VACUUM FULL, CLUSTER, and REPACK block on other sessions' temp tables

On Mar 24, 2026, at 23:35, Jim Jones <jim.jones@uni-muenster.de> wrote:

Hi,

While testing another patch [1], I noticed that REPACK is blocked when a
temporary table is locked in another session. It also turns out that the
same behaviour occurs with VACUUM FULL and CLUSTER:

== session 1 ==

$ psql postgres
psql (19devel)
Type "help" for help.

postgres=# CREATE TEMPORARY TABLE tmp (id int);
CREATE TABLE
postgres=# BEGIN;
LOCK TABLE tmp IN SHARE MODE;
BEGIN
LOCK TABLE
postgres=*#

== session 2 ==

$ psql postgres
psql (19devel)
Type "help" for help.

postgres=# REPACK;
^CCancel request sent
ERROR: canceling statement due to user request
CONTEXT: waiting for AccessExclusiveLock on relation 38458 of database 5
postgres=# VACUUM FULL;
^CCancel request sent
ERROR: canceling statement due to user request
CONTEXT: waiting for AccessExclusiveLock on relation 38458 of database 5

Skipping temporary relations in get_tables_to_repack() and
get_all_vacuum_rels() before they're appended to the list seems to do
the trick -- see attached draft.

I can reproduce the same behaviour with CLUSTER and VACUUM FULL in
PG14-PG18. I took a quick look at the code in PG17 and PG18 and the fix
appears to be straightforward, but before I start working on it, I'd
like to hear your thoughts. Is it worth the effort?

Best, Jim

1 - /messages/by-id/13637.1774342137@localhost&lt;v1-0001-Skip-other-sessions-temp-tables-in-REPACK-CLUSTER.patch&gt;

I think skipping temp tables of another session is reasonable, because anyway they are not accessible from the current session, though visible via pg_class.

Looking at the patch:
```
+			/* Skip temp relations belonging to other sessions */
+			if (class->relpersistence == RELPERSISTENCE_TEMP &&
+				isOtherTempNamespace(class->relnamespace))
```

It uses isOtherTempNamespace(), but I noticed that the header comment of the function says:
```
* isOtherTempNamespace - is the given namespace some other backend's
* temporary-table namespace (including temporary-toast-table namespaces)?
*
* Note: for most purposes in the C code, this function is obsolete. Use
* RELATION_IS_OTHER_TEMP() instead to detect non-local temp relations.
```

Then looking at RELATION_IS_OTHER_TEMP():
```
#define RELATION_IS_OTHER_TEMP(relation) \
((relation)->rd_rel->relpersistence == RELPERSISTENCE_TEMP && \
!(relation)->rd_islocaltemp)
```

It takes a relation as parameter and check relation->rd_islocaltemp, however in the context of this patch, we have only Form_pg_class.

Then checking how rd_islocaltemp is set:
```
case RELPERSISTENCE_TEMP:
if (isTempOrTempToastNamespace(relation->rd_rel->relnamespace))
{
relation->rd_backend = ProcNumberForTempRelations();
relation->rd_islocaltemp = true;
}
else
{
/*
* If it's a temp table, but not one of ours, we have to use
* the slow, grotty method to figure out the owning backend.
*
* Note: it's possible that rd_backend gets set to
* MyProcNumber here, in case we are looking at a pg_class
* entry left over from a crashed backend that coincidentally
* had the same ProcNumber we're using. We should *not*
* consider such a table to be "ours"; this is why we need the
* separate rd_islocaltemp flag. The pg_class entry will get
* flushed if/when we clean out the corresponding temp table
* namespace in preparation for using it.
*/
relation->rd_backend =
GetTempNamespaceProcNumber(relation->rd_rel->relnamespace);
Assert(relation->rd_backend != INVALID_PROC_NUMBER);
relation->rd_islocaltemp = false;
}
break;
```

It uses isTempOrTempToastNamespace(relation->rd_rel->relnamespace) to decide relation->rd_islocaltemp.

So, I think this patch should also use "!isTempOrTempToastNamespace(classForm->relnamespace)" instead of isOtherTempNamespace(class->relnamespace). I tried that locally, and it works for me.

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/

Hi Chao,

Thanks for the thorough review.

On 25/03/2026 02:55, Chao Li wrote:

It uses isTempOrTempToastNamespace(relation->rd_rel->relnamespace) to decide relation->rd_islocaltemp.

So, I think this patch should also use "!isTempOrTempToastNamespace(classForm->relnamespace)" instead of isOtherTempNamespace(class->relnamespace). I tried that locally, and it works for me.

I agree. In get_all_vacuum_rels() and the pg_class scan branch of
get_tables_to_repack(), relpersistence == RELPERSISTENCE_TEMP already
establishes the relation is a temp table, so
!isTempOrTempToastNamespace() alone is sufficient to identify
other-session temp tables.

In the usingindex path of get_tables_to_repack(), Form_pg_class is not
available, so there is no relpersistence to use as a pre-filter. The
explicit isAnyTempNamespace() check is required to avoid incorrectly
skipping permanent tables. This is pretty much what
isOtherTempNamespace() does internally -- the only change is inlining it
to avoid the obsolete wrapper.

* Skip temp relations belonging to other sessions */
{
Oid nsp = get_rel_namespace(index->indrelid);

if (!isTempOrTempToastNamespace(nsp) && isAnyTempNamespace(nsp))
{
UnlockRelationOid(index->indrelid, AccessShareLock);
continue;
}
}

v2 attached.

Thanks!

Best, Jim

I forgot to create a CF entry. Here it is:
https://commitfest.postgresql.org/patch/6616/

Best, Jim

Hello!

Shouldn't the patch also include a tap test to verify that the change
works / fails without it?

+ /* Skip temp relations belonging to other sessions */
+ {
+ Oid nsp = get_rel_namespace(index->indrelid);
+
+ if (!isTempOrTempToastNamespace(nsp) && isAnyTempNamespace(nsp))
+ {

Doesn't this result in several repeated syscache lookups?

There's already a SearchSysCacheExsists1 directly above this, then a
get_rel_namespace, then an isAnyTempNamespace. While this probably
isn't performance critical, this should be doable with a single
SearchSysCache1(RELOID...) and then a few conditions, similarly to the
else branch below this?

Hi

On 25/03/2026 21:38, Zsolt Parragi wrote:

Shouldn't the patch also include a tap test to verify that the change
works / fails without it?

Definitely. I just didn't want to invest much time on tests before
getting feedback on the issue itself.

+ /* Skip temp relations belonging to other sessions */
+ {
+ Oid nsp = get_rel_namespace(index->indrelid);
+
+ if (!isTempOrTempToastNamespace(nsp) && isAnyTempNamespace(nsp))
+ {

Doesn't this result in several repeated syscache lookups?

There's already a SearchSysCacheExsists1 directly above this, then a
get_rel_namespace, then an isAnyTempNamespace. While this probably
isn't performance critical, this should be doable with a single
SearchSysCache1(RELOID...) and then a few conditions, similarly to the
else branch below this?

You're right. Although it is not performance critical we can solve it
with a single SearchSysCache1.

PFA v3 with the improved fix (0001) and tests (0002).

Thanks for the review!

Best, Jim

On Mar 26, 2026, at 07:00, Jim Jones <jim.jones@uni-muenster.de> wrote:

Hi

On 25/03/2026 21:38, Zsolt Parragi wrote:

Shouldn't the patch also include a tap test to verify that the change
works / fails without it?

Definitely. I just didn't want to invest much time on tests before
getting feedback on the issue itself.

+ /* Skip temp relations belonging to other sessions */
+ {
+ Oid nsp = get_rel_namespace(index->indrelid);
+
+ if (!isTempOrTempToastNamespace(nsp) && isAnyTempNamespace(nsp))
+ {

Doesn't this result in several repeated syscache lookups?

There's already a SearchSysCacheExsists1 directly above this, then a
get_rel_namespace, then an isAnyTempNamespace. While this probably
isn't performance critical, this should be doable with a single
SearchSysCache1(RELOID...) and then a few conditions, similarly to the
else branch below this?

You're right. Although it is not performance critical we can solve it
with a single SearchSysCache1.

PFA v3 with the improved fix (0001) and tests (0002).

Thanks for the review!

Best, Jim<v3-0001-Skip-other-sessions-temp-tables-in-REPACK-CLUSTER.patch><v3-0002-Test-VACUUM-FULL-CLUSTER-and-REPACK-with-locked-t.patch>

I don't think such a TAP test is necessary.

One reason is that, if we look at the check right above the new one:
```
/*
* We include partitioned tables here; depending on which operation is
* to be performed, caller will decide whether to process or ignore
* them.
*/
if (classForm->relkind != RELKIND_RELATION &&
classForm->relkind != RELKIND_MATVIEW &&
classForm->relkind != RELKIND_PARTITIONED_TABLE)
continue;
```

I don't see a test specifically for that check either. So I don't think we need a test for every individual path.

Second, based on [1]/messages/by-id/mtkrkkcn2tlhytumitpch5ubxiprv2jzvprf5r5m3mjeczvq4q@p6wkzbfxuyv2 </messages/by-id/&gt; and [2]/messages/by-id/1449781.1773948276@sss.pgh.pa.us, I got the impression that adding new tests is not always welcome considering overall test runtime. Anyway, maybe I’m wrong, let the committers judge that.

[1]: /messages/by-id/mtkrkkcn2tlhytumitpch5ubxiprv2jzvprf5r5m3mjeczvq4q@p6wkzbfxuyv2 </messages/by-id/&gt;
[2]: /messages/by-id/1449781.1773948276@sss.pgh.pa.us

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/

Chao Li <li.evan.chao@gmail.com> wrote:

I don't think such a TAP test is necessary.

+1

--
Antonin Houska
Web: https://www.cybertec-postgresql.com

On 26/03/2026 12:25, Antonin Houska wrote:

Chao Li <li.evan.chao@gmail.com> wrote:

I don't think such a TAP test is necessary.

+1

I've kept the tests in a separate file so the committer can easily skip
them if needed.

Thanks for the feedback, everyone!

Best, Jim

rebase

Jim

rebase

Jim