the efficiency of the DB interface. On the rsyslog side, moving to prepared queries allows you to remove the escaping that needs to be done currently before attempting to insert the data into the SQL backend as well as removing the parsing and planning time from the insert. This is a big win for high insert rates, which is what we are talking about. The escaping process is also a big CPU user in rsyslog which then hands the escaped string to the backend which then has to undo everything that had been done and parse/plan the resulting query. This can use a surprising amount of additional CPU. Even if you cannot support a general prepared query interface, by specifying what the query should look like you can handle much of the low-hanging fruit query-wise. We are currently using a date based trigger to use a new partition each day and keep 2 months of logs currently. This can be usefully managed on the backend database, but if rsyslog supported changing the insert to the new table on a time basis, the CPU used by the trigger to support this on the backend could be reclaimed. This would be a win for any DB backend. As you move to the new partition, issuing a truncate to clear the table would simplify the DB interfaces. Another performance enhancement already mentioned, would be to allow certain extra fields in the DB to be automatically populated as a function of the log messages. For example, logging the mail queue id for messages from mail systems would make it much easier to locate particular mail transactions in large amounts of data. To sum up, eliminating the escaping in rsyslog through the use of prepared queries would reduce the CPU load on the DB backend. Batching the inserts will also net you a big performance increase. Some DB-based applications allow for the specification of several types of queries, one for single inserts and then a second to support multiple inserts (copy). Rsyslog already supports the queuing pieces to allow you to batch inserts. Just some ideas. Regards, Ken On Tue, Apr 21, 2009 at 09:56:23AM +0100, Richard Huxton wrote: da...@lang.hm wrote: On Tue, 21 Apr 2009, Stephen Frost wrote: * da...@lang.hm (da...@lang.hm) wrote: while I fully understand the 'benchmark your situation' need, this isn't that simple. It really is. You know your application, you know it's primary use cases, and probably have some data to play with. You're certainly in a much better situation to at least *try* and benchmark it than we are. rsyslog is a syslog server. it replaces (or for debian and fedora, has replaced) your standard syslog daemon. it recieves log messages from every app on your system (and possibly others), filters, maniulates them, and then stores them somewhere. among the places that it can store the logs are database servers (native support for MySQL, PostgreSQL, and Oracle. plus libdbi for others) Well, from a performance standpoint the obvious things to do are: 1. Keep a connection open, do NOT reconnect for each log-statement 2. Batch log statements together where possible 3. Use prepared statements 4. Partition the tables by day/week/month/year (configurable I suppose) The first two are vital, the third takes you a step further. The fourth is a long-term admin thing. And possibly 5. Have two connections, one for fatal/error etc and one for info/debug level log statements (configurable split?). Then you can use the synchronous_commit setting on the less important ones. Might buy you some performance on a busy system. http://www.postgresql.org/docs/8.3/interactive/runtime-config-wal.html#RUNTIME-CONFIG-WAL-SETTINGS other apps then search and report on the data after it is stored. what apps?, I don't know either. pick your favorite reporting tool and you'll be a
Re: [PERFORM] SQL With Dates
On Mon, Apr 20, 2009 at 9:55 AM, Rafael Domiciano rafael.domici...@gmail.com wrote: Hello People, I have initiated a work to review the sqls of our internal software. Lot of them he problem are about sql logic, or join with table unecessary, and so on. But software has lot of sql with date, doing thinks like: [..] date = '2009-04-01' AND date = '2009-04-15' [..] Redoing the SQL with fix date (date = '2009-04-01') o cost in explain always still about 200 or less. But with a period the cost is high, about 6000 or more. Select is using Index and the date is using index too. There is some way to use date period with less cost? If you have an actual performance problem (as opposed to a big number in EXPLAIN), then it's possible that the planner isn't estimating the number of rows that will be in that range very accurately. In that case, you might need to increase the statistics target for that column, or your default_statistics_target. In 8.3, the default default_statistics_target = 10. In 8.4, it will be 100, so you might try that for a starting point. But date columns can sometimes have highly skewed data, so you might find that you need an even higher value for that particular column. I wouldn't recommend raising the database-wide setting above 100 though (though I know some people have used 200 or 400 without too much pain, especially on Really Big Databases where longer planning time isn't a big deal because the execution times are measured in minutes - it doesn't sound like that's your situation though). The first thing, to do, is see how fast the query actually runs. Try setting \timing in psql and running the query to see how long it actually takes. If it's fast enough, you're done. If not, run EXPLAIN ANALYZE and compare the estimated row counts to t he actual row counts. If they're pretty close, you're out of luck - as others have already said, TANSTAAFL. If they're way off, the try the above. ...Robert -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
Re: [PERFORM] performance for high-volume log insertion
On Mon, 20 Apr 2009, da...@lang.hm wrote: one huge advantage of putting the sql into the configuration is the ability to work around other users of the database. +1 on this. We've always found tools much easier to work with when they could be adapted to our schema, as opposed to changing our process for the tool. -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
Re: [PERFORM] performance for high-volume log insertion
* Ben Chobot (be...@silentmedia.com) wrote: On Mon, 20 Apr 2009, da...@lang.hm wrote: one huge advantage of putting the sql into the configuration is the ability to work around other users of the database. +1 on this. We've always found tools much easier to work with when they could be adapted to our schema, as opposed to changing our process for the tool. I think we're all in agreement that we should allow the user to define their schema and support loading the data into it. The question has been if the user really needs the flexibility to define arbitrary SQL to be used to do the inserts. Something I'm a bit confused about, still, is if this is really even a problem. It sounds like rsyslog already allows arbitrary SQL in the config file with some kind of escape syntax for the variables. Why not just keep that, but split it into a prepared query (where you change the variables to $NUM vars for the prepared statement) and an array of values (to pass to PQexecPrepared)? If you already know how to figure out what the variables in the arbitrary SQL statement are, this shouldn't be any more limiting than today, except where a prepared query can't have a variable argument but a non-prepared query can (eg, table name). You could deal with that with some kind of configuration variable that lets the user choose to use prepared queries or not though, or some additional syntax that indicates certain variables shouldn't be translated to $NUM vars (eg: $*blah instead of $blah). Thanks, Stephen signature.asc Description: Digital signature
Re: [PERFORM] performance for high-volume log insertion
On Tue, 21 Apr 2009, Stephen Frost wrote: * Ben Chobot (be...@silentmedia.com) wrote: On Mon, 20 Apr 2009, da...@lang.hm wrote: one huge advantage of putting the sql into the configuration is the ability to work around other users of the database. +1 on this. We've always found tools much easier to work with when they could be adapted to our schema, as opposed to changing our process for the tool. I think we're all in agreement that we should allow the user to define their schema and support loading the data into it. The question has been if the user really needs the flexibility to define arbitrary SQL to be used to do the inserts. Something I'm a bit confused about, still, is if this is really even a problem. It sounds like rsyslog already allows arbitrary SQL in the config file with some kind of escape syntax for the variables. Why not just keep that, but split it into a prepared query (where you change the variables to $NUM vars for the prepared statement) and an array of values (to pass to PQexecPrepared)? If you already know how to figure out what the variables in the arbitrary SQL statement are, this shouldn't be any more limiting than today, except where a prepared query can't have a variable argument but a non-prepared query can (eg, table name). You could deal with that with some kind of configuration variable that lets the user choose to use prepared queries or not though, or some additional syntax that indicates certain variables shouldn't be translated to $NUM vars (eg: $*blah instead of $blah). I think the key thing is that rsyslog today doesn't know anything about SQL variables, it just creates a string that the user and the database say looks like a SQL statement. an added headache is that the rsyslog config does not have the concept of arrays (the closest that it has is one special-case hack to let you specify one variable multiple times) if the performance win of the prepared statement is significant, then it's probably worth the complication of changing these things. David Lang -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
Re: [PERFORM] performance for high-volume log insertion
On Mon, 20 Apr 2009, da...@lang.hm wrote: while I fully understand the 'benchmark your situation' need, this isn't that simple. in this case we are trying to decide what API/interface to use in a infrastructure tool that will be distributed in common distros (it's now the default syslog package of debian and fedora), there are so many variables in hardware, software, and load that trying to benchmark it becomes effectivly impossible. From your later comments, you're wandering a bit outside of what you were asking about here. Benchmarking the *query* side of things can be extremely complicated. You have to worry about memory allocation, cold vs. warm cache, scale of database relative to RAM, etc. You were asking specifically about *insert* performance, which isn't nearly as complicated. There are basically three setups: 1) Disk/controller has a proper write cache. Writes and fsync will be fast. You can insert a few thousand individual transactions per second. 2) Disk/controller has a lying write cache. Writes and fsync will be fast, but it's not safe for database use. But since (1) is expensive and this one you can get for free jut by using a regular SATA drive with its write cache enabled, you can use this case as a proxy for approximately how (1) would act. You'll still get a few thousand transactions per second, sustained writes may slow down relative to (1) if you insert enough that you hit a checkpoint (triggering lots of random I/O). 3) All write caches have been disabled because they were not battery-backed. This is the case if you have a regular SATA drive and you disable its write cache because you care about write durability. You'll get a bit less than RPM/60 writes/second, so 120 inserts/second with a typical 7200RPM drive. Here batching multiple INSERTs together is critical to get any sort of reasonable performance. In (3), I'd expect that trivia like INSERT vs. COPY and COPY BINARY vs. COPY TEXT would be overwhelmed by the overhead of the commit itself. Therefore you probably want to test with case (2) instead, as it doesn't require any additional hardware but has similar performance to a production-worthy (1). All of the other things you're worried about really don't matter here; you can get an approximate measure of what the performance of the various INSERT/COPY schemes are that is somewhat platform dependant, but the results should be good enough to give you some rule of thumb suggestions for whether optimizations are significant enough to justify the coding effort to implement them or not. I'm not sure whether you're familiar with all the fsync trivia here. In normal syslog use, there's an fsync call after every write. You can disable that by placing a - before the file name in /etc/syslog.conf The thing that is going to make database-based writes very different is that syslog's fsync'd writes are unlikely to leave you in a bad state if the drive lies about them, while database writes can. So someone using syslog on a standard SATA drive isn't getting the write guarantee they think they are, but the downside on a crash is minimal. If you've got a high-volume syslog environment (100 lines/second), you can't support those as individual database writes unless you've got a battery-backed write controller. A regular disk just can't process genuine fsync calls any faster than that. A serious syslog deployment that turns fsync on and expects it to really do its thing is already exposed to this issue though. I think it may be a the case that a lot of people think they have durable writes in their configuration but really don't. -- * Greg Smith gsm...@gregsmith.com http://www.gregsmith.com Baltimore, MD -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
Re: [PERFORM] performance for high-volume log insertion
* da...@lang.hm (da...@lang.hm) wrote: I think the key thing is that rsyslog today doesn't know anything about SQL variables, it just creates a string that the user and the database say looks like a SQL statement. err, what SQL variables? You mean the $NUM stuff? They're just placeholders.. You don't really need to *do* anything with them.. Or are you worried that users would provide something that would break as a prepared query? If so, you just need to figure out how to handle that cleanly.. an added headache is that the rsyslog config does not have the concept of arrays (the closest that it has is one special-case hack to let you specify one variable multiple times) Argh. The array I'm talking about is a C array, and has nothing to do with the actual config syntax.. I swear, I think you're making this more difficult by half. Alright, looking at the documentation on rsyslog.com, I see something like: $template MySQLInsert,insert iut, message, receivedat values ('%iut%', '%msg:::UPPERCASE%', '%timegenerated:::date-mysql%') into systemevents\r\n, SQL Ignoring the fact that this is horrible, horrible non-SQL, I see that you use %blah% to define variables inside your string. That's fine. There's no reason why you can't use this exact syntax to build a prepared query. No user-impact changes are necessary. Here's what you do: build your prepared query by doing: copy user string newstring = replace all %blah% strings with $1, $2, $3, etc. myvars = dynamically created C array with the %blah%'s in it call PQprepare(newstring) when a record comes in: allocate a myvalues array of pointers loop through myvars for each myvar set the corresponding pointer in myvalues to the string which it corresponds to from the record call PQexecPrepared(myvalues) That's pretty much it. I assume you already deal with escaping %'s somehow during the config load so that the prepared statement will be what the user expects. As I mentioned before, the only obvious issue I see with doing this implicitly is that the user might want to put variables in places that you can't have variables in prepared queries. You could deal with that by having the user indicate per template, using another template option, if the query can be prepared or not. Another options is adding to your syntax something like '%*blah%' which would tell the system to pre-populate that variable before issuing PQprepare on the resultant string. Of course, you might just use PQexecParams there, unless you want to be gung-ho and actually keep a hash around of prepared queries on the assumption that the variable the user gave you doesn't change very often (eg, '%*month%') and it's cheap to keep a small list of them around to use when they do match up. Thanks, Stephen signature.asc Description: Digital signature
Re: [PERFORM] performance for high-volume log insertion
On Tue, 21 Apr 2009, Greg Smith wrote: On Mon, 20 Apr 2009, da...@lang.hm wrote: while I fully understand the 'benchmark your situation' need, this isn't that simple. in this case we are trying to decide what API/interface to use in a infrastructure tool that will be distributed in common distros (it's now the default syslog package of debian and fedora), there are so many variables in hardware, software, and load that trying to benchmark it becomes effectivly impossible. From your later comments, you're wandering a bit outside of what you were asking about here. Benchmarking the *query* side of things can be extremely complicated. You have to worry about memory allocation, cold vs. warm cache, scale of database relative to RAM, etc. You were asking specifically about *insert* performance, which isn't nearly as complicated. There are basically three setups: 1) Disk/controller has a proper write cache. Writes and fsync will be fast. You can insert a few thousand individual transactions per second. 2) Disk/controller has a lying write cache. Writes and fsync will be fast, but it's not safe for database use. But since (1) is expensive and this one you can get for free jut by using a regular SATA drive with its write cache enabled, you can use this case as a proxy for approximately how (1) would act. You'll still get a few thousand transactions per second, sustained writes may slow down relative to (1) if you insert enough that you hit a checkpoint (triggering lots of random I/O). 3) All write caches have been disabled because they were not battery-backed. This is the case if you have a regular SATA drive and you disable its write cache because you care about write durability. You'll get a bit less than RPM/60 writes/second, so 120 inserts/second with a typical 7200RPM drive. Here batching multiple INSERTs together is critical to get any sort of reasonable performance. in case #1 would you expect to get significant gains from batching? doesn't it suffer from problems similar to #2 when checkpoints hit? In (3), I'd expect that trivia like INSERT vs. COPY and COPY BINARY vs. COPY TEXT would be overwhelmed by the overhead of the commit itself. Therefore you probably want to test with case (2) instead, as it doesn't require any additional hardware but has similar performance to a production-worthy (1). All of the other things you're worried about really don't matter here; you can get an approximate measure of what the performance of the various INSERT/COPY schemes are that is somewhat platform dependant, but the results should be good enough to give you some rule of thumb suggestions for whether optimizations are significant enough to justify the coding effort to implement them or not. I'll see about setting up a test in the next day or so. should I be able to script this through psql? or do I need to write a C program to test this? I'm not sure whether you're familiar with all the fsync trivia here. In normal syslog use, there's an fsync call after every write. You can disable that by placing a - before the file name in /etc/syslog.conf The thing that is going to make database-based writes very different is that syslog's fsync'd writes are unlikely to leave you in a bad state if the drive lies about them, while database writes can. So someone using syslog on a standard SATA drive isn't getting the write guarantee they think they are, but the downside on a crash is minimal. If you've got a high-volume syslog environment (100 lines/second), you can't support those as individual database writes unless you've got a battery-backed write controller. A regular disk just can't process genuine fsync calls any faster than that. A serious syslog deployment that turns fsync on and expects it to really do its thing is already exposed to this issue though. I think it may be a the case that a lot of people think they have durable writes in their configuration but really don't. rsyslog is a little different, instead of just input - disk it does input - queue - output (where output can be many things, including disk or database) it's default is to use memory-based queues (and no fsync), but has config options to do disk based queues with a fsync after each update David Lang -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
Re: [PERFORM] WHERE condition not being pushed down to union parts
On Tue, Apr 21, 2009 at 12:05 PM, John L. Clark j...@po.cwru.edu wrote: On Tue, Apr 21, 2009 at 10:50 AM, Tom Lane t...@sss.pgh.pa.us wrote: In that case you're going to need to provide a reproducible test case, 'cause it worksforme. Ok. I scaled back my example by just selecting 1000 random rows from each of the component tables. The resulting database dump should be attached to this email. I tried a very small subset (just 10 rows), but the resulting tables were small enough that the query plans were changing to use scans. Note that I haven't actually run sample queries with this smaller dataset. I have only been inspecting the query plans of the two queries that I listed in my original message, and the results are the same, except that the magnitude of the costs are scaled down. This scaling leads to a smaller performance penalty, but the query plan still shows that the join filter is still not being pushed down in the case of the view (built from a union). I posted this earlier, but I haven't seen it come through the mailing list, perhaps because of the attachment. I have also posted the attachment at http://infinitesque.net/temp/union_performance_2009-04-21.postgresql.dump.gz. The MD5 checksum is 3942fee39318aa5d9f18ac2ef3c298cf. If the original does end up coming through, I'm sorry about the redundant post. Take care, John L. Clark -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
Re: [PERFORM] performance for high-volume log insertion
On Tue, Apr 21, 2009 at 11:09:18AM -0700, da...@lang.hm wrote: On Tue, 21 Apr 2009, Greg Smith wrote: On Mon, 20 Apr 2009, da...@lang.hm wrote: while I fully understand the 'benchmark your situation' need, this isn't that simple. in this case we are trying to decide what API/interface to use in a infrastructure tool that will be distributed in common distros (it's now the default syslog package of debian and fedora), there are so many variables in hardware, software, and load that trying to benchmark it becomes effectivly impossible. From your later comments, you're wandering a bit outside of what you were asking about here. Benchmarking the *query* side of things can be extremely complicated. You have to worry about memory allocation, cold vs. warm cache, scale of database relative to RAM, etc. You were asking specifically about *insert* performance, which isn't nearly as complicated. There are basically three setups: 1) Disk/controller has a proper write cache. Writes and fsync will be fast. You can insert a few thousand individual transactions per second. 2) Disk/controller has a lying write cache. Writes and fsync will be fast, but it's not safe for database use. But since (1) is expensive and this one you can get for free jut by using a regular SATA drive with its write cache enabled, you can use this case as a proxy for approximately how (1) would act. You'll still get a few thousand transactions per second, sustained writes may slow down relative to (1) if you insert enough that you hit a checkpoint (triggering lots of random I/O). 3) All write caches have been disabled because they were not battery-backed. This is the case if you have a regular SATA drive and you disable its write cache because you care about write durability. You'll get a bit less than RPM/60 writes/second, so 120 inserts/second with a typical 7200RPM drive. Here batching multiple INSERTs together is critical to get any sort of reasonable performance. in case #1 would you expect to get significant gains from batching? doesn't it suffer from problems similar to #2 when checkpoints hit? Even with a disk controller with a proper write cache, the latency for single-insert-at-a-time will keep the number of updates to the low thousands per second (on the controllers I have used). If you can batch them, it would not be unreasonable to increase performance by an order of magnitude or more. At the high end, other issues like CPU usage can restrict performance. Ken In (3), I'd expect that trivia like INSERT vs. COPY and COPY BINARY vs. COPY TEXT would be overwhelmed by the overhead of the commit itself. Therefore you probably want to test with case (2) instead, as it doesn't require any additional hardware but has similar performance to a production-worthy (1). All of the other things you're worried about really don't matter here; you can get an approximate measure of what the performance of the various INSERT/COPY schemes are that is somewhat platform dependant, but the results should be good enough to give you some rule of thumb suggestions for whether optimizations are significant enough to justify the coding effort to implement them or not. I'll see about setting up a test in the next day or so. should I be able to script this through psql? or do I need to write a C program to test this? I'm not sure whether you're familiar with all the fsync trivia here. In normal syslog use, there's an fsync call after every write. You can disable that by placing a - before the file name in /etc/syslog.conf The thing that is going to make database-based writes very different is that syslog's fsync'd writes are unlikely to leave you in a bad state if the drive lies about them, while database writes can. So someone using syslog on a standard SATA drive isn't getting the write guarantee they think they are, but the downside on a crash is minimal. If you've got a high-volume syslog environment (100 lines/second), you can't support those as individual database writes unless you've got a battery-backed write controller. A regular disk just can't process genuine fsync calls any faster than that. A serious syslog deployment that turns fsync on and expects it to really do its thing is already exposed to this issue though. I think it may be a the case that a lot of people think they have durable writes in their configuration but really don't. rsyslog is a little different, instead of just input - disk it does input - queue - output (where output can be many things, including disk or database) it's default is to use memory-based queues (and no fsync), but has config options to do disk based queues with a fsync after each update David Lang -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance -- Sent
Re: [PERFORM] performance for high-volume log insertion
On Tue, 21 Apr 2009, Stephen Frost wrote: * da...@lang.hm (da...@lang.hm) wrote: I think the key thing is that rsyslog today doesn't know anything about SQL variables, it just creates a string that the user and the database say looks like a SQL statement. err, what SQL variables? You mean the $NUM stuff? They're just placeholders.. You don't really need to *do* anything with them.. Or are you worried that users would provide something that would break as a prepared query? If so, you just need to figure out how to handle that cleanly.. an added headache is that the rsyslog config does not have the concept of arrays (the closest that it has is one special-case hack to let you specify one variable multiple times) Argh. The array I'm talking about is a C array, and has nothing to do with the actual config syntax.. I swear, I think you're making this more difficult by half. not intentinally, but you may be right. Alright, looking at the documentation on rsyslog.com, I see something like: $template MySQLInsert,insert iut, message, receivedat values ('%iut%', '%msg:::UPPERCASE%', '%timegenerated:::date-mysql%') into systemevents\r\n, SQL Ignoring the fact that this is horrible, horrible non-SQL, that example is for MySQL, nuff said ;-) or are you referring to the modifiers that rsyslog has to manipulate the strings before inserting them? (as opposed to using sql to manipulate the strings) I see that you use %blah% to define variables inside your string. That's fine. There's no reason why you can't use this exact syntax to build a prepared query. No user-impact changes are necessary. Here's what you do: snip psudocode to replace %blah% with $num for some reason I was stuck on the idea of the config specifying the statement and variables seperatly, so I wasn't thinking this way, however there are headaches doing this will require changes to the structure of rsyslog, today the string manipulation is done before calling the output (database) module, so all the database module currently gets is a string. in a (IMHO misguided) attempt at security in a multi-threaded program, the output modules are not given access to the full data, only to the distiled result. also, this approach won't work if the user wants to combine fixed text with the variable into a column. an example of doing that would be to have a filter to match specific lines, and then use a slightly different template for those lines. I guess that could be done in SQL instead of in the rsyslog string manipulation (i.e. instead of 'blah-%host%' do 'blah-'||'%host') As I mentioned before, the only obvious issue I see with doing this implicitly is that the user might want to put variables in places that you can't have variables in prepared queries. this problem space would be anywhere except the column contents, right? You could deal with that by having the user indicate per template, using another template option, if the query can be prepared or not. Another options is adding to your syntax something like '%*blah%' which would tell the system to pre-populate that variable before issuing PQprepare on the resultant string. Of course, you might just use PQexecParams there, unless you want to be gung-ho and actually keep a hash around of prepared queries on the assumption that the variable the user gave you doesn't change very often (eg, '%*month%') and it's cheap to keep a small list of them around to use when they do match up. rsyslog supports something similar for writing to disk where you can use variables as part of the filename/path (referred to as 'dynafiles' in the documentation). that's a little easier to deal with as the filename is specified seperatly from the format of the data to write. If we end up doing prepared statements I suspect they initially won't support variables outside of the columns. David Lang -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
Re: [PERFORM] performance for high-volume log insertion
On Tue, 21 Apr 2009, da...@lang.hm wrote: I see that you use %blah% to define variables inside your string. That's fine. There's no reason why you can't use this exact syntax to build a prepared query. No user-impact changes are necessary. Here's what you do: snip psudocode to replace %blah% with $num for some reason I was stuck on the idea of the config specifying the statement and variables seperatly, so I wasn't thinking this way, however there are headaches doing this will require changes to the structure of rsyslog, today the string manipulation is done before calling the output (database) module, so all the database module currently gets is a string. in a (IMHO misguided) attempt at security in a multi-threaded program, the output modules are not given access to the full data, only to the distiled result. also, this approach won't work if the user wants to combine fixed text with the variable into a column. an example of doing that would be to have a filter to match specific lines, and then use a slightly different template for those lines. I guess that could be done in SQL instead of in the rsyslog string manipulation (i.e. instead of 'blah-%host%' do 'blah-'||'%host') by the way, now that I understand how you were viewing this, I see why you were saying that there would need to be a SQL parser. I was missing that headache, by going the direction of having the user specify the individual components (which has it's own headache) David Lang -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
Re: [PERFORM] performance for high-volume log insertion
* da...@lang.hm (da...@lang.hm) wrote: Ignoring the fact that this is horrible, horrible non-SQL, that example is for MySQL, nuff said ;-) indeed. for some reason I was stuck on the idea of the config specifying the statement and variables seperatly, so I wasn't thinking this way, however there are headaches Nothing worth doing is ever completely without complications. :) doing this will require changes to the structure of rsyslog, today the string manipulation is done before calling the output (database) module, so all the database module currently gets is a string. in a (IMHO misguided) attempt at security in a multi-threaded program, the output modules are not given access to the full data, only to the distiled result. Ah, yes, that's definitely a problem and I agree- a very misguided approach to doing things. Certainly, to use prepared queries, you will have to pass the data to whatever is talking to the database in some kind of structured way. There's not much advantage if you're getting it as a string and having to parse it out yourself before using a prepared query with the database. In a multi-threaded program, I think it would at least be reasonably easy/cheap to provide the output modules with the full data? Of course, you would need to teach the string manipulation logic to not do its escaping and other related work for prepared queries which are just going to use the full data anyway. also, this approach won't work if the user wants to combine fixed text with the variable into a column. an example of doing that would be to have a filter to match specific lines, and then use a slightly different template for those lines. I guess that could be done in SQL instead of in the rsyslog string manipulation (i.e. instead of 'blah-%host%' do 'blah-'||'%host') It would be more like: 'blah-' || %host% Just to be clear (if you put the %host% in quotes, and then convert that to '$1', it won't be considered a variable, at least in PG). That might be an issue going forward, but on the flip side, I can see some reasons for supporting both prepared and unprepared queries, so if you implement that through an additional template option, you can document that the user needs to ensure the prepared query is structured correctly with the correct quoting. This gives you the flexibility of the unprepared query for users who don't care about performance, and the benefits of prepared queries, where they can be used, for users who do need that performance. Or you could just force your users to move everything to prepared queries but it's probably too late for that. :) Maybe if things had started out that way.. As I mentioned before, the only obvious issue I see with doing this implicitly is that the user might want to put variables in places that you can't have variables in prepared queries. this problem space would be anywhere except the column contents, right? Well, it depends on the query.. You can have variables in the column contents, sure, but you can also have them in where clauses if you're doing something like: insert into blah select $1,$2,$3,b from mytable where $2 = c; I believe, in PG at least, you can use them pretty much anywhere you can use a constant. rsyslog supports something similar for writing to disk where you can use variables as part of the filename/path (referred to as 'dynafiles' in the documentation). that's a little easier to deal with as the filename is specified seperatly from the format of the data to write. If we end up doing prepared statements I suspect they initially won't support variables outside of the columns. That sounds reasonable, to me at least. Thanks, Stephen signature.asc Description: Digital signature
Re: [PERFORM] performance for high-volume log insertion
* da...@lang.hm (da...@lang.hm) wrote: by the way, now that I understand how you were viewing this, I see why you were saying that there would need to be a SQL parser. I was missing that headache, by going the direction of having the user specify the individual components (which has it's own headache) Right, but really, you're already parsing the SQL to the extent that you need to, and whatever limitations and headaches that causes you've already had to deal with through proper escaping and whatnot of your variables.. So I'm not sure that it'll be all that bad in the end. If you add this as a new feature that users essentially have to opt-in to, then I think you can offload alot of the work on to the users for doing things like fixing quoting (so the $NUM vars aren't quoted). Thanks, Stephen signature.asc Description: Digital signature
Re: [PERFORM] performance for high-volume log insertion
da...@lang.hm wrote: 2. insert into table values (),(),(),() Using this structure would be more database agnostic, but won't perform as well as the COPY options I don't believe. It might be interesting to do a large insert into table values (),(),() as a prepared statement, but then you'd have to have different sizes for each different number of items you want inserted. on the other hand, when you have a full queue (lots of stuff to insert) is when you need the performance the most. if it's enough of a win on the database side, it could be worth more effort on the applicaiton side. Are you sure preparing a simple insert is really worthwhile? I'd check if I were you. It shouldn't take long to plan. Note that this structure (above) is handy but not universal. You might want to try: insert into table select (...) union select (...) union select (...) ... as well, since its more univeral. Works on Sybase and SQLServer for example (and v.quickly too - much more so than a TSQL batch with lots of inserts or execs of stored procs) James -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
Re: [PERFORM] performance for high-volume log insertion
* James Mansion (ja...@mansionfamily.plus.com) wrote: da...@lang.hm wrote: on the other hand, when you have a full queue (lots of stuff to insert) is when you need the performance the most. if it's enough of a win on the database side, it could be worth more effort on the applicaiton side. Are you sure preparing a simple insert is really worthwhile? I'd check if I were you. It shouldn't take long to plan. Using prepared queries, at least if you use PQexecPrepared or PQexecParams, also reduces the work required on the client to build the whole string, and the parsing overhead on the database side to pull it apart again. That's where the performance is going to be improved by going that route, not so much in eliminating the planning. Thanks, Stephen signature.asc Description: Digital signature
Re: [PERFORM] WHERE condition not being pushed down to union parts
John L. Clark j...@po.cwru.edu writes: I posted this earlier, but I haven't seen it come through the mailing list, perhaps because of the attachment. I have also posted the attachment at http://infinitesque.net/temp/union_performance_2009-04-21.postgresql.dump.gz. Ah. The problem is that your view contains constants in the UNION arms: CREATE VIEW uri_or_literal_object AS SELECT literalproperties.subject, literalproperties.subject_term, literalproperties.predicate, literalproperties.predicate_term, literalproperties.object, 'L'::character(1) AS object_term, literalproperties.context, literalproperties.context_term, literalproperties.data_type, literalproperties.language FROM literalproperties UNION ALL SELECT relations.subject, relations.subject_term, relations.predicate, relations.predicate_term, relations.object, relations.object_term, relations.context, relations.context_term, NULL::bigint AS data_type, NULL::character varying(3) AS language FROM relations; In 8.2 and 8.3, the planner is only smart enough to generate inner-indexscan nestloop plans on UNIONs if all the elements of the SELECT lists are simple variables (that is, table columns). 8.4 will be smarter about this. regards, tom lane -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
Re: [PERFORM] performance for high-volume log insertion
On Tue, 21 Apr 2009, da...@lang.hm wrote: 1) Disk/controller has a proper write cache. Writes and fsync will be fast. You can insert a few thousand individual transactions per second. in case #1 would you expect to get significant gains from batching? doesn't it suffer from problems similar to #2 when checkpoints hit? Typically controllers with a write cache are doing elevator sorting across a much larger chunk of working memory (typically =256MB instead of 32MB on the disk itself) which means a mix of random writes will average better performance--on top of being able to aborb a larger chunk of them before blocking on writes. You get some useful sorting in the OS itself, but every layer of useful additional cache helps significantly here. Batching is always a win because even a write-cached commit is still pretty expensive, from the server on down the chain. I'll see about setting up a test in the next day or so. should I be able to script this through psql? or do I need to write a C program to test this? You can easily compare things with psql, like in the COPY BINARY vs. TEXT example I gave earlier, that's why I was suggesting you run your own tests here just to get a feel for things on your data set. -- * Greg Smith gsm...@gregsmith.com http://www.gregsmith.com Baltimore, MD -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
Re: [PERFORM] performance for high-volume log insertion
On Tue, 21 Apr 2009, Stephen Frost wrote: * James Mansion (ja...@mansionfamily.plus.com) wrote: da...@lang.hm wrote: on the other hand, when you have a full queue (lots of stuff to insert) is when you need the performance the most. if it's enough of a win on the database side, it could be worth more effort on the applicaiton side. Are you sure preparing a simple insert is really worthwhile? I'd check if I were you. It shouldn't take long to plan. Using prepared queries, at least if you use PQexecPrepared or PQexecParams, also reduces the work required on the client to build the whole string, and the parsing overhead on the database side to pull it apart again. That's where the performance is going to be improved by going that route, not so much in eliminating the planning. in a recent thread about prepared statements, where it was identified that since the planning took place at the time of the prepare you sometimes have worse plans than for non-prepared statements, a proposal was made to have a 'pre-parsed, but not pre-planned' version of a prepared statement. This was dismissed as a waste of time (IIRC by Tom L) as the parsing time was negligable. was that just because it was a more complex query to plan? David Lang -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
Re: [PERFORM] performance for high-volume log insertion
On Tue, Apr 21, 2009 at 8:12 PM, da...@lang.hm wrote: Using prepared queries, at least if you use PQexecPrepared or PQexecParams, also reduces the work required on the client to build the whole string, and the parsing overhead on the database side to pull it apart again. That's where the performance is going to be improved by going that route, not so much in eliminating the planning. in a recent thread about prepared statements, where it was identified that since the planning took place at the time of the prepare you sometimes have worse plans than for non-prepared statements, a proposal was made to have a 'pre-parsed, but not pre-planned' version of a prepared statement. This was dismissed as a waste of time (IIRC by Tom L) as the parsing time was negligable. was that just because it was a more complex query to plan? Joins are expensive to plan; a simple insert is not. I also disagree that pre-parsed but not pre-planned is a waste of time, whoever said it. Sometimes it's what you want, especially in PL/pgsql. ...Robert -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
Re: [PERFORM] performance for high-volume log insertion
Stephen Frost wrote: apart again. That's where the performance is going to be improved by going that route, not so much in eliminating the planning. Fine. But like I said, I'd suggest measuring the fractional improvement for this when sending multi-row inserts before writing something complex. I think the big will will be doing multi-row inserts at all. If you are going to prepare then you'll need a collection of different prepared statements for different batch sizes (say 1,2,3,4,5,10,20,50) and things will get complicated. A multi-row insert with unions and dynamic SQL is actually rather universal. Personally I'd implement that first (and it should be easy to do across multiple dbms types) and then return to it to have a more complex client side with prepared statements etc if (and only if) necessary AND the performance improvement were measurably worthwhile, given the indexing and storage overheads. There is no point optimising away the CPU of the simple parse if you are just going to get hit with a lot of latency from round trips, and forming a generic multi-insert SQL string is much, much easier to get working as a first step. Server CPU isn't a bottleneck all that often - and with something as simple as this you'll hit IO performance bottlenecks rather easily. James -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance