Re: [PERFORM] Insert performance vs Table size
I assume you took size to mean the row size? What I really meant was does the number of rows a table has affect the performance of new inserts into the table (just INSERTs) all other things remaining constant. Sorry for the confusion. I know that having indexes on the table adds an overhead but again does this overhead increase (for an INSERT operation) with the number of rows the table contains? My instinct says no to both. If I'm wrong can someone explain why the number of rows in a table affects INSERT performance? Thanks again -Original Message- From: Jacques Caron [mailto:[EMAIL PROTECTED] Sent: 27 June 2005 14:05 To: Praveen Raja Cc: pgsql-performance@postgresql.org Subject: RE: [PERFORM] Insert performance vs Table size Hi, At 13:50 27/06/2005, Praveen Raja wrote: Just to clear things up a bit, the scenario that I'm interested in is a table with a large number of indexes on it (maybe 7-8). If you're after performance you'll want to carefully consider which indexes are really useful and/or redesign your schema so that you can have less indexes on that table. 7 or 8 indexes is quite a lot, and that really has a cost. In this scenario other than the overhead of having to maintain the indexes (which I'm guessing is the same regardless of the size of the table) Definitely not: indexes grow with the size of the table. Depending on what columns you index (and their types), the indexes may be a fraction of the size of the table, or they may be very close in size (in extreme cases they may even be larger). With 7 or 8 indexes, that can be quite a large volume of data to manipulate, especially if the values of the columns inserted can span the whole range of the index (rather than being solely id- or time-based, for instance, in which case index updates are concentrated in a small area of each of the indexes), as this means you'll need to have a majority of the indexes in RAM if you want to maintain decent performance. does the size of the table play a role in determining insert performance (and I mean only insert performance)? In this case, it's really the indexes that'll cause you trouble, though heavily fragmented tables (due to lots of deletes or updates) will also incur a penalty just for the data part of the inserts. Also, don't forget the usual hints if you are going to do lots of inserts: - batch them in large transactions, don't do them one at a time - better yet, use COPY rather than INSERT - in some situations, you might be better of dropping the indexes, doing large batch inserts, then re-creating the indexes. YMMV depending on the existing/new ratio, whether you need to maintain indexed access to the tables, etc. - pay attention to foreign keys Jacques. ---(end of broadcast)--- TIP 7: don't forget to increase your free space map settings
Re: [PERFORM] Insert performance vs Table size
Hi, At 11:50 28/06/2005, Praveen Raja wrote: I assume you took size to mean the row size? Nope, the size of the table. What I really meant was does the number of rows a table has affect the performance of new inserts into the table (just INSERTs) all other things remaining constant. Sorry for the confusion. As I said previously, in most cases it does. One of the few cases where it doesn't would be an append-only table, no holes, no indexes, no foreign keys... I know that having indexes on the table adds an overhead but again does this overhead increase (for an INSERT operation) with the number of rows the table contains? It depends on what you are indexing. If the index key is something that grows monotonically (e.g. a unique ID or a timestamp), then the size of the table (and hence of the indexes) should have a very limited influence on the INSERTs. If the index key is anything else (and that must definitely be the case if you have 7 or 8 indexes!), then that means updates will happen all over the indexes, which means a lot of read and write activity, and once the total size of your indexes exceeds what can be cached in RAM, performance will decrease quite a bit. Of course if your keys are concentrated in a few limited areas of the key ranges it might help. My instinct says no to both. If I'm wrong can someone explain why the number of rows in a table affects INSERT performance? As described above, maintaining indexes when you hit anywhere in said indexes is very costly. The larger the table, the larger the indexes, the higher the number of levels in the trees, etc. As long as it fits in RAM, it shouldn't be a problem. Once you exceed that threshold, you start getting a lot of random I/O, and that's expensive. Again, it depends a lot on your exact schema, the nature of the data, the spread of the different values, etc, but I would believe it's more often the case than not. Jacques. ---(end of broadcast)--- TIP 5: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faq
Re: [PERFORM] Insert performance vs Table size
Praveen Raja [EMAIL PROTECTED] writes: I know that having indexes on the table adds an overhead but again does this overhead increase (for an INSERT operation) with the number of rows the table contains? Typical index implementations (such as b-tree) have roughly O(log N) cost to insert or lookup a key in an N-entry index. So yes, it grows, though slowly. regards, tom lane ---(end of broadcast)--- TIP 1: subscribe and unsubscribe commands go to [EMAIL PROTECTED]
Re: [PERFORM] Insert performance vs Table size
Praveen Raja: I think the size of a table don't affect the speed of inserts into it.Because PostgreSQL just doing something like append on the data files. But the index do speed-down the inserts. Because PostgreSQL should maintain the index when doing inserts. hope this is useful for your question. === 2005-06-27 19:24:06 you wrote:=== Hi all I'm wondering if and how the size of a table affects speed of inserts into it? What if the table has indexes, does that alter the answer? Thanks ---(end of broadcast)--- TIP 1: subscribe and unsubscribe commands go to [EMAIL PROTECTED] = = = = = = = = = = = = = = = = = = = = Best regards! 李江华 Seamus Dean Alibaba.com TEL:0571-85022088-2287 [EMAIL PROTECTED] 2005-06-27 ---(end of broadcast)--- TIP 3: if posting/reading through Usenet, please send an appropriate subscribe-nomail command to [EMAIL PROTECTED] so that your message can get through to the mailing list cleanly
Re: [PERFORM] Insert performance vs Table size
Hi, At 13:24 27/06/2005, Praveen Raja wrote: I'm wondering if and how the size of a table affects speed of inserts into it? What if the table has indexes, does that alter the answer? Many parameters will affect the result: - whether there are any indexes (including the primary key, unique constraints...) to update or not - whether there are any foreign keys from or to that table - the size of the rows - whether the table (or at least the bits being updated) fit in RAM or not - whether the table has holes (due to former updates/deletes and vacuum) and how they are placed - and probably a bunch of other things... Obviously, if you have an append-only (no updates, no deletes) table with no indexes and no foreign keys, the size of the table should not matter much. As soon as one of those conditions is not met table size will have an impact, probably small as long as whatever is needed can be held in RAM, a lot bigger once it's not the case. Hope that helps, Jacques. ---(end of broadcast)--- TIP 4: Don't 'kill -9' the postmaster
Re: [PERFORM] Insert performance vs Table size
Just to clear things up a bit, the scenario that I'm interested in is a table with a large number of indexes on it (maybe 7-8). In this scenario other than the overhead of having to maintain the indexes (which I'm guessing is the same regardless of the size of the table), does the size of the table play a role in determining insert performance (and I mean only insert performance)? -Original Message- From: Jacques Caron [mailto:[EMAIL PROTECTED] Sent: 27 June 2005 13:40 To: Praveen Raja Cc: pgsql-performance@postgresql.org Subject: Re: [PERFORM] Insert performance vs Table size Hi, At 13:24 27/06/2005, Praveen Raja wrote: I'm wondering if and how the size of a table affects speed of inserts into it? What if the table has indexes, does that alter the answer? Many parameters will affect the result: - whether there are any indexes (including the primary key, unique constraints...) to update or not - whether there are any foreign keys from or to that table - the size of the rows - whether the table (or at least the bits being updated) fit in RAM or not - whether the table has holes (due to former updates/deletes and vacuum) and how they are placed - and probably a bunch of other things... Obviously, if you have an append-only (no updates, no deletes) table with no indexes and no foreign keys, the size of the table should not matter much. As soon as one of those conditions is not met table size will have an impact, probably small as long as whatever is needed can be held in RAM, a lot bigger once it's not the case. Hope that helps, Jacques. ---(end of broadcast)--- TIP 8: explain analyze is your friend
Re: [PERFORM] Insert performance vs Table size
Hi, At 13:50 27/06/2005, Praveen Raja wrote: Just to clear things up a bit, the scenario that I'm interested in is a table with a large number of indexes on it (maybe 7-8). If you're after performance you'll want to carefully consider which indexes are really useful and/or redesign your schema so that you can have less indexes on that table. 7 or 8 indexes is quite a lot, and that really has a cost. In this scenario other than the overhead of having to maintain the indexes (which I'm guessing is the same regardless of the size of the table) Definitely not: indexes grow with the size of the table. Depending on what columns you index (and their types), the indexes may be a fraction of the size of the table, or they may be very close in size (in extreme cases they may even be larger). With 7 or 8 indexes, that can be quite a large volume of data to manipulate, especially if the values of the columns inserted can span the whole range of the index (rather than being solely id- or time-based, for instance, in which case index updates are concentrated in a small area of each of the indexes), as this means you'll need to have a majority of the indexes in RAM if you want to maintain decent performance. does the size of the table play a role in determining insert performance (and I mean only insert performance)? In this case, it's really the indexes that'll cause you trouble, though heavily fragmented tables (due to lots of deletes or updates) will also incur a penalty just for the data part of the inserts. Also, don't forget the usual hints if you are going to do lots of inserts: - batch them in large transactions, don't do them one at a time - better yet, use COPY rather than INSERT - in some situations, you might be better of dropping the indexes, doing large batch inserts, then re-creating the indexes. YMMV depending on the existing/new ratio, whether you need to maintain indexed access to the tables, etc. - pay attention to foreign keys Jacques. ---(end of broadcast)--- TIP 9: In versions below 8.0, the planner will ignore your desire to choose an index scan if your joining column's datatypes do not match