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Avoid using tuple from syscache for update of pg_database.datfrozenxid
author Michael Paquier <michael@paquier.xyz>
Tue, 8 Dec 2020 03:13:19 +0000 (12:13 +0900)
committer Michael Paquier <michael@paquier.xyz>
Tue, 8 Dec 2020 03:13:19 +0000 (12:13 +0900)
pg_database.datfrozenxid gets updated using an in-place update at the
end of vacuum or autovacuum. Since 96cdeae, as pg_database has a toast
relation, it is possible for a pg_database tuple to have toast values
if there is a large set of ACLs in place. In such a case, the in-place
update would fail because of the flattening of the toast values done for
the catcache entry fetched. Instead of using a copy from the catcache,
this changes the logic to fetch the copy of the tuple by directly
scanning pg_database.

Per the lack of complaints on the matter, no backpatch is done. Note
that before 96cdeae, attempting to insert such a tuple to pg_database
would cause a "row is too big" error, so the end-of-vacuum problem was
not reachable.

Author: Ashwin Agrawal, Junfeng Yang
Discussion: https://postgr.es/m/DM5PR0501MB38800D9E4605BCA72DD35557CCE10@DM5PR0501MB3880.namprd05.prod.outlook.com

src/backend/access/heap/heapam.c patch | blob | blame | history
src/backend/commands/vacuum.c patch | blob | blame | history

diff --git a/src/backend/access/heap/heapam.c b/src/backend/access/heap/heapam.c
index 1b2f70499e54aee4679316f8c3d53da02d980483..a9583f31036884b4aafd544750204697fde06f30 100644 (file)
--- a/src/backend/access/heap/heapam.c
+++ b/src/backend/access/heap/heapam.c
@@ -5710,6 +5710,10 @@ heap_abort_speculative(Relation relation, ItemPointer tid)
*
* tuple is an in-memory tuple structure containing the data to be written
* over the target tuple. Also, tuple->t_self identifies the target tuple.
+ *
+ * Note that the tuple updated here had better not come directly from the
+ * syscache if the relation has a toast relation as this tuple could
+ * include toast values that have been expanded, causing a failure here.
*/
void
heap_inplace_update(Relation relation, HeapTuple tuple)
diff --git a/src/backend/commands/vacuum.c b/src/backend/commands/vacuum.c
index f1112111de86d0bf5c5c8ed4b0903265524524e9..98270a10495d75e608074acaa494697c83a050cf 100644 (file)
--- a/src/backend/commands/vacuum.c
+++ b/src/backend/commands/vacuum.c
@@ -1361,6 +1361,7 @@ vac_update_datfrozenxid(void)
MultiXactId lastSaneMinMulti;
bool bogus = false;
bool dirty = false;
+ ScanKeyData key[1];
/*
* Restrict this task to one backend per database. This avoids race
@@ -1479,10 +1480,25 @@ vac_update_datfrozenxid(void)
/* Now fetch the pg_database tuple we need to update. */
relation = table_open(DatabaseRelationId, RowExclusiveLock);
- /* Fetch a copy of the tuple to scribble on */
- tuple = SearchSysCacheCopy1(DATABASEOID, ObjectIdGetDatum(MyDatabaseId));
+ /*
+ * Get the pg_database tuple to scribble on. Note that this does not
+ * directly rely on the syscache to avoid issues with flattened toast
+ * values for the in-place update.
+ */
+ ScanKeyInit(&key[0],
+ Anum_pg_database_oid,
+ BTEqualStrategyNumber, F_OIDEQ,
+ ObjectIdGetDatum(MyDatabaseId));
+
+ scan = systable_beginscan(relation, DatabaseOidIndexId, true,
+ NULL, 1, key);
+ tuple = systable_getnext(scan);
+ tuple = heap_copytuple(tuple);
+ systable_endscan(scan);
+
if (!HeapTupleIsValid(tuple))
elog(ERROR, "could not find tuple for database %u", MyDatabaseId);
+
dbform = (Form_pg_database) GETSTRUCT(tuple);
/*
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