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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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19
src/backend/commands/vacuum.c
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@ -1110,25 +1110,6 @@ vacuum_get_cutoffs(Relation rel, const VacuumParams *params,
*/
cutoffs->OldestXmin = GetOldestNonRemovableTransactionId(rel);
if (OldSnapshotThresholdActive())
{
TransactionId limit_xmin;
TimestampTz limit_ts;
if (TransactionIdLimitedForOldSnapshots(cutoffs->OldestXmin, rel,
&limit_xmin, &limit_ts))
{
/*
* TODO: We should only set the threshold if we are pruning on the
* basis of the increased limits. Not as crucial here as it is
* for opportunistic pruning (which often happens at a much higher
* frequency), but would still be a significant improvement.
*/
SetOldSnapshotThresholdTimestamp(limit_ts, limit_xmin);
cutoffs->OldestXmin = limit_xmin;
}
}
Assert(TransactionIdIsNormal(cutoffs->OldestXmin));
/* Acquire OldestMxact */