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Remove the "snapshot too old" feature.

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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
This commit is contained in:
Thomas Munro
2023-09-05 18:26:12 +12:00
parent aa0d350456
commit f691f5b80a
50 changed files with 21 additions and 1425 deletions
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28
src/backend/catalog/index.c
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@@ -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;