Re: Global temporary tables

Hi, Dean

On Thu, 02 Jul 2026 at 17:11, Japin Li <japinli(at)hotmail(dot)com> wrote:
> Hi, Dean
>
> On Wed, 01 Jul 2026 at 08:32, Dean Rasheed <dean(dot)a(dot)rasheed(at)gmail(dot)com> wrote:
>> On Wed, 1 Jul 2026 at 07:24, Japin Li <japinli(at)hotmail(dot)com> wrote:
>>>
>>> Thanks for working on this.
>>
>> Thank you for testing it!
>>
>>> While testing the v5 patch, I encountered a lock wait.
>>>
>>> 2026-07-01 14:18:43.603 CST [1486593] LOG: process 1486593 still waiting for AccessExclusiveLock on relation 16384 of database 5 after 1000.106 ms
>>> 2026-07-01 14:18:43.603 CST [1486593] DETAIL: Process holding the lock: 1486484. Wait queue: 1486593.
>>> 2026-07-01 14:18:43.603 CST [1486593] CONTEXT: waiting for AccessExclusiveLock on relation 16384 of database 5
>>> 2026-07-01 14:18:43.603 CST [1486593] STATEMENT: SELECT * FROM gtt_delete;
>>>
>>> Is this expected?
>>
>> Ah yes, you're right. That's not good.
>>
>> The root cause looks to be the same issue that Pavel reported -- the
>> ON COMMIT DELETE ROWS does a TRUNCATE when the transaction is
>> committed, which requires an AccessExclusiveLock. I think the patch
>> should reduce the lock level required for TRUNCATE on a global
>> temporary relation to RowExclusiveLock, like DELETE.
>>
>
> Here is my review on v5-0002. Please take a look.
>
> 1.
> $ git am ~/Patches/gtt/*.patch
> Applying: Save temporary table ON COMMIT actions to pg_class.
> Applying: Basic support for global temporary tables.
> Applying: Support indexes on global temporary tables.
> Applying: Support global temporary sequences.
> Applying: Support global temporary catalog tables and add pg_temp_class.
> Applying: Add relation statistics columns to pg_temp_class.
> Applying: Add relfrozenxid and relminmxid columns to pg_temp_class.
> .git/rebase-apply/patch:940: new blank line at EOF.
> +
> warning: 1 line adds whitespace errors.
> Applying: Add pg_temp_statistic global temporary catalog table.
> Applying: Add pg_temp_statistic_ext_data global temporary catalog table.
> Applying: Add pg_temp_index global temporary catalog table.
>
> 2.
> +static void
> +gtr_delete_all_storage_on_exit(int code, Datum arg)
> +{
> + ProcNumber backend;
> + HASH_SEQ_STATUS status;
> + GtrStorageEntry *entry;
> + SMgrRelation srel;
> +
> + /* Loop over all storage created and delete it */
> + backend = ProcNumberForTempRelations();
> + hash_seq_init(&status, gtr_local_storage);
> + while ((entry = hash_seq_search(&status)) != NULL)
> + {
> + srel = smgropen(entry->rlocator, backend);
> + smgrdounlinkall(&srel, 1, false);
> + smgrclose(srel);
> + }
> +}
>
> We can restrict srel to the loop scope.
>
> 3.
> +gtr_remove_all_usage_on_exit(int code, Datum arg)
> +{
> + HASH_SEQ_STATUS status;
> + GtrUsageEntry *local_entry;
> + GtrSharedUsageKey key;
> + GtrSharedUsageEntry *shared_entry;
> +
> + /* Loop over all the global temporary relations we were using */
> + hash_seq_init(&status, gtr_local_usage);
> + while ((local_entry = hash_seq_search(&status)) != NULL)
> + {
> + /* Find the shared usage entry */
> + key.dbid = MyDatabaseId;
> + key.relid = local_entry->relid;
> + shared_entry = dshash_find(gtr_shared_usage, &key, true);
>
> Same as above: move the declarations of key and shared_entry inside the loop.
>
> 4.
> +static void
> +gtr_init_usage_tables(void)
> +{
> + HASHCTL ctl;
> + MemoryContext oldcontext;
> +
> + /* Local usage table */
> + if (gtr_local_usage == NULL)
> + {
> + ctl.keysize = sizeof(Oid);
> + ctl.entrysize = sizeof(GtrUsageEntry);
> +
> + gtr_local_usage = hash_create("Global temporary relations in use locally",
> + 128, &ctl, HASH_ELEM | HASH_BLOBS);
> + }
> +
> + /* Shared usage table */
> + if (gtr_shared_usage == NULL)
> + {
> + /* Use a lock to ensure only one process creates the table */
> + LWLockAcquire(GlobalTempRelControlLock, LW_EXCLUSIVE);
> +
> + /* Be sure any local memory allocated by DSA routines is persistent */
> + oldcontext = MemoryContextSwitchTo(TopMemoryContext);
>
> Likewise, narrow ctl to the first if and oldContext to the second.
>
> 5.
> + * the usage hash table entries for it. Otherwise, reset the hash entry's
> + * subids to zero.
>
> Instead of zero, how about using an invalid sub-transaction ID?
>
> 6.
> +static void
> +gtr_process_invalidated_gtrs(void)
> +{
> + MemoryContext oldcontext;
> + Oid relid;
> +
> + /*
> + * Scan the gtrs_invalidated list and add any dropped relations to the
> + * gtrs_dropped list. Since the transaction might fail later on, we need
> + * the gtrs_dropped list to persist until we can successfully process it.
> + */
> + oldcontext = MemoryContextSwitchTo(TopMemoryContext);
> +
> + /*
> + * As we scan the gtrs_invalidated list, more invalidation messages might
> + * arrive, so keep going until it is empty.
> + */
> + while (gtrs_invalidated != NIL)
> + {
> + /* Pop the last relid from the list */
> + relid = llast_oid(gtrs_invalidated);
>
> Restrict relid to the while loop scope.
>
> 7.
> +void
> +TrackGlobalTempRelationStorage(Oid relid, RelFileLocator rlocator,
> + ProcNumber backend, bool create)
> +{
> + GtrStorageEntry *entry;
> + bool found;
> + SMgrRelation srel;
> +
> + if (create)
> + {
> + /* Initialize the storage table, if necessary */
> + gtr_init_storage_table();
> +
> + /* Insert an entry to track the storage */
> + entry = hash_search(gtr_local_storage, &rlocator, HASH_ENTER, &found);
>
> Likewise, restrict found and srel to the if statement.
>
> 8.
> + /* Register the ON COMMIT action for relation, if it's DELETE ROWS */
> + Assert(relation->rd_rel->reloncommit == RELONCOMMIT_PRESERVE_ROWS ||
> + relation->rd_rel->reloncommit == RELONCOMMIT_DELETE_ROWS);
> +
> + if (relation->rd_rel->reloncommit == RELONCOMMIT_DELETE_ROWS)
> + register_on_commit_action(relation->rd_id, ONCOMMIT_DELETE_ROWS);
>
> How about moving the comment to just before the if statement?
>
> 9.
> +void
> +InvalidateGlobalTempRelation(Oid relid)
> +{
> + MemoryContext oldcontext;
> + HASH_SEQ_STATUS status;
> + GtrUsageEntry *entry;
> +
> + /* Quick exit if we haven't used any global temporary relations */
> + if (gtr_local_usage == NULL)
> + return;
>
> Restrict status and entry to the else statement.
>
> 10.
> +void
> +GlobalTempRelationDropped(Oid relid)
>
> Not a fan of this name, but no better idea yet.
>
> 11.
> +void
> +PreCommit_GlobalTempRelation(void)
> +{
> + HASH_SEQ_STATUS status;
> + GtrStorageEntry *storage_entry;
> + Relation rel;
> +
>
> Restrict rel to the while loop.
>
> 12.
> +List *
> +AllGlobalTempRelationsInUse(Oid dbid)
>
> How about GetGlobalTempReplationsInUse()?
>
> 13.
> + ereport(ERROR,
> + (errcode(ERRCODE_INVALID_TABLE_DEFINITION),
> + errmsg("cannot create relations in temporary schemas of other sessions")));
>
> For consistency with the above, I suggest changing the error message to:
>
> cannot create global temporary relation in temporary schema of other sessions
>
> 14.
> + * If they're global temp relations, reassign rel2's storage to rel1.
>
> We use "global temporary relations" elsewhere – should we keep the same
> terminology here?
>
> 15.
> + errmsg(RELATION_IS_GLOBAL_TEMP(relation)
> + ? "cannot create a local temporary relation as partition of global temporary relation \"%s\""
> + : "cannot create a temporary relation as partition of permanent relation \"%s\"",
>
> Should we use the following error message:
>
> cannot create a local temporary relation as partition of permanent relation \"%s\"
>
> 16.
> + : relpersistence == RELPERSISTENCE_GLOBAL_TEMP
> + ? "cannot create a global temporary relation as partition of local temporary relation \"%s\""
> : "cannot create a permanent relation as partition of temporary relation \"%s\"",
>
> Same as above:
>
> cannot create a permanent relation as partition of local temporary relation \"%s\"
>
> 17.
> + if (RELATION_IS_GLOBAL_TEMP(OldHeap) &&
> + IsOtherUsingGlobalTempRelation(RelationGetRelid(OldHeap)))
> + ereport(ERROR,
> + errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
> + errmsg("cannot rewrite global temporary table \"%s\" because it is being used in another session",
> + RelationGetRelationName(OldHeap)));
>
>
> 18.
> + errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
> + errmsg("cannot add or alter constraints of global temporary table \"%s\" because it is being used in another session",
> + get_rel_name(tab->relid)));
>
> 19.
> if (pkrel->rd_rel->relpersistence != RELPERSISTENCE_TEMP)
> ereport(ERROR,
> (errcode(ERRCODE_INVALID_TABLE_DEFINITION),
> - errmsg("constraints on temporary tables may reference only temporary tables")));
> + errmsg("constraints on local temporary tables may reference only local temporary tables")));
> if (!pkrel->rd_islocaltemp || !rel->rd_islocaltemp)
> ereport(ERROR,
> (errcode(ERRCODE_INVALID_TABLE_DEFINITION),
> - errmsg("constraints on temporary tables must involve temporary tables of this session")));
> + errmsg("constraints on local temporary tables must involve local temporary tables of this session")));
> + break;
> + case RELPERSISTENCE_GLOBAL_TEMP:
> + if (!RELATION_IS_GLOBAL_TEMP(pkrel))
> + ereport(ERROR,
> + (errcode(ERRCODE_INVALID_TABLE_DEFINITION),
> + errmsg("constraints on global temporary tables may reference only global temporary tables")));
>
> s/global temporary table/global temporary relation/g
> s/local temporary table/local temporary relation/g
>
> 20.
> attachrel->rd_rel->relpersistence == RELPERSISTENCE_TEMP)
> ereport(ERROR,
> (errcode(ERRCODE_WRONG_OBJECT_TYPE),
> - errmsg("cannot attach a temporary relation as partition of permanent relation \"%s\"",
> + errmsg(RELATION_IS_GLOBAL_TEMP(rel)
> + ? "cannot attach a local temporary relation as partition of global temporary relation \"%s\""
> + : "cannot attach a temporary relation as partition of permanent relation \"%s\"",
> RelationGetRelationName(rel))));
>
> Same as 15:
>
> cannot attach a local temporary relation as partition of permanent relation \"%s\"
>
> 21.
> attachrel->rd_rel->relpersistence != RELPERSISTENCE_TEMP)
> ereport(ERROR,
> (errcode(ERRCODE_WRONG_OBJECT_TYPE),
> - errmsg("cannot attach a permanent relation as partition of temporary relation \"%s\"",
> + errmsg(RELATION_IS_GLOBAL_TEMP(attachrel)
> + ? "cannot attach a global temporary relation as partition of local temporary relation \"%s\""
> + : "cannot attach a permanent relation as partition of temporary relation \"%s\"",
> RelationGetRelationName(rel))));
>
> Same as 16:
>
> cannot attach a permanent relation as partition of local temporary relation \"%s\
>
> 22.
> @@ -22776,7 +22828,9 @@ createPartitionTable(List **wqueue, RangeVar *newPartName,
> newPartName->relpersistence == RELPERSISTENCE_TEMP)
> ereport(ERROR,
> errcode(ERRCODE_WRONG_OBJECT_TYPE),
> - errmsg("cannot create a temporary relation as partition of permanent relation \"%s\"",
> + errmsg(parent_relform->relpersistence == RELPERSISTENCE_GLOBAL_TEMP
> + ? "cannot create a local temporary relation as partition of global temporary relation \"%s\""
> + : "cannot create a temporary relation as partition of permanent relation \"%s\"",
> RelationGetRelationName(parent_rel)));
>
> /* Permanent rels cannot be partitions belonging to a temporary parent. */
>
> Suggestion:
>
> cannot create a local temporary relation as partition of permanent relation \"%s\"
>
> 23.
> @@ -22784,7 +22838,9 @@ createPartitionTable(List **wqueue, RangeVar *newPartName,
> parent_relform->relpersistence == RELPERSISTENCE_TEMP)
> ereport(ERROR,
> errcode(ERRCODE_WRONG_OBJECT_TYPE),
> - errmsg("cannot create a permanent relation as partition of temporary relation \"%s\"",
> + errmsg(newPartName->relpersistence == RELPERSISTENCE_GLOBAL_TEMP
> + ? "cannot create a global temporary relation as partition of local temporary relation \"%s\""
> + : "cannot create a permanent relation as partition of temporary relation \"%s\"",
> RelationGetRelationName(parent_rel)));
>
> Suggest:
>
> cannot create a permanent relation as partition of local temporary relation \"%s\"
>
>

While testing the v5 patch, I noticed that the index of a table with ON COMMIT
DELETE ROWS has the PRESERVE ROWS flag. See the example below.

postgres=# CREATE EXTENSION pageinspect;
CREATE EXTENSION
postgres=# CREATE GLOBAL TEMPORARY TABLE gtt_delete (id int primary key, info text) ON COMMIT DELETE ROWS;
CREATE TABLE
postgres=# CREATE GLOBAL TEMPORARY TABLE gtt_preserve (id int primary key, info text);
CREATE TABLE
postgres=# SELECT relname, reloncommit FROM pg_class WHERE relname ~ '^gtt_.*';
relname | reloncommit
-------------------+-------------
gtt_delete | d
gtt_delete_pkey | p <-- See here.
gtt_preserve | p
gtt_preserve_pkey | p
(4 rows)
postgres=# BEGIN;
BEGIN
postgres=*# INSERT INTO gtt_delete SELECT id FROM generate_series(1, 10000) id;
INSERT 0 10000
postgres=*# SELECT * FROM bt_metap('gtt_delete_pkey');
magic | version | root | level | fastroot | fastlevel | last_cleanup_num_delpages | last_cleanup_num_tuples | allequalimage
--------+---------+------+-------+----------+-----------+---------------------------+-------------------------+---------------
340322 | 4 | 3 | 1 | 3 | 1 | 0 | -1 | t
(1 row)

postgres=*# SELECT * FROM bt_page_items('gtt_delete_pkey', 1) limit 2;
itemoffset | ctid | itemlen | nulls | vars | data | dead | htid | tids
------------+-------+---------+-------+------+-------------------------+------+-------+------
1 | (1,1) | 16 | f | f | 6f 01 00 00 00 00 00 00 | | |
2 | (0,1) | 16 | f | f | 01 00 00 00 00 00 00 00 | f | (0,1) |
(2 rows)

postgres=*# END;
COMMIT
postgres=# SELECT * FROM bt_metap('gtt_delete_pkey');
magic | version | root | level | fastroot | fastlevel | last_cleanup_num_delpages | last_cleanup_num_tuples | allequalimage
--------+---------+------+-------+----------+-----------+---------------------------+-------------------------+---------------
340322 | 4 | 0 | 0 | 0 | 0 | 0 | -1 | t
(1 row)

postgres=# SELECT * FROM bt_page_items('gtt_delete_pkey', 1) limit 2;
ERROR: block number 1 is out of range
postgres=# BEGIN;
BEGIN
postgres=*# INSERT INTO gtt_preserve SELECT id FROM generate_series(1, 10000) id;
INSERT 0 10000
postgres=*# SELECT * FROM bt_metap('gtt_preserve_pkey');
magic | version | root | level | fastroot | fastlevel | last_cleanup_num_delpages | last_cleanup_num_tuples | allequalimage
--------+---------+------+-------+----------+-----------+---------------------------+-------------------------+---------------
340322 | 4 | 3 | 1 | 3 | 1 | 0 | -1 | t
(1 row)

postgres=*# SELECT * FROM bt_page_items('gtt_preserve_pkey', 1) limit 2;
itemoffset | ctid | itemlen | nulls | vars | data | dead | htid | tids
------------+-------+---------+-------+------+-------------------------+------+-------+------
1 | (1,1) | 16 | f | f | 6f 01 00 00 00 00 00 00 | | |
2 | (0,1) | 16 | f | f | 01 00 00 00 00 00 00 00 | f | (0,1) |
(2 rows)

postgres=*# COMMIT;
COMMIT
postgres=# SELECT * FROM bt_metap('gtt_preserve_pkey');
magic | version | root | level | fastroot | fastlevel | last_cleanup_num_delpages | last_cleanup_num_tuples | allequalimage
--------+---------+------+-------+----------+-----------+---------------------------+-------------------------+---------------
340322 | 4 | 3 | 1 | 3 | 1 | 0 | -1 | t
(1 row)

postgres=# SELECT * FROM bt_page_items('gtt_preserve_pkey', 1) limit 2;
itemoffset | ctid | itemlen | nulls | vars | data | dead | htid | tids
------------+-------+---------+-------+------+-------------------------+------+-------+------
1 | (1,1) | 16 | f | f | 6f 01 00 00 00 00 00 00 | | |
2 | (0,1) | 16 | f | f | 01 00 00 00 00 00 00 00 | f | (0,1) |
(2 rows)

Is this expected behavior?

Since the index data for gtt_delete is cleared upon transaction commit, I would
expect its reloncommit flag to be 'd'.

--
Regards,
Japin Li
ChengDu WenWu Information Technology Co., Ltd.