Re: [HACKERS] Re: Should we have an optional limit on the recursion depth of recursive CTEs?
Greg Stark wrote: On Mon, Aug 15, 2011 at 9:31 PM, Tom Lane t...@sss.pgh.pa.us wrote: ... and that would be a seriously bad API. ?There are not SUSET restrictions on other resources such as work_mem. ?Why do we need one for this? I think a better analogy would be imposing a maximum number of rows a query can output. That might be a sane thing to have for some circumstances but it's not useful in general. Consider for instance my favourite recursive query application, displaying the lock dependency graph for pg_locks. What arbitrary maximum number of locks would you like to impose at which the query should error out? There is a situation though that I think is motivating this though where it would be nice to detect a problem: when the query is such that it *can't* produce a record because there's an infinite loop before the first record. Ideally you want some way to detect that you've recursed and haven't changed anything that could lead to a change in the recursion condition. But that seems like a pretty hard thing to detect, probably impossible. Actually, using UNION instead of UNION ALL does prevent some infinite loops: WITH RECURSIVE source AS ( SELECT 'Hello' UNION SELECT 'Hello' FROM source ) SELECT * FROM source; Change that to UNION ALL and you have an infinite loop. -- Bruce Momjian br...@momjian.ushttp://momjian.us EnterpriseDB http://enterprisedb.com + It's impossible for everything to be true. + -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
Re: [HACKERS] Re: Should we have an optional limit on the recursion depth of recursive CTEs?
On 20 August 2011 15:34, Bruce Momjian br...@momjian.us wrote: Actually, using UNION instead of UNION ALL does prevent some infinite loops: While that is worth pointing out, it cannot be recommended as a way of preventing infinite recursion; after all, all 5 WITH RECURSIVE examples in the docs use UNION ALL. It's just a different way of specifying a terminating condition that isn't likely to be applicable to more complicated rCTEs. -- Peter Geoghegan http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training and Services -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
Re: [HACKERS] Re: Should we have an optional limit on the recursion depth of recursive CTEs?
On Mon, Aug 15, 2011 at 23:49, Greg Stark st...@mit.edu wrote: On Mon, Aug 15, 2011 at 9:31 PM, Tom Lane t...@sss.pgh.pa.us wrote: ... and that would be a seriously bad API. There are not SUSET restrictions on other resources such as work_mem. Why do we need one for this? I think a better analogy would be imposing a maximum number of rows a query can output. That might be a sane thing to have for some circumstances but it's not useful in general. Uh. You mean like LIMIT, which we already have? -- Magnus Hagander Me: http://www.hagander.net/ Work: http://www.redpill-linpro.com/ -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
Re: [HACKERS] Re: Should we have an optional limit on the recursion depth of recursive CTEs?
On 08/16/2011 04:56 AM, Magnus Hagander wrote: On Mon, Aug 15, 2011 at 23:49, Greg Starkst...@mit.edu wrote: On Mon, Aug 15, 2011 at 9:31 PM, Tom Lanet...@sss.pgh.pa.us wrote: ... and that would be a seriously bad API. There are not SUSET restrictions on other resources such as work_mem. Why do we need one for this? I think a better analogy would be imposing a maximum number of rows a query can output. That might be a sane thing to have for some circumstances but it's not useful in general. Uh. You mean like LIMIT, which we already have? There is no LIMIT imposed on a query by a server setting, which would be the right analogy here. cheers andrew -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
Re: [HACKERS] Re: Should we have an optional limit on the recursion depth of recursive CTEs?
On Tue, Aug 16, 2011 at 7:23 AM, Andrew Dunstan and...@dunslane.net wrote: There is no LIMIT imposed on a query by a server setting, which would be the right analogy here. I am not sure I understand any of these analogies. I think Peter's point is that it's not very difficult to write (perhaps accidentally) a CTE that goes into infinite recursion. In general, we can't detect that situation, because it's equivalent to the halting problem. But there's an old joke about a Turing test (where a computer program must try to fool a human into believing that it is also human) where the person asks the computer: What would the following program do? 10 PRINT HELLO 20 GOTO 10 And gets back an infinite stream of HELLO HELLO HELLO HELLO HELLO I don't think it's going to be feasible to implement a security restriction that keeps untrusted users from hosing the machine with a long running CTE; there are nearly infinitely many ways for an untrusted user who can run queries to hose the machine, and plugging one of them imperfectly is going to get us pretty much nowhere. On the other hand, there is perhaps a reasonable argument to be made that we should cut off CTE processing at some point to prevent *inadvertent* exhaustion of system resources. Or even query processing more generally. In fact, we already have some things sort of like this: you can use statement_timeout to kill queries that run for too long, and we just recently added temp_file_limit to kill those that eat too much temp file space. I can see a good case for memory_limit and query_cpu_limit and maybe some others. cte_recursion_depth_limit wouldn't be all that high on my personal list, I guess, but the concept doesn't seem completely insane. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
Re: [HACKERS] Re: Should we have an optional limit on the recursion depth of recursive CTEs?
Robert Haas robertmh...@gmail.com writes: I don't think it's going to be feasible to implement a security restriction that keeps untrusted users from hosing the machine with a long running CTE; there are nearly infinitely many ways for an untrusted user who can run queries to hose the machine, and plugging one of them imperfectly is going to get us pretty much nowhere. On the other hand, there is perhaps a reasonable argument to be made that we should cut off CTE processing at some point to prevent *inadvertent* exhaustion of system resources. Or even query processing more generally. Indeed: the real question here is why a recursive CTE is any worse than, say, an accidentally unconstrained join (or three or four...). However, we already have a perfectly suitable general mechanism for that; it's called statement_timeout. I think we've already had the discussion about whether there should be a system-wide SUSET maximum statement_timeout, and rejected it on the grounds that there was not a very clear need for it. In fact, we already have some things sort of like this: you can use statement_timeout to kill queries that run for too long, and we just recently added temp_file_limit to kill those that eat too much temp file space. I can see a good case for memory_limit and query_cpu_limit and maybe some others. temp_file_limit got accepted because it was constraining a resource not closely related to run time. I don't think that it provides a precedent in support of any of these other ideas. regards, tom lane -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
Re: [HACKERS] Re: Should we have an optional limit on the recursion depth of recursive CTEs?
On Tue, Aug 16, 2011 at 10:26 AM, Tom Lane t...@sss.pgh.pa.us wrote: In fact, we already have some things sort of like this: you can use statement_timeout to kill queries that run for too long, and we just recently added temp_file_limit to kill those that eat too much temp file space. I can see a good case for memory_limit and query_cpu_limit and maybe some others. temp_file_limit got accepted because it was constraining a resource not closely related to run time. I don't think that it provides a precedent in support of any of these other ideas. Well, CPU usage might be somewhat closely related to query runtime, but memory usage sure isn't. But we digress from $SUBJECT... -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
Re: [HACKERS] Re: Should we have an optional limit on the recursion depth of recursive CTEs?
On 16 August 2011 14:43, Robert Haas robertmh...@gmail.com wrote: What would the following program do? 10 PRINT HELLO 20 GOTO 10 And gets back an infinite stream of HELLO HELLO HELLO HELLO HELLO heh, that's pretty funny. It also compliments my view, because the Turing test is only failed because the human eventually thinks hmm, he's taking way too long to get to the '...and so on infinitum' bit. I don't think it's going to be feasible to implement a security restriction that keeps untrusted users from hosing the machine with a long running CTE; there are nearly infinitely many ways for an untrusted user who can run queries to hose the machine, and plugging one of them imperfectly is going to get us pretty much nowhere. Unless that happens to be the exact area that is a problem for you, due perhaps to a poorly written application. We're protecting against Murphy, not Machiavelli - if your users are malicious, or are motivated by seeing if they can somehow hose the machine for kicks, clearly all bets are off. This mindset happens to pretty well meet the needs of industry, IMHO. That said, I admit the case for making a separate SUSET GUC is the least compelling one I've made on this thread, if only because of the glaring inconsistency with other areas. On the other hand, there is perhaps a reasonable argument to be made that we should cut off CTE processing at some point to prevent *inadvertent* exhaustion of system resources. Or even query processing more generally. In fact, we already have some things sort of like this: you can use statement_timeout to kill queries that run for too long, and we just recently added temp_file_limit to kill those that eat too much temp file space. statement_timeout is far too blunt an instrument to deal with this problem. For one thing, it may vary based on many external factors, whereas number of iterations is a consistent, useful metric for the WITH query in isolation. For another, it prevents the DBA from managing known problems with deployed apps per database - maybe they have a reporting query that is expected to take a really long time. Sure, they can increase statement_timeout when that it run, but that's another thing to remember. I can see a good case for memory_limit and query_cpu_limit and maybe some others. cte_recursion_depth_limit wouldn't be all that high on my personal list, I guess, but the concept doesn't seem completely insane. I agree that those things would be much better than this. This is still a useful, easy-to-implement feature though. On 16 August 2011 15:26, Tom Lane t...@sss.pgh.pa.us wrote: Indeed: the real question here is why a recursive CTE is any worse than, say, an accidentally unconstrained join (or three or four...). It's much worse because an unconstrained join query will not all-of-a-sudden fail to have a terminating condition. It will, for the most part, take forever or practically forever predictably and consistently, even as the contents of tables changes over time. -- Peter Geoghegan http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training and Services -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
[HACKERS] Re: Should we have an optional limit on the recursion depth of recursive CTEs?
On Mon, Aug 15, 2011 at 9:31 PM, Tom Lane t...@sss.pgh.pa.us wrote: ... and that would be a seriously bad API. There are not SUSET restrictions on other resources such as work_mem. Why do we need one for this? I think a better analogy would be imposing a maximum number of rows a query can output. That might be a sane thing to have for some circumstances but it's not useful in general. Consider for instance my favourite recursive query application, displaying the lock dependency graph for pg_locks. What arbitrary maximum number of locks would you like to impose at which the query should error out? There is a situation though that I think is motivating this though where it would be nice to detect a problem: when the query is such that it *can't* produce a record because there's an infinite loop before the first record. Ideally you want some way to detect that you've recursed and haven't changed anything that could lead to a change in the recursion condition. But that seems like a pretty hard thing to detect, probably impossible. -- greg -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
