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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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6
src/backend/access/hash/hashsearch.c
View File

@@ -71,7 +71,6 @@ _hash_next(IndexScanDesc scan, ScanDirection dir)
if (BlockNumberIsValid(blkno))
{
buf = _hash_getbuf(rel, blkno, HASH_READ, LH_OVERFLOW_PAGE);
TestForOldSnapshot(scan->xs_snapshot, rel, BufferGetPage(buf));
if (!_hash_readpage(scan, &buf, dir))
end_of_scan = true;
}
@@ -91,7 +90,6 @@ _hash_next(IndexScanDesc scan, ScanDirection dir)
{
buf = _hash_getbuf(rel, blkno, HASH_READ,
LH_BUCKET_PAGE | LH_OVERFLOW_PAGE);
TestForOldSnapshot(scan->xs_snapshot, rel, BufferGetPage(buf));
/*
* We always maintain the pin on bucket page for whole scan
@@ -186,7 +184,6 @@ _hash_readnext(IndexScanDesc scan,
if (block_found)
{
*pagep = BufferGetPage(*bufp);
TestForOldSnapshot(scan->xs_snapshot, rel, *pagep);
*opaquep = HashPageGetOpaque(*pagep);
}
}
@@ -232,7 +229,6 @@ _hash_readprev(IndexScanDesc scan,
*bufp = _hash_getbuf(rel, blkno, HASH_READ,
LH_BUCKET_PAGE | LH_OVERFLOW_PAGE);
*pagep = BufferGetPage(*bufp);
TestForOldSnapshot(scan->xs_snapshot, rel, *pagep);
*opaquep = HashPageGetOpaque(*pagep);
/*
@@ -351,7 +347,6 @@ _hash_first(IndexScanDesc scan, ScanDirection dir)
buf = _hash_getbucketbuf_from_hashkey(rel, hashkey, HASH_READ, NULL);
PredicateLockPage(rel, BufferGetBlockNumber(buf), scan->xs_snapshot);
page = BufferGetPage(buf);
TestForOldSnapshot(scan->xs_snapshot, rel, page);
opaque = HashPageGetOpaque(page);
bucket = opaque->hasho_bucket;
@@ -387,7 +382,6 @@ _hash_first(IndexScanDesc scan, ScanDirection dir)
LockBuffer(buf, BUFFER_LOCK_UNLOCK);
old_buf = _hash_getbuf(rel, old_blkno, HASH_READ, LH_BUCKET_PAGE);
TestForOldSnapshot(scan->xs_snapshot, rel, BufferGetPage(old_buf));
/*
* remember the split bucket buffer so as to use it later for