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Fix hash index vs "snapshot too old" problemms

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Hash indexes are not WAL-logged, and so do not maintain the LSN of
index pages.  Since the "snapshot too old" feature counts on
detecting error conditions using the LSN of a table and all indexes
on it, this makes it impossible to safely do early vacuuming on any
table with a hash index, so add this to the tests for whether the
xid used to vacuum a table can be adjusted based on
old_snapshot_threshold.

While at it, add a paragraph to the docs for old_snapshot_threshold
which specifically mentions this and other aspects of the feature
which may otherwise surprise users.

Problem reported and patch reviewed by Amit Kapila
This commit is contained in:
Kevin Grittner
2016-05-06 07:47:12 -05:00
parent 9b66aa006f
commit 2cc41acd8f
6 changed files with 62 additions and 6 deletions
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1
src/backend/access/hash/hash.c
View File

@@ -279,7 +279,6 @@ hashgettuple(IndexScanDesc scan, ScanDirection dir)
buf = so->hashso_curbuf;
Assert(BufferIsValid(buf));
page = BufferGetPage(buf);
TestForOldSnapshot(scan->xs_snapshot, rel, page);
maxoffnum = PageGetMaxOffsetNumber(page);
for (offnum = ItemPointerGetOffsetNumber(current);
offnum <= maxoffnum;

4
src/backend/access/hash/hashsearch.c
View File

@@ -189,7 +189,6 @@ _hash_first(IndexScanDesc scan, ScanDirection dir)
/* Read the metapage */
metabuf = _hash_getbuf(rel, HASH_METAPAGE, HASH_READ, LH_META_PAGE);
page = BufferGetPage(metabuf);
TestForOldSnapshot(scan->xs_snapshot, rel, page);
metap = HashPageGetMeta(page);
/*
@@ -243,7 +242,6 @@ _hash_first(IndexScanDesc scan, ScanDirection dir)
/* Fetch the primary bucket page for the bucket */
buf = _hash_getbuf(rel, blkno, HASH_READ, LH_BUCKET_PAGE);
page = BufferGetPage(buf);
TestForOldSnapshot(scan->xs_snapshot, rel, page);
opaque = (HashPageOpaque) PageGetSpecialPointer(page);
Assert(opaque->hasho_bucket == bucket);
@@ -350,7 +348,6 @@ _hash_step(IndexScanDesc scan, Buffer *bufP, ScanDirection dir)
_hash_readnext(rel, &buf, &page, &opaque);
if (BufferIsValid(buf))
{
TestForOldSnapshot(scan->xs_snapshot, rel, page);
maxoff = PageGetMaxOffsetNumber(page);
offnum = _hash_binsearch(page, so->hashso_sk_hash);
}
@@ -392,7 +389,6 @@ _hash_step(IndexScanDesc scan, Buffer *bufP, ScanDirection dir)
_hash_readprev(rel, &buf, &page, &opaque);
if (BufferIsValid(buf))
{
TestForOldSnapshot(scan->xs_snapshot, rel, page);
maxoff = PageGetMaxOffsetNumber(page);
offnum = _hash_binsearch_last(page, so->hashso_sk_hash);
}