Remove the "snapshot too old" feature.

Remove the old_snapshot_threshold setting and mechanism for producing
the error "snapshot too old", originally added by commit 848ef42b.
Unfortunately it had a number of known problems in terms of correctness
and performance, mostly reported by Andres in the course of his work on
snapshot scalability.  We agreed to remove it, after a long period
without an active plan to fix it.

This is certainly a desirable feature, and someone might propose a new
or improved implementation in the future.

Reported-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CACG%3DezYV%2BEvO135fLRdVn-ZusfVsTY6cH1OZqWtezuEYH6ciQA%40mail.gmail.com
Discussion: https://postgr.es/m/20200401064008.qob7bfnnbu4w5cw4%40alap3.anarazel.de
Discussion: https://postgr.es/m/CA%2BTgmoY%3Daqf0zjTD%2B3dUWYkgMiNDegDLFjo%2B6ze%3DWtpik%2B3XqA%40mail.gmail.com

1 files changed, 0 insertions, 49 deletions

diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h
index 980d37a1947..4dc94e7ec98 100644
--- a/src/include/utils/snapmgr.h
+++ b/src/include/utils/snapmgr.h
@@ -19,40 +19,6 @@
 #include "utils/snapshot.h"
 
 
-/*
- * The structure used to map times to TransactionId values for the "snapshot
- * too old" feature must have a few entries at the tail to hold old values;
- * otherwise the lookup will often fail and the expected early pruning or
- * vacuum will not usually occur.  It is best if this padding is for a number
- * of minutes greater than a thread would normally be stalled, but it's OK if
- * early vacuum opportunities are occasionally missed, so there's no need to
- * use an extreme value or get too fancy.  10 minutes seems plenty.
- */
-#define OLD_SNAPSHOT_PADDING_ENTRIES 10
-#define OLD_SNAPSHOT_TIME_MAP_ENTRIES (old_snapshot_threshold + OLD_SNAPSHOT_PADDING_ENTRIES)
-
-/*
- * Common definition of relation properties that allow early pruning/vacuuming
- * when old_snapshot_threshold >= 0.
- */
-#define RelationAllowsEarlyPruning(rel) \
-( \
-	 RelationIsPermanent(rel) && !IsCatalogRelation(rel) \
-  && !RelationIsAccessibleInLogicalDecoding(rel) \
-)
-
-#define EarlyPruningEnabled(rel) (old_snapshot_threshold >= 0 && RelationAllowsEarlyPruning(rel))
-
-/* GUC variables */
-extern PGDLLIMPORT int old_snapshot_threshold;
-
-
-extern Size SnapMgrShmemSize(void);
-extern void SnapMgrInit(void);
-extern TimestampTz GetSnapshotCurrentTimestamp(void);
-extern TimestampTz GetOldSnapshotThresholdTimestamp(void);
-extern void SnapshotTooOldMagicForTest(void);
-
 extern PGDLLIMPORT bool FirstSnapshotSet;
 
 extern PGDLLIMPORT TransactionId TransactionXmin;
@@ -97,14 +63,6 @@ extern PGDLLIMPORT SnapshotData CatalogSnapshotData;
 	((snapshot)->snapshot_type == SNAPSHOT_MVCC || \
 	 (snapshot)->snapshot_type == SNAPSHOT_HISTORIC_MVCC)
 
-#ifndef FRONTEND
-static inline bool
-OldSnapshotThresholdActive(void)
-{
-	return old_snapshot_threshold >= 0;
-}
-#endif
-
 extern Snapshot GetTransactionSnapshot(void);
 extern Snapshot GetLatestSnapshot(void);
 extern void SnapshotSetCommandId(CommandId curcid);
@@ -138,13 +96,6 @@ extern void DeleteAllExportedSnapshotFiles(void);
 extern void WaitForOlderSnapshots(TransactionId limitXmin, bool progress);
 extern bool ThereAreNoPriorRegisteredSnapshots(void);
 extern bool HaveRegisteredOrActiveSnapshot(void);
-extern bool TransactionIdLimitedForOldSnapshots(TransactionId recentXmin,
-												Relation relation,
-												TransactionId *limit_xid,
-												TimestampTz *limit_ts);
-extern void SetOldSnapshotThresholdTimestamp(TimestampTz ts, TransactionId xlimit);
-extern void MaintainOldSnapshotTimeMapping(TimestampTz whenTaken,
-										   TransactionId xmin);
 
 extern char *ExportSnapshot(Snapshot snapshot);