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 <[email protected]>
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, 10 insertions, 18 deletions

diff --git a/src/backend/catalog/index.c b/src/backend/catalog/index.c
index fd09378848e..72f476b51de 100644
--- a/src/backend/catalog/index.c
+++ b/src/backend/catalog/index.c
@@ -3048,12 +3048,11 @@ index_build(Relation heapRelation,
 	/*
 	 * If we found any potentially broken HOT chains, mark the index as not
 	 * being usable until the current transaction is below the event horizon.
-	 * See src/backend/access/heap/README.HOT for discussion.  Also set this
-	 * if early pruning/vacuuming is enabled for the heap relation.  While it
-	 * might become safe to use the index earlier based on actual cleanup
-	 * activity and other active transactions, the test for that would be much
-	 * more complex and would require some form of blocking, so keep it simple
-	 * and fast by just using the current transaction.
+	 * See src/backend/access/heap/README.HOT for discussion.  While it might
+	 * become safe to use the index earlier based on actual cleanup activity
+	 * and other active transactions, the test for that would be much more
+	 * complex and would require some form of blocking, so keep it simple and
+	 * fast by just using the current transaction.
 	 *
 	 * However, when reindexing an existing index, we should do nothing here.
 	 * Any HOT chains that are broken with respect to the index must predate
@@ -3065,7 +3064,7 @@ index_build(Relation heapRelation,
 	 *
 	 * We also need not set indcheckxmin during a concurrent index build,
 	 * because we won't set indisvalid true until all transactions that care
-	 * about the broken HOT chains or early pruning/vacuuming are gone.
+	 * about the broken HOT chains are gone.
 	 *
 	 * Therefore, this code path can only be taken during non-concurrent
 	 * CREATE INDEX.  Thus the fact that heap_update will set the pg_index
@@ -3074,7 +3073,7 @@ index_build(Relation heapRelation,
 	 * about any concurrent readers of the tuple; no other transaction can see
 	 * it yet.
 	 */
-	if ((indexInfo->ii_BrokenHotChain || EarlyPruningEnabled(heapRelation)) &&
+	if (indexInfo->ii_BrokenHotChain &&
 		!isreindex &&
 		!indexInfo->ii_Concurrent)
 	{
@@ -3759,11 +3758,6 @@ reindex_index(Oid indexId, bool skip_constraint_checks, char persistence,
 	 * reindexing pg_index itself, we must not try to update tuples in it.
 	 * pg_index's indexes should always have these flags in their clean state,
 	 * so that won't happen.
-	 *
-	 * If early pruning/vacuuming is enabled for the heap relation, the
-	 * usability horizon must be advanced to the current transaction on every
-	 * build or rebuild.  pg_index is OK in this regard because catalog tables
-	 * are not subject to early cleanup.
 	 */
 	if (!skipped_constraint)
 	{
@@ -3771,7 +3765,6 @@ reindex_index(Oid indexId, bool skip_constraint_checks, char persistence,
 		HeapTuple	indexTuple;
 		Form_pg_index indexForm;
 		bool		index_bad;
-		bool		early_pruning_enabled = EarlyPruningEnabled(heapRelation);
 
 		pg_index = table_open(IndexRelationId, RowExclusiveLock);
 
@@ -3785,12 +3778,11 @@ reindex_index(Oid indexId, bool skip_constraint_checks, char persistence,
 					 !indexForm->indisready ||
 					 !indexForm->indislive);
 		if (index_bad ||
-			(indexForm->indcheckxmin && !indexInfo->ii_BrokenHotChain) ||
-			early_pruning_enabled)
+			(indexForm->indcheckxmin && !indexInfo->ii_BrokenHotChain))
 		{
-			if (!indexInfo->ii_BrokenHotChain && !early_pruning_enabled)
+			if (!indexInfo->ii_BrokenHotChain)
 				indexForm->indcheckxmin = false;
-			else if (index_bad || early_pruning_enabled)
+			else if (index_bad)
 				indexForm->indcheckxmin = true;
 			indexForm->indisvalid = true;
 			indexForm->indisready = true;