Re: [HACKERS] Make relation_openrv atomic wrt DDL
On Thu, Jul 07, 2011 at 09:30:47PM -0400, Robert Haas wrote: > On Thu, Jul 7, 2011 at 3:54 PM, Noah Misch wrote: > > Yes. DDL-DDL concurrency is a much smaller practical concern, but it is a > > quality-of-implementation hole. > > Agreed, although I'm not too pleased about the fact that this doesn't > fix nextval(). That seems like a fairly significant hole (but one to > be addressed by a later patch). True. > > The third sentence is true for all lock levels. The fourth sentence is true > > for lock levels _except_ NoLock. > > I rewrote this whole blurb. See what you think. Seems better: > + * DDL operations can change the results of a name lookup. Since all > + * such operations will generate invalidation messages, we keep track > + * of whether any such messages show up while we're performing the > + * operation, and retry until either (1) no more invalidation messages > + * show up or (2) the answer doesn't change. > + * > + * But if lockmode = NoLock, then we assume that either the caller is OK > + * with the answer changing under them, or that they already hold some > + * appropriate lock, and therefore return the first answer we get > without > + * checking for invalidation messages. Also, if the requested lock is > + * already held, no LockRelationOid will not AcceptInvalidationMessages, Extra word in that sentence. > + * so we may fail to notice a change. We could protect against that > case I failed to note it last time, but it might be worth mentioning that failing to notice a change only happens due to search_path interposition. > + * by calling AcceptInvalidationMessages() before beginning this loop, > + * but that would add a significant amount overhead, so for now we > don't. > >> + /* > >> + * If no lock requested, we assume the caller knows what > >> they're > >> + * doing. They should have already acquired a heavyweight > >> lock on > >> + * this relation earlier in the processing of this same > >> statement, > >> + * so it wouldn't be appropriate to > >> AcceptInvalidationMessages() > >> + * here, as that might pull the rug out from under them. > >> + */ > > > > What sort of rug-pullout do you have in mind? Also, I don't think it > > matters > > if the caller acquired the lock during this _statement_. It just needs to > > hold a lock, somehow. > > What I'm specifically concerned about here is the possibility that we > have code which does RangeVarGetRelid() multiple times and expects to > get the same relation every time. Now, granted, any such places are > bugs. But I am not eager to change the logic here without looking > harder for them (and also for performance reasons). Yeah, I think a key point is that we're not promising to avoid calling AcceptInvalidationMessages() to prop up code that relies on it not getting called. We just know it's not needed in this case, so we save that expense. > > ... also making it cleaner to preserve this optimization. > > That optimization is now gone anyway. Okay. > > Incidentally, you've added in many places this pattern of commenting that a > > lock level must match another lock level used elsewhere. Would it be better > > to migrate away from looking up a relation oid in one function and opening > > the > > relation in another function, instead passing either a Relation or a > > RangeVar? > > You did substitute passing a Relation in a couple of places. [reasons this is a can of worms] > At any rate, I'm not inventing this problem; there are plenty of > existing instances where this same phenomenon occurs. At least I'm > documenting the ones I'm adding. There's probably room for further > improvement and restructuring of this code, but right at the moment I > feel like the reasonable alternatives are (a) to pass a lock level > that matches what will ultimately be taken later on or (b) pass > NoLock. I'm voting for the former. Works for me. -- 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] Make relation_openrv atomic wrt DDL
On Thu, Jul 7, 2011 at 3:54 PM, Noah Misch wrote: > Yes. DDL-DDL concurrency is a much smaller practical concern, but it is a > quality-of-implementation hole. Agreed, although I'm not too pleased about the fact that this doesn't fix nextval(). That seems like a fairly significant hole (but one to be addressed by a later patch). > The third sentence is true for all lock levels. The fourth sentence is true > for lock levels _except_ NoLock. I rewrote this whole blurb. See what you think. >> + /* >> + * If no lock requested, we assume the caller knows what >> they're >> + * doing. They should have already acquired a heavyweight >> lock on >> + * this relation earlier in the processing of this same >> statement, >> + * so it wouldn't be appropriate to >> AcceptInvalidationMessages() >> + * here, as that might pull the rug out from under them. >> + */ > > What sort of rug-pullout do you have in mind? Also, I don't think it matters > if the caller acquired the lock during this _statement_. It just needs to > hold a lock, somehow. What I'm specifically concerned about here is the possibility that we have code which does RangeVarGetRelid() multiple times and expects to get the same relation every time. Now, granted, any such places are bugs. But I am not eager to change the logic here without looking harder for them (and also for performance reasons). > ... also making it cleaner to preserve this optimization. That optimization is now gone anyway. > Incidentally, you've added in many places this pattern of commenting that a > lock level must match another lock level used elsewhere. Would it be better > to migrate away from looking up a relation oid in one function and opening the > relation in another function, instead passing either a Relation or a RangeVar? > You did substitute passing a Relation in a couple of places. Well, I've got these: - ReindexTable() must match reindex_relation() - ExecRenameStmt() must match RenameRelation() - DropTrigger() must match RemoveTriggerById() - DefineRule() must match DefineQueryRewrite() - RemoveRewriteRule() must match RemoveRewriteRuleById() (Whoever came up with these names was clearly a genius...) RemoveTriggerById() and RemoveRewriteRuleById() are part of the drop-statement machinery - they can be called either because that rule/trigger itself gets dropped, or because some other object gets dropped and it cascades to the rule/trigger. I'm pretty sure I don't want to go start mucking with that machinery. On the other hand, RenameRelation() is only called in one place other than ExecRenameStmt(), and it looks to me like the other caller could just as well use RenameRelationInternal(). If we change that, then we can redefine RenameRelation() to take a RangeVar instead of an Oid and push the rest of the logic from ExecRenameStmt() into it. That would probably be better all around. The situation with DefineRule and DefineQueryRewrite is a bit more nuanced. As you suggest, those could probably be merged into one function taking both an Oid and a RangeVar. If the Oid is not InvalidOid, we do heap_open(); else, we do heap_openrv(). That's slightly ugly, but there are only two callers, so maybe it's not so bad. Offhand, I don't see any good way to rearrange reindex_relation() along those lines. ReindexTable() wants to check permissions, not just open the relation. And passing a Relation to reindex_relation() rather than an Oid or RangeVar is no good either; you still end up with multiple people needing to know what lock level to use. At any rate, I'm not inventing this problem; there are plenty of existing instances where this same phenomenon occurs. At least I'm documenting the ones I'm adding. There's probably room for further improvement and restructuring of this code, but right at the moment I feel like the reasonable alternatives are (a) to pass a lock level that matches what will ultimately be taken later on or (b) pass NoLock. I'm voting for the former. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company atomic-rangevargetrelid-v2.patch Description: Binary data -- 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] Make relation_openrv atomic wrt DDL
On Thu, Jul 07, 2011 at 11:43:30AM -0400, Robert Haas wrote: > On Wed, Jul 6, 2011 at 10:44 PM, Noah Misch wrote: > > Attached. I made the counter 64 bits wide, handled the nothing-found case > > per > > your idea, and improved a few comments cosmetically. I have not attempted > > to > > improve the search_path interposition case. We can recommend the workaround > > above, and doing better looks like an excursion much larger than the one > > represented by this patch. > > I looked at this some more and started to get uncomfortable with the > whole idea of having RangeVarLockRelid() be a wrapper around > RangeVarGetRelid(). This hazard exists everywhere the latter function > gets called, not just in relation_open(). So it doesn't seem right to > fix the problem only in those places. Yes; I wished to focus on the major case for this round, then address the other callers later. We can do it this way, though. It does make long-term sense to expose only the lock-taking form, making otherwise-unaffected callers say NoLock explicitly. > So I went through and incorporated the logic proposed for > RangeVarLockRelid() into RangeVarGetRelid() itself, and then went > through and examined all the callers of RangeVarGetRelid(). There are > some, such as has_table_privilege(), where it's really impractical to > take any lock, first because we might have no privileges at all on > that table and second because that could easily lead to a massive > amount of locking for no particular good reason. I believe Tom > suggested that the right fix for these functions is to have them > index-scan the system catalogs using the caller's MVCC snapshot, which > would be right at least for pg_dump. And there are other callers that > cannot acquire the lock as part of RangeVarGetRelid() for a variety of > other reasons. However, having said that, there do appear to be a > number of cases that are can be fixed fairly easily. > > So here's a (heavily) updated patch that tries to do that, along with > adding comments to the places where things still need more fixing. In > addition to the problems corrected by your last version, this fixes > LOCK TABLE, ALTER SEQUENCE, ALTER TABLE .. RENAME, the whole-table > variant of REINDEX, CREATE CONSTRAINT TRIGGER (which is flat-out wrong > as it stands, since it acquires *no lock at all* on the table > specified in the FROM clause, never mind the question of doing so > atomically), CREATE RULE, and (partially) DROP TRIGGER and DROP RULE. Looks basically sound; see a few code comments below. > Regardless of exactly how we decide to proceed here, it strikes me > that there is a heck of a lot more work that could stand to be done in > this area... :-( Yes. DDL-DDL concurrency is a much smaller practical concern, but it is a quality-of-implementation hole. > --- a/src/backend/catalog/namespace.c > +++ b/src/backend/catalog/namespace.c > @@ -238,37 +246,121 @@ RangeVarGetRelid(const RangeVar *relation, bool failOK) > } > > /* > - * Some non-default relpersistence value may have been specified. The > - * parser never generates such a RangeVar in simple DML, but it can > happen > - * in contexts such as "CREATE TEMP TABLE foo (f1 int PRIMARY KEY)". > Such > - * a command will generate an added CREATE INDEX operation, which must > be > - * careful to find the temp table, even when pg_temp is not first in the > - * search path. > + * If lockmode = NoLock, the caller is assumed to already hold some sort > + * of heavyweight lock on the target relation. Otherwise, we're > preceding > + * here with no lock at all, which means that any answers we get must be > + * viewed with a certain degree of suspicion. In particular, any time > we > + * AcceptInvalidationMessages(), the anwer might change. We handle that > + * case by retrying the operation until either (1) no more invalidation > + * messages show up or (2) the answer doesn't change. The third sentence is true for all lock levels. The fourth sentence is true for lock levels _except_ NoLock. > + /* > + * If no lock requested, we assume the caller knows what they're > + * doing. They should have already acquired a heavyweight lock > on > + * this relation earlier in the processing of this same > statement, > + * so it wouldn't be appropriate to AcceptInvalidationMessages() > + * here, as that might pull the rug out from under them. > + */ What sort of rug-pullout do you have in mind? Also, I don't think it matters if the caller acquired the lock during this _statement_. It just needs to hold a lock, somehow. > --- a/src/backend/commands/lockcmds.c > +++ b/src/backend/commands/lockcmds.c > @@ -69,67 +70,10 @@ LockTableCommand(LockStmt *lockstmt) > * "rv" is NULL and we should just silently ignore any dropped child rel. This comment refers to a now
Re: [HACKERS] Make relation_openrv atomic wrt DDL
On Wed, Jul 6, 2011 at 10:44 PM, Noah Misch wrote: > Attached. I made the counter 64 bits wide, handled the nothing-found case per > your idea, and improved a few comments cosmetically. I have not attempted to > improve the search_path interposition case. We can recommend the workaround > above, and doing better looks like an excursion much larger than the one > represented by this patch. I looked at this some more and started to get uncomfortable with the whole idea of having RangeVarLockRelid() be a wrapper around RangeVarGetRelid(). This hazard exists everywhere the latter function gets called, not just in relation_open(). So it doesn't seem right to fix the problem only in those places. So I went through and incorporated the logic proposed for RangeVarLockRelid() into RangeVarGetRelid() itself, and then went through and examined all the callers of RangeVarGetRelid(). There are some, such as has_table_privilege(), where it's really impractical to take any lock, first because we might have no privileges at all on that table and second because that could easily lead to a massive amount of locking for no particular good reason. I believe Tom suggested that the right fix for these functions is to have them index-scan the system catalogs using the caller's MVCC snapshot, which would be right at least for pg_dump. And there are other callers that cannot acquire the lock as part of RangeVarGetRelid() for a variety of other reasons. However, having said that, there do appear to be a number of cases that are can be fixed fairly easily. So here's a (heavily) updated patch that tries to do that, along with adding comments to the places where things still need more fixing. In addition to the problems corrected by your last version, this fixes LOCK TABLE, ALTER SEQUENCE, ALTER TABLE .. RENAME, the whole-table variant of REINDEX, CREATE CONSTRAINT TRIGGER (which is flat-out wrong as it stands, since it acquires *no lock at all* on the table specified in the FROM clause, never mind the question of doing so atomically), CREATE RULE, and (partially) DROP TRIGGER and DROP RULE. Regardless of exactly how we decide to proceed here, it strikes me that there is a heck of a lot more work that could stand to be done in this area... :-( -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company atomic-rangevargetrelid.patch Description: Binary data -- 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] Make relation_openrv atomic wrt DDL
On Wed, Jul 06, 2011 at 08:35:55PM -0400, Robert Haas wrote:
> On Wed, Jul 6, 2011 at 6:32 PM, Noah Misch wrote:
> > While a mere "LOCK bar.x" is sufficient to get a clean cutover with respect
> > to
> > parsing, it fails to invalidate plans. To really cover all bases, you need
> > some no-op action that invalidates "bar.x". For actual practical use, I'd
> > recommend something like:
> >
> > BEGIN;
> > ALTER TABLE bar.x RENAME TO x0;
> > ALTER TABLE bar.x0 RENAME TO x;
> > CREATE TABLE foo.x ...
> > COMMIT;
> >
> > Probably worth making it more intuitive to DTRT here.
>
> Well, what would be really nice is if it just worked.
Yes.
> Care to submit an updated patch?
Attached. I made the counter 64 bits wide, handled the nothing-found case per
your idea, and improved a few comments cosmetically. I have not attempted to
improve the search_path interposition case. We can recommend the workaround
above, and doing better looks like an excursion much larger than the one
represented by this patch.
Thanks,
nm
diff --git a/src/backend/access/heap/heapam.c b/src/backend/access/heap/heapam.c
index c9b1d5f..a345e39 100644
*** a/src/backend/access/heap/heapam.c
--- b/src/backend/access/heap/heapam.c
***
*** 975,1000 relation_openrv(const RangeVar *relation, LOCKMODE lockmode)
{
Oid relOid;
! /*
!* Check for shared-cache-inval messages before trying to open the
!* relation. This is needed to cover the case where the name
identifies a
!* rel that has been dropped and recreated since the start of our
!* transaction: if we don't flush the old syscache entry then we'll
latch
!* onto that entry and suffer an error when we do RelationIdGetRelation.
!* Note that relation_open does not need to do this, since a relation's
!* OID never changes.
!*
!* We skip this if asked for NoLock, on the assumption that the caller
has
!* already ensured some appropriate lock is held.
!*/
! if (lockmode != NoLock)
! AcceptInvalidationMessages();
!
! /* Look up the appropriate relation using namespace search */
! relOid = RangeVarGetRelid(relation, false);
/* Let relation_open do the rest */
! return relation_open(relOid, lockmode);
}
/*
--- 975,985
{
Oid relOid;
! /* Look up and lock the appropriate relation using namespace search */
! relOid = RangeVarLockRelid(relation, lockmode, false);
/* Let relation_open do the rest */
! return relation_open(relOid, NoLock);
}
/*
***
*** 1012,1041 relation_openrv_extended(const RangeVar *relation, LOCKMODE
lockmode,
{
Oid relOid;
! /*
!* Check for shared-cache-inval messages before trying to open the
!* relation. This is needed to cover the case where the name
identifies a
!* rel that has been dropped and recreated since the start of our
!* transaction: if we don't flush the old syscache entry then we'll
latch
!* onto that entry and suffer an error when we do RelationIdGetRelation.
!* Note that relation_open does not need to do this, since a relation's
!* OID never changes.
!*
!* We skip this if asked for NoLock, on the assumption that the caller
has
!* already ensured some appropriate lock is held.
!*/
! if (lockmode != NoLock)
! AcceptInvalidationMessages();
!
! /* Look up the appropriate relation using namespace search */
! relOid = RangeVarGetRelid(relation, missing_ok);
/* Return NULL on not-found */
if (!OidIsValid(relOid))
return NULL;
/* Let relation_open do the rest */
! return relation_open(relOid, lockmode);
}
/*
--- 997,1011
{
Oid relOid;
! /* Look up and lock the appropriate relation using namespace search */
! relOid = RangeVarLockRelid(relation, lockmode, missing_ok);
/* Return NULL on not-found */
if (!OidIsValid(relOid))
return NULL;
/* Let relation_open do the rest */
! return relation_open(relOid, NoLock);
}
/*
diff --git a/src/backend/catalog/namespindex ce795a6..f75fcef 100644
*** a/src/backend/catalog/namespace.c
--- b/src/backend/catalog/namespace.c
***
*** 44,49
--- 44,51
#include "parser/parse_func.h"
#include "storage/backendid.h"
#include "storage/ipc.h"
+ #include "storage/lmgr.h"
+ #include "storage/sinval.h"
#include "utils/acl.h"
#include "utils/builtins.h"
#include "utils/guc.h"
***
*** 285,290 RangeVarGetRelid(const RangeVar *relation, bool failOK)
--- 287,372
}
/*
+ * RangeVarLockR
Re: [HACKERS] Make relation_openrv atomic wrt DDL
On Wed, Jul 6, 2011 at 6:32 PM, Noah Misch wrote: >> Maybe this is a stupid idea, but what about changing the logic so >> that, if we get back InvalidOid, we AcceptInvalidationMessages() and >> retry if the counter has advanced? ISTM that might cover the example >> you mentioned in your last post, where we fail to detect a relation >> that has come into existence since our last call to >> AcceptInvalidationMessages(). It would cost an extra >> AcceptInvalidationMessages() only in the case where we haven't found >> the relation, which (a) seems like a good time to worry about whether >> we're missing something, since users generally try not to reference >> nonexistent tables and (b) should be rare enough to be ignorable from >> a performance perspective. > > Agreed on all points. Good idea. That improves our guarantee from "any > client-issued command will see tables committed before its submission" to > "_any command_ will see tables committed before its _parsing_". In > particular, commands submitted using SPI will no longer be subject to this > source of déją vu. I, too, doubt that looking up nonexistent relations is a > performance-critical operation for anyone. > >> In the department of minor nitpicking, why not use a 64-bit counter >> for SharedInvalidMessageCounter? Then we don't have to think very >> hard about whether overflow can ever pose a problem. > > Overflow is fine because I only ever compare values for equality, and I use an > unsigned int to give defined behavior at overflow. However, the added cost of > a 64-bit counter should be negligible, and future use cases (including > external code) might appreciate it. No strong preference. Yeah, that's what I was thinking. I have a feeling we may want to use this mechanism in other places, including places where it would be nice to be able to assume that > has sensible semantics. >> It strikes me that, even with this patch, there is a fair amount of >> room for wonky behavior. For example, as your comment notes, if >> search_path = foo, bar, and we've previously referenced "x", getting >> "bar.x", the creation of "foo.x" will generate invalidation messages, >> but a subsequent reference - within the same transaction - to "x" will >> not cause us to read them. It would be nice to >> AcceptInvalidationMessages() unconditionally at the beginning of >> RangeVarGetRelid() [and then redo as necessary to get a stable >> answer], but that might have some performance consequence for >> transactions that repeatedly read the same tables. > > A user doing that should "LOCK bar.x" in the transaction that creates "foo.x", > giving a clean cutover. (I thought of documenting that somewhere, but it > seemed a tad esoteric.) In the absence of such a lock, an extra unconditional > call to AcceptInvalidationMessages() narrows the window in which his commands > parse as using the "wrong" table. However, commands that have already parsed > will still use the old table without interruption, with no particular bound on > when they may finish. I've failed to come up with a use case where the > narrower window for parse inconsistencies is valuable but the remaining > exposure is acceptable. There may very well be one I'm missing, though. > > While a mere "LOCK bar.x" is sufficient to get a clean cutover with respect to > parsing, it fails to invalidate plans. To really cover all bases, you need > some no-op action that invalidates "bar.x". For actual practical use, I'd > recommend something like: > > BEGIN; > ALTER TABLE bar.x RENAME TO x0; > ALTER TABLE bar.x0 RENAME TO x; > CREATE TABLE foo.x ... > COMMIT; > > Probably worth making it more intuitive to DTRT here. Well, what would be really nice is if it just worked. Care to submit an updated patch? -- 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] Make relation_openrv atomic wrt DDL
On Wed, Jul 06, 2011 at 03:06:40PM -0400, Robert Haas wrote: > On Sun, Jun 19, 2011 at 11:42 PM, Noah Misch wrote: > > On Sun, Jun 19, 2011 at 11:44:35PM +0100, Greg Stark wrote: > >> So I was the victim assigned to review this patch. > > > > Thanks for doing so. > > This discussion seems to have died off. Let's see if we can drive > this forward to some conclusion. > > I took a look at this patch and found that it had bit-rotted slightly. > I am attaching a rebased version. Thanks. > Maybe this is a stupid idea, but what about changing the logic so > that, if we get back InvalidOid, we AcceptInvalidationMessages() and > retry if the counter has advanced? ISTM that might cover the example > you mentioned in your last post, where we fail to detect a relation > that has come into existence since our last call to > AcceptInvalidationMessages(). It would cost an extra > AcceptInvalidationMessages() only in the case where we haven't found > the relation, which (a) seems like a good time to worry about whether > we're missing something, since users generally try not to reference > nonexistent tables and (b) should be rare enough to be ignorable from > a performance perspective. Agreed on all points. Good idea. That improves our guarantee from "any client-issued command will see tables committed before its submission" to "_any command_ will see tables committed before its _parsing_". In particular, commands submitted using SPI will no longer be subject to this source of déjà vu. I, too, doubt that looking up nonexistent relations is a performance-critical operation for anyone. > In the department of minor nitpicking, why not use a 64-bit counter > for SharedInvalidMessageCounter? Then we don't have to think very > hard about whether overflow can ever pose a problem. Overflow is fine because I only ever compare values for equality, and I use an unsigned int to give defined behavior at overflow. However, the added cost of a 64-bit counter should be negligible, and future use cases (including external code) might appreciate it. No strong preference. > It strikes me that, even with this patch, there is a fair amount of > room for wonky behavior. For example, as your comment notes, if > search_path = foo, bar, and we've previously referenced "x", getting > "bar.x", the creation of "foo.x" will generate invalidation messages, > but a subsequent reference - within the same transaction - to "x" will > not cause us to read them. It would be nice to > AcceptInvalidationMessages() unconditionally at the beginning of > RangeVarGetRelid() [and then redo as necessary to get a stable > answer], but that might have some performance consequence for > transactions that repeatedly read the same tables. A user doing that should "LOCK bar.x" in the transaction that creates "foo.x", giving a clean cutover. (I thought of documenting that somewhere, but it seemed a tad esoteric.) In the absence of such a lock, an extra unconditional call to AcceptInvalidationMessages() narrows the window in which his commands parse as using the "wrong" table. However, commands that have already parsed will still use the old table without interruption, with no particular bound on when they may finish. I've failed to come up with a use case where the narrower window for parse inconsistencies is valuable but the remaining exposure is acceptable. There may very well be one I'm missing, though. While a mere "LOCK bar.x" is sufficient to get a clean cutover with respect to parsing, it fails to invalidate plans. To really cover all bases, you need some no-op action that invalidates "bar.x". For actual practical use, I'd recommend something like: BEGIN; ALTER TABLE bar.x RENAME TO x0; ALTER TABLE bar.x0 RENAME TO x; CREATE TABLE foo.x ... COMMIT; Probably worth making it more intuitive to DTRT here. Thanks, nm -- 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] Make relation_openrv atomic wrt DDL
On Sun, Jun 19, 2011 at 11:42 PM, Noah Misch wrote: > On Sun, Jun 19, 2011 at 11:44:35PM +0100, Greg Stark wrote: >> So I was the victim assigned to review this patch. > > Thanks for doing so. This discussion seems to have died off. Let's see if we can drive this forward to some conclusion. I took a look at this patch and found that it had bit-rotted slightly. I am attaching a rebased version. Maybe this is a stupid idea, but what about changing the logic so that, if we get back InvalidOid, we AcceptInvalidationMessages() and retry if the counter has advanced? ISTM that might cover the example you mentioned in your last post, where we fail to detect a relation that has come into existence since our last call to AcceptInvalidationMessages(). It would cost an extra AcceptInvalidationMessages() only in the case where we haven't found the relation, which (a) seems like a good time to worry about whether we're missing something, since users generally try not to reference nonexistent tables and (b) should be rare enough to be ignorable from a performance perspective. In the department of minor nitpicking, why not use a 64-bit counter for SharedInvalidMessageCounter? Then we don't have to think very hard about whether overflow can ever pose a problem. It strikes me that, even with this patch, there is a fair amount of room for wonky behavior. For example, as your comment notes, if search_path = foo, bar, and we've previously referenced "x", getting "bar.x", the creation of "foo.x" will generate invalidation messages, but a subsequent reference - within the same transaction - to "x" will not cause us to read them. It would be nice to AcceptInvalidationMessages() unconditionally at the beginning of RangeVarGetRelid() [and then redo as necessary to get a stable answer], but that might have some performance consequence for transactions that repeatedly read the same tables. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company atomic-openrv-v3.patch Description: Binary data -- 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] Make relation_openrv atomic wrt DDL
Hi Greg,
On Sun, Jun 19, 2011 at 11:44:35PM +0100, Greg Stark wrote:
> So I was the victim assigned to review this patch.
Thanks for doing so.
> The code is pretty much impeccable as usual from Noah. But I have
> questions about the semantics of it.
>
> Firstly this bit makes me wonder:
>
> + /* Not-found is always final. */
> + if (!OidIsValid(relOid1))
> + return relOid1;
>
> If someone does
>
> BEGIN; DROP TABLE foo; CREATE TABLE foo; COMMIT;
>
> Then what prevents this logic from finding the doomed relation,
> blocking until the transaction commits, then finding it's deleted and
> returning InvalidOid?
> RangeVarGetRelid is just going to complete its index scan of pg_class
> and may not come across the newly inserted row.
RangeVarGetRelid() always runs its index scan to completion, and the blocking
happens in LockRelationOid(). You will get a sequence like this:
RangeVarGetRelid("foo") => 2
LockRelationOid(2) [... blocks ...]
AcceptInvalidationMessages() [process a message]
RangeVarGetRelid("foo") => 20001
[restart loop]
LockRelationOid(20001)
AcceptInvalidationMessages() [no new messages - done]
RangeVarGetRelid() *is* vulnerable to the problem Simon just reported in the
"ALTER TABLE lock strength reduction patch is unsafe" thread, which arises
when the DDL transaction actually commits in the middle of a concurrent system
table scan. I don't think this patch makes things better or worse in that
regard, but I haven't thought it through in great depth.
> Am I missing something? I would have expected to have to loop around
> and retry if no valid record is found. But this raises the question --
> if no lock was acquired then what would have triggered an
> AcceptInvalidatationMessages and how would we know we waited long
> enough to find out about the newly created table?
Good question. I think characterizing "long enough" quickly leads to defining
one or more sequence points after which all backends must recognize a new
table as existing. My greenfield definition would be "a command should see
precisely the tables visible to its MVCC snapshot", but that has practical
problems. Let's see what implementation concerns would suggest...
This leads to a case I had not considered explicitly: CREATE TABLE on a name
that has not recently mapped to any table. If the catcache has a negative
entry on the key in question, we will rely on that and miss the new table
until we call AcceptInvalidationMessages() somehow. To hit this, you need a
command that dynamically chooses to query a table that has been created since
the command started running. DROP/CREATE of the same name in a single
transaction can't hit the problem. Consider this test script:
psql -X <<\_EOSQL &
-- Cleanup from last run
DROP TABLE IF EXISTS public.foo;
BEGIN;
-- Create the neg catcache entry.
SAVEPOINT q;
SELECT 1 FROM public.foo;
ROLLBACK to q;
--SET client_min_messages = debug5; -- use with CACHEDEBUG for insight
DO $$
BEGIN
EXECUTE 'SELECT 1 FROM pg_am'; -- prime basic catcache entries
PERFORM pg_sleep(11);
EXECUTE 'SELECT 1 FROM public.foo';
END
$$;
_EOSQL
sleep 1
psql -Xc 'CREATE TABLE public.foo ()'
wait
The first backend fails to see the new table despite its creating transaction
having committed ~10s ago. Starting a transaction, beginning to process a new
client-issued command, or successfully locking any relation prevents the miss.
We could narrow the window in most cases by re-adding a call to
AcceptInvalidationMessages() before RangeVarLockRelid()'s first call to
RangeVarGetRelid(). My current thinking is that it's not worth adding that
cost to every RangeVarLockRelid(). Thus, specify that, minimally, each
client-issued command will see all tables whose names were occupied at the
time the command started. I would add a comment to that effect. Thoughts?
> As a side note, if there are a long stream of such concurrent DDL then
> this code will leave all the old versions locked. This is consistent
> with our "hold locks until end of transaction" semantics but it seems
> weird for tables that we locked "accidentally" and didn't really end
> up using at all. I'm not sure it's really bad though.
Yes. If that outcome were more common, this would be a good place to try
relaxing the rule.
nm
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Re: [HACKERS] Make relation_openrv atomic wrt DDL
So I was the victim assigned to review this patch. The code is pretty much impeccable as usual from Noah. But I have questions about the semantics of it. Firstly this bit makes me wonder: + /* Not-found is always final. */ + if (!OidIsValid(relOid1)) + return relOid1; If someone does BEGIN; DROP TABLE foo; CREATE TABLE foo; COMMIT; Then what prevents this logic from finding the doomed relation, blocking until the transaction commits, then finding it's deleted and returning InvalidOid? RangeVarGetRelid is just going to complete its index scan of pg_class and may not come across the newly inserted row. Am I missing something? I would have expected to have to loop around and retry if no valid record is found. But this raises the question -- if no lock was acquired then what would have triggered an AcceptInvalidatationMessages and how would we know we waited long enough to find out about the newly created table? As a side note, if there are a long stream of such concurrent DDL then this code will leave all the old versions locked. This is consistent with our "hold locks until end of transaction" semantics but it seems weird for tables that we locked "accidentally" and didn't really end up using at all. I'm not sure it's really bad though. -- 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] Make relation_openrv atomic wrt DDL
On Mon, Jun 13, 2011 at 4:04 PM, Noah Misch wrote: > On Mon, Jun 13, 2011 at 08:21:05AM -0400, Robert Haas wrote: >> On Mon, Jun 13, 2011 at 1:12 AM, Noah Misch wrote: >> > This probably would not replace a backend-local counter of processed >> > messages >> > for RangeVarLockRelid()'s purposes. ?It's quite possibly a good way to >> > reduce >> > SInvalReadLock traffic, though. > >> I was imagining one shared global counter, not one per backend, and >> thinking that each backend could do something like: >> >> volatile uint32 *the_global_counter = &global_counter; >> uint32 latest_counter; >> mfence(); >> latest_counter = *the_global_counter; >> if (latest_counter != previous_value_of_global_counter || >> myprocstate->isReset) >> really_do_it(); >> previous_value_of_global_counter = latest_counter; >> >> I'm not immediately seeing why that wouldn't work for your purposes as well. > > That takes us back to the problem of answering the (somewhat rephrased) > question > "Did any call to AcceptInvalidationMessages() between code point A and code > point B call really_do_it()?" in a way not prone to breaking when new calls to > AcceptInvalidationMessages(), perhaps indirectly, get added. That's what the > local counter achieved. To achieve that, previous_value_of_global_counter > would > need to be exposed outside sinval.c. That leaves us with a backend-local > counter updated in a different fashion. I might be missing something... I see your point. -- 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] Make relation_openrv atomic wrt DDL
On Mon, Jun 13, 2011 at 08:21:05AM -0400, Robert Haas wrote: > On Mon, Jun 13, 2011 at 1:12 AM, Noah Misch wrote: > > This probably would not replace a backend-local counter of processed > > messages > > for RangeVarLockRelid()'s purposes. ?It's quite possibly a good way to > > reduce > > SInvalReadLock traffic, though. > I was imagining one shared global counter, not one per backend, and > thinking that each backend could do something like: > > volatile uint32 *the_global_counter = &global_counter; > uint32 latest_counter; > mfence(); > latest_counter = *the_global_counter; > if (latest_counter != previous_value_of_global_counter || > myprocstate->isReset) >really_do_it(); > previous_value_of_global_counter = latest_counter; > > I'm not immediately seeing why that wouldn't work for your purposes as well. That takes us back to the problem of answering the (somewhat rephrased) question "Did any call to AcceptInvalidationMessages() between code point A and code point B call really_do_it()?" in a way not prone to breaking when new calls to AcceptInvalidationMessages(), perhaps indirectly, get added. That's what the local counter achieved. To achieve that, previous_value_of_global_counter would need to be exposed outside sinval.c. That leaves us with a backend-local counter updated in a different fashion. I might be missing something... -- 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] Make relation_openrv atomic wrt DDL
On Mon, Jun 13, 2011 at 1:12 AM, Noah Misch wrote: > That might be a start, but it's not a complete replacement for the global > counter. AcceptInvalidationMessages() is actually called in > LockRelationOid(), > but the comparison needs to happen a level up in RangeVarLockRelid(). So, we > would be adding encapsulation in one place to lose it in another. Also, in > the > uncontended case, the patch only calls AcceptInvalidationMessages() once per > relation_openrv. It compares the counter after that call with a counter as > the > last caller left it -- RangeVarLockRelid() doesn't care who that caller was. Hmm, OK. >> Taking that a bit further, what if we put that counter in >> shared-memory? After writing new messages into the queue, a writer >> would bump this count (only one process can be doing this at a time >> because SInvalWriteLock is held) and memory-fence. Readers would >> memory-fence and then read the count before acquiring the lock. If it >> hasn't changed since we last read it, then don't bother acquiring >> SInvalReadLock, because no new messages have arrived. Or maybe an >> exact multiple of 2^32 messages have arrived, but there's probably >> someway to finesse around that issue, like maybe also using some kind >> of memory barrier to allow resetState to be checked without the lock. > > This probably would not replace a backend-local counter of processed messages > for RangeVarLockRelid()'s purposes. It's quite possibly a good way to reduce > SInvalReadLock traffic, though. > > Exact multiples of 2^32 messages need not be a problem, because the queue is > limited to MAXNUMMESSAGES (4096, currently). I think you will need to pack > into > one 32-bit value all data each backend needs to decide whether to proceed with > the full process. Given that queue offsets fit into 13 bits (easily reduced > to > 12) and resetState is a bit, that seems practical enough at first glance. I was imagining one shared global counter, not one per backend, and thinking that each backend could do something like: volatile uint32 *the_global_counter = &global_counter; uint32 latest_counter; mfence(); latest_counter = *the_global_counter; if (latest_counter != previous_value_of_global_counter || myprocstate->isReset) really_do_it(); previous_value_of_global_counter = latest_counter; I'm not immediately seeing why that wouldn't work for your purposes as well. -- 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] Make relation_openrv atomic wrt DDL
On Sun, Jun 12, 2011 at 10:56:41PM -0400, Robert Haas wrote: > On Sun, Jun 12, 2011 at 9:23 PM, Noah Misch wrote: > >> I haven't reviewed > >> your patch in detail, but is there a way we can encapsulate the > >> knowledge of the invalidation system down inside the sinval machinery, > >> rather than letting the heap code have to know directly about the > >> counter? ?Perhaps AIV() could return true or false depending on > >> whether any invalidation messages were processed, or somesuch. > > > > I actually did it exactly that way originally. ?The problem was the return > > value > > only applying to the given call, while I wished to answer a question like > > "Did > > any call to AcceptInvalidationMessages() between code point A and code > > point B > > process a message?" ?Adding AcceptInvalidationMessages() calls to code > > between A > > and B would make the return value test yield a false negative. ?A global > > counter > > was the best thing I could come up with that avoided this hazard. > > Oh, interesting point. What if AcceptInvalidationMessages returns the > counter? Maybe with typedef uint32 InvalidationPositionId or > something like that, to make it partially self-documenting, and > greppable. That might be a start, but it's not a complete replacement for the global counter. AcceptInvalidationMessages() is actually called in LockRelationOid(), but the comparison needs to happen a level up in RangeVarLockRelid(). So, we would be adding encapsulation in one place to lose it in another. Also, in the uncontended case, the patch only calls AcceptInvalidationMessages() once per relation_openrv. It compares the counter after that call with a counter as the last caller left it -- RangeVarLockRelid() doesn't care who that caller was. > Taking that a bit further, what if we put that counter in > shared-memory? After writing new messages into the queue, a writer > would bump this count (only one process can be doing this at a time > because SInvalWriteLock is held) and memory-fence. Readers would > memory-fence and then read the count before acquiring the lock. If it > hasn't changed since we last read it, then don't bother acquiring > SInvalReadLock, because no new messages have arrived. Or maybe an > exact multiple of 2^32 messages have arrived, but there's probably > someway to finesse around that issue, like maybe also using some kind > of memory barrier to allow resetState to be checked without the lock. This probably would not replace a backend-local counter of processed messages for RangeVarLockRelid()'s purposes. It's quite possibly a good way to reduce SInvalReadLock traffic, though. Exact multiples of 2^32 messages need not be a problem, because the queue is limited to MAXNUMMESSAGES (4096, currently). I think you will need to pack into one 32-bit value all data each backend needs to decide whether to proceed with the full process. Given that queue offsets fit into 13 bits (easily reduced to 12) and resetState is a bit, that seems practical enough at first glance. nm -- 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] Make relation_openrv atomic wrt DDL
On Sun, Jun 12, 2011 at 9:23 PM, Noah Misch wrote: >> I haven't reviewed >> your patch in detail, but is there a way we can encapsulate the >> knowledge of the invalidation system down inside the sinval machinery, >> rather than letting the heap code have to know directly about the >> counter? Perhaps AIV() could return true or false depending on >> whether any invalidation messages were processed, or somesuch. > > I actually did it exactly that way originally. The problem was the return > value > only applying to the given call, while I wished to answer a question like "Did > any call to AcceptInvalidationMessages() between code point A and code point B > process a message?" Adding AcceptInvalidationMessages() calls to code > between A > and B would make the return value test yield a false negative. A global > counter > was the best thing I could come up with that avoided this hazard. Oh, interesting point. What if AcceptInvalidationMessages returns the counter? Maybe with typedef uint32 InvalidationPositionId or something like that, to make it partially self-documenting, and greppable. Taking that a bit further, what if we put that counter in shared-memory? After writing new messages into the queue, a writer would bump this count (only one process can be doing this at a time because SInvalWriteLock is held) and memory-fence. Readers would memory-fence and then read the count before acquiring the lock. If it hasn't changed since we last read it, then don't bother acquiring SInvalReadLock, because no new messages have arrived. Or maybe an exact multiple of 2^32 messages have arrived, but there's probably someway to finesse around that issue, like maybe also using some kind of memory barrier to allow resetState to be checked without the lock. -- 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] Make relation_openrv atomic wrt DDL
On Sun, Jun 12, 2011 at 06:20:53PM -0400, Robert Haas wrote:
> On Sun, Jun 12, 2011 at 3:18 PM, Noah Misch wrote:
> > Indeed, the original patch slowed it by about 50%. ?I improved the patch,
> > adding a global SharedInvalidMessageCounter to increment as we process
> > messages. ?If this counter does not change between the RangeVarGetRelid()
> > call
> > and the post-lock AcceptInvalidationMessages() call, we can skip the second
> > RangeVarGetRelid() call. ?With the updated patch, I get these timings (in
> > ms)
> > for runs of "SELECT nmtest(1000)":
> >
> > master: 19697.642, 20087.477, 19748.995
> > patched: 19723.716, 19879.538, 20257.671
> >
> > In other words, no significant difference. ?Since the patch removes the
> > no-longer-needed pre-lock call to AcceptInvalidationMessages(), changing to
> > "relation_close(r, NoLock)" in the test case actually reveals a 15%
> > performance improvement. ?Granted, nothing to get excited about in light of
> > the artificiality.
>
> In point of fact, given the not-so-artificial results I just posted on
> another thread ("lazy vxid locks") I'm *very* excited about trying to
> reduce the cost of AcceptInvalidationMessages().
Quite interesting. A quick look suggests there is room for optimization there.
> I haven't reviewed
> your patch in detail, but is there a way we can encapsulate the
> knowledge of the invalidation system down inside the sinval machinery,
> rather than letting the heap code have to know directly about the
> counter? Perhaps AIV() could return true or false depending on
> whether any invalidation messages were processed, or somesuch.
I actually did it exactly that way originally. The problem was the return value
only applying to the given call, while I wished to answer a question like "Did
any call to AcceptInvalidationMessages() between code point A and code point B
process a message?" Adding AcceptInvalidationMessages() calls to code between A
and B would make the return value test yield a false negative. A global counter
was the best thing I could come up with that avoided this hazard.
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Re: [HACKERS] Make relation_openrv atomic wrt DDL
On Sun, Jun 12, 2011 at 3:18 PM, Noah Misch wrote:
> Indeed, the original patch slowed it by about 50%. I improved the patch,
> adding a global SharedInvalidMessageCounter to increment as we process
> messages. If this counter does not change between the RangeVarGetRelid() call
> and the post-lock AcceptInvalidationMessages() call, we can skip the second
> RangeVarGetRelid() call. With the updated patch, I get these timings (in ms)
> for runs of "SELECT nmtest(1000)":
>
> master: 19697.642, 20087.477, 19748.995
> patched: 19723.716, 19879.538, 20257.671
>
> In other words, no significant difference. Since the patch removes the
> no-longer-needed pre-lock call to AcceptInvalidationMessages(), changing to
> "relation_close(r, NoLock)" in the test case actually reveals a 15%
> performance improvement. Granted, nothing to get excited about in light of
> the artificiality.
In point of fact, given the not-so-artificial results I just posted on
another thread ("lazy vxid locks") I'm *very* excited about trying to
reduce the cost of AcceptInvalidationMessages(). I haven't reviewed
your patch in detail, but is there a way we can encapsulate the
knowledge of the invalidation system down inside the sinval machinery,
rather than letting the heap code have to know directly about the
counter? Perhaps AIV() could return true or false depending on
whether any invalidation messages were processed, or somesuch.
> This semantic improvement would be hard to test with the current pg_regress
> suite, so I do not include any test case addition in the patch. If
> sufficiently important, it could be done with isolationtester.
I haven't had a chance to look closely at the isolation tester yet,
but I'm excited about the possibilities for testing this sort of
thing. Not sure whether it's worth including this or not, but it
doesn't seem like a bad idea.
--
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EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
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[HACKERS] Make relation_openrv atomic wrt DDL
On Thu, Jan 20, 2011 at 09:48:12PM -0500, Robert Haas wrote:
> On Thu, Jan 20, 2011 at 6:19 PM, Noah Misch wrote:
> > On Thu, Jan 20, 2011 at 09:36:11PM +, Simon Riggs wrote:
> >> I agree that the DDL behaviour is wrong and should be fixed. Thank you
> >> for championing that alternative view.
> >>
> >> Swapping based upon names only works and is very flexible, much more so
> >> than EXCHANGE could be.
> >>
> >> A separate utility might be worth it, but the feature set of that should
> >> be defined in terms of correctly-working DDL behaviour. It's possible
> >> that no further requirement exists. I remove my own patch from
> >> consideration for this release.
> >>
> >> I'll review your patch and commit it, problems or objections excepted. I
> >> haven't looked at it in any detail.
> >
> > Thanks. ?I wouldn't be very surprised if that patch is even the wrong way to
> > achieve these semantics, but it's great that we're on the same page as to
> > which
> > semantics they are.
>
> I think Noah's patch is a not a good idea, because it will result in
> calling RangeVarGetRelid twice even in the overwhelmingly common case
> where no relevant invalidation occurs. That'll add several syscache
> lookups per table to very common operations.
Valid concern.
[Refresher: this was a patch to improve behavior for this test case:
psql -c "CREATE TABLE t AS VALUES ('old'); CREATE TABLE new_t AS VALUES
('new')"
psql -c 'SELECT pg_sleep(2) FROM t' & # block the ALTER or DROP briefly
sleep 1 #
give prev time to take AccessShareLock
# Do it this way, and the next SELECT gets data from the old table.
psql -c 'ALTER TABLE t RENAME TO old_t; ALTER TABLE new_t RENAME TO t' &
# Do it this way, and get: ERROR: could not open relation with OID
41380
#psql -c 'DROP TABLE t; ALTER TABLE new_t RENAME TO t' &
psql -c 'SELECT * FROM t' # I get 'old' or an error,
never 'new'.
psql -c 'DROP TABLE IF EXISTS t, old_t, new_t'
It did so by rechecking the RangeVar->oid resolution after locking the found
relation, by which time concurrent DDL could no longer be interfering.]
I benchmarked the original patch with this function:
Datum
nmtest(PG_FUNCTION_ARGS)
{
int32 n = PG_GETARG_INT32(0);
int i;
RangeVar *rv = makeRangeVar(NULL, "pg_am", 0);
for (i = 0; i < n; ++i)
{
Relationr = relation_openrv(rv,
AccessShareLock);
relation_close(r, AccessShareLock);
}
PG_RETURN_INT32(4);
}
(Releasing the lock before transaction end makes for an unrealistic benchmark,
but so would opening the same relation millions of times in a single
transaction. I'm trying to isolate the cost that would be spread over
millions of transactions opening relations a handful of times. See attached
shar archive for a complete extension wrapping that test function.)
Indeed, the original patch slowed it by about 50%. I improved the patch,
adding a global SharedInvalidMessageCounter to increment as we process
messages. If this counter does not change between the RangeVarGetRelid() call
and the post-lock AcceptInvalidationMessages() call, we can skip the second
RangeVarGetRelid() call. With the updated patch, I get these timings (in ms)
for runs of "SELECT nmtest(1000)":
master: 19697.642, 20087.477, 19748.995
patched: 19723.716, 19879.538, 20257.671
In other words, no significant difference. Since the patch removes the
no-longer-needed pre-lock call to AcceptInvalidationMessages(), changing to
"relation_close(r, NoLock)" in the test case actually reveals a 15%
performance improvement. Granted, nothing to get excited about in light of
the artificiality.
This semantic improvement would be hard to test with the current pg_regress
suite, so I do not include any test case addition in the patch. If
sufficiently important, it could be done with isolationtester.
Thanks,
nm
diff --git a/src/backend/access/heap/heapam.c b/src/backend/access/heap/heapam.c
index 01a492e..63537fd 100644
*** a/src/backend/access/heap/heapam.c
--- b/src/backend/access/heap/heapam.c
***
*** 979,1004 relation_openrv(const RangeVar *relation, LOCKMODE lockmode)
{
Oid relOid;
! /*
!* Check for shared-cache-inval messages before trying to open the
!* relation. This is needed to cover the case where the name
identifies a
!* rel that has been dropped and recreated since the start of our
!* transaction: if we don't flush the old syscache entry then we'll
latch
!* onto that entry and suffer an error when we do RelationIdGetRelation.
!* Note that relation_open does not need to do this, since a relation's
