Re: LOCK TABLE and DROP TABLE on temp tables of other sessions
On Fri, Feb 14, 2020 at 05:59:34PM +0530, Ashutosh Bapat wrote: > On Fri, Feb 14, 2020 at 11:35 AM Michael Paquier wrote: >> One thing that we need to consider is if there are applications which >> take advantage of LOCK allowed on temp relations from other backends >> or not. One downside is that if one backend takes a lock on a temp >> table from a different session, then this other session would not >> completely shut down (still report the shutdown to the client), >> and would remain blocked during the temp schema cleanup until the >> transaction of the session locking the temp relation commits. This >> blocks access to one connection slot, still we are talking about an >> operation where the owner of the temp schema wants to do the lock. > > That might be disastrous if happens by accident eating up most of the > available connection slots. Well, that would be an owner doing that. > Whatever the user wants to achieve using LOCK [temp] TABLE of other > session, I guess can be achieved by other means or can be shown to have > disastrous effect. So that kind of usage pattern would better be forced to > change. Anyway, don't take me wrong. I would be rather in favor of restricting LOCK but that does not seem like something enough for a backpatch. One recent example in this area I had to deal with is REINDEX on temp tables. We have some assumptions which involve lock upgrades (ShareUpdateExclusiveLock => AccessExclusiveLock between the moment we take the relation lock using its RangeVar until the moment the reindex is actually done), so being able to take a conflicting lock on the temp relation could cause reindex to deadlock. -- Michael signature.asc Description: PGP signature
Re: LOCK TABLE and DROP TABLE on temp tables of other sessions
On Fri, Feb 14, 2020 at 11:35 AM Michael Paquier wrote: > On Thu, Feb 13, 2020 at 09:05:01PM +0530, Ashutosh Bapat wrote: > > That looks odd. Other sessions are able to see temporary tables of a > given > > session because they are stored in the same catalog which is accessible > to > > all the sessions. But ideally, a temporary table should be visible only > to > > the session which created it (GTT is an exception). So LOCK and DROP > table > > should not succeed. > > One thing that we need to consider is if there are applications which > take advantage of LOCK allowed on temp relations from other backends > or not. One downside is that if one backend takes a lock on a temp > table from a different session, then this other session would not > completely shut down (still report the shutdown to the client), > and would remain blocked during the temp schema cleanup until the > transaction of the session locking the temp relation commits. This > blocks access to one connection slot, still we are talking about an > operation where the owner of the temp schema wants to do the lock. > That might be disastrous if happens by accident eating up most of the available connection slots. Whatever the user wants to achieve using LOCK [temp] TABLE of other session, I guess can be achieved by other means or can be shown to have disastrous effect. So that kind of usage pattern would better be forced to change. > > > Thinking from a different perspective, DROP TABLE being able to drop a > > temporary table seems a good tool in case a temporary table is left > behind > > by a finished session. But that doesn't seem like a good reason to have > it > > and I don't see much use of LOCK TABLE there. > > Yep. Robert had actually this argument with DROP SCHEMA pg_temp not > so long ago with me. > > DROP SCHEMA might be better for mass cleanup. That actually makes DROP [other session temp] TABLE useless. -- -- Best Wishes, Ashutosh Bapat
Re: LOCK TABLE and DROP TABLE on temp tables of other sessions
On Thu, Feb 13, 2020 at 09:05:01PM +0530, Ashutosh Bapat wrote: > That looks odd. Other sessions are able to see temporary tables of a given > session because they are stored in the same catalog which is accessible to > all the sessions. But ideally, a temporary table should be visible only to > the session which created it (GTT is an exception). So LOCK and DROP table > should not succeed. One thing that we need to consider is if there are applications which take advantage of LOCK allowed on temp relations from other backends or not. One downside is that if one backend takes a lock on a temp table from a different session, then this other session would not completely shut down (still report the shutdown to the client), and would remain blocked during the temp schema cleanup until the transaction of the session locking the temp relation commits. This blocks access to one connection slot, still we are talking about an operation where the owner of the temp schema wants to do the lock. > Thinking from a different perspective, DROP TABLE being able to drop a > temporary table seems a good tool in case a temporary table is left behind > by a finished session. But that doesn't seem like a good reason to have it > and I don't see much use of LOCK TABLE there. Yep. Robert had actually this argument with DROP SCHEMA pg_temp not so long ago with me. -- Michael signature.asc Description: PGP signature
Re: LOCK TABLE and DROP TABLE on temp tables of other sessions
On Thu, Feb 13, 2020 at 6:40 PM Fujii Masao wrote: > Hi, > > TRUNCATE command on the temporary tables of other sessions fails > with the following error. This behavior looks expected to me. > > ERROR: cannot truncate temporary tables of other sessions > > However I found that LOCK TABLE and DROP TABLE commands on > the temporary tables of other sessions are successfully processed, > if users (like superusers) have enough access privileges on them. > Is this a bug? ISTM that the similar check that TRUNCATE command > does needs to be added even in LOCK TABLE and DROP TABLE cases. > That looks odd. Other sessions are able to see temporary tables of a given session because they are stored in the same catalog which is accessible to all the sessions. But ideally, a temporary table should be visible only to the session which created it (GTT is an exception). So LOCK and DROP table should not succeed. Thinking from a different perspective, DROP TABLE being able to drop a temporary table seems a good tool in case a temporary table is left behind by a finished session. But that doesn't seem like a good reason to have it and I don't see much use of LOCK TABLE there. > > BTW, even SELECT has the same issue. Basically SELECT on > the temporary tables of other sessions fails with the following > error. > > ERROR: cannot access temporary tables of other sessions > > However if the temporary table is empty, SELECT doesn't reach > the above check, is successfully processed and the relation lock > is taken. This lock can prevent the backend process that created > the temporary table from exiting even when the client that > the backend is connecting to quits. Seems it's problematic. > > Regards, > > -- > Fujii Masao > NTT DATA CORPORATION > Advanced Platform Technology Group > Research and Development Headquarters > > > -- -- Best Wishes, Ashutosh Bapat
