From 7da83415e5bc01bba0093f34f4f612b70c70b678 Mon Sep 17 00:00:00 2001 From: Noah Misch Date: Sat, 30 Jan 2021 00:12:18 -0800 Subject: [PATCH] Revive "snapshot too old" with wal_level=minimal and SET TABLESPACE. Given a permanent relation rewritten in the current transaction, the old_snapshot_threshold mechanism assumed the relation had never been subject to early pruning. Hence, a query could fail to report "snapshot too old" when the rewrite followed an early truncation. ALTER TABLE SET TABLESPACE is probably the only rewrite mechanism capable of exposing this bug. REINDEX sets indcheckxmin, avoiding the problem. CLUSTER has zeroed page LSNs since before old_snapshot_threshold existed, so old_snapshot_threshold has never cooperated with it. ALTER TABLE ... SET DATA TYPE makes the table look empty to every past snapshot, which is strictly worse. Back-patch to v13, where commit c6b92041d38512a4176ed76ad06f713d2e6c01a8 broke this. Kyotaro Horiguchi and Noah Misch Discussion: https://postgr.es/m/20210113.160705.2225256954956139776.horikyota.ntt@gmail.com --- src/backend/utils/time/snapmgr.c | 6 +++++- src/include/utils/snapmgr.h | 2 +- 2 files changed, 6 insertions(+), 2 deletions(-) diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index ae16c3ed7d6..95704265b67 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -1764,7 +1764,11 @@ TransactionIdLimitedForOldSnapshots(TransactionId recentXmin, Assert(OldSnapshotThresholdActive()); Assert(limit_ts != NULL && limit_xid != NULL); - if (!RelationAllowsEarlyPruning(relation)) + /* + * TestForOldSnapshot() assumes early pruning advances the page LSN, so we + * can't prune early when skipping WAL. + */ + if (!RelationAllowsEarlyPruning(relation) || !RelationNeedsWAL(relation)) return false; ts = GetSnapshotCurrentTimestamp(); diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h index 579be352c5f..c21ee3c289c 100644 --- a/src/include/utils/snapmgr.h +++ b/src/include/utils/snapmgr.h @@ -37,7 +37,7 @@ */ #define RelationAllowsEarlyPruning(rel) \ ( \ - RelationNeedsWAL(rel) \ + (rel)->rd_rel->relpersistence == RELPERSISTENCE_PERMANENT \ && !IsCatalogRelation(rel) \ && !RelationIsAccessibleInLogicalDecoding(rel) \ )