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Truncate line pointer array during VACUUM.

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Teach VACUUM to truncate the line pointer array of each heap page when a
contiguous group of LP_UNUSED line pointers appear at the end of the
array -- these unused and unreferenced items are excluded.  This process
occurs during VACUUM's second pass over the heap, right after LP_DEAD
line pointers on the page (those encountered/pruned during the first
pass) are marked LP_UNUSED.

Truncation avoids line pointer bloat with certain workloads,
particularly those involving continual range DELETEs and bulk INSERTs
against the same table.

Also harden heapam code to check for an out-of-range page offset number
in places where we weren't already doing so.

Author: Matthias van de Meent <boekewurm+postgres@gmail.com>
Author: Peter Geoghegan <pg@bowt.ie>
Reviewed-By: Masahiko Sawada <sawada.mshk@gmail.com>
Reviewed-By: Peter Geoghegan <pg@bowt.ie>
Discussion: https://postgr.es/m/CAEze2WjgaQc55Y5f5CQd3L=eS5CZcff2Obxp=O6pto8-f0hC4w@mail.gmail.com
Discussion: https://postgr.es/m/CAH2-Wzn6a64PJM1Ggzm=uvx2otsopJMhFQj_g1rAj4GWr3ZSzw@mail.gmail.com
This commit is contained in:
Peter Geoghegan
2021-04-07 08:47:15 -07:00
parent 3db826bd55
commit 3c3b8a4b26
5 changed files with 144 additions and 11 deletions
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22
src/backend/access/heap/heapam.c
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@ -635,8 +635,15 @@ heapgettup(HeapScanDesc scan,
}
else
{
/*
* The previous returned tuple may have been vacuumed since the
* previous scan when we use a non-MVCC snapshot, so we must
* re-establish the lineoff <= PageGetMaxOffsetNumber(dp)
* invariant
*/
lineoff = /* previous offnum */
OffsetNumberPrev(ItemPointerGetOffsetNumber(&(tuple->t_self)));
Min(lines,
OffsetNumberPrev(ItemPointerGetOffsetNumber(&(tuple->t_self))));
}
/* page and lineoff now reference the physically previous tid */
@ -678,6 +685,13 @@ heapgettup(HeapScanDesc scan,
lpp = PageGetItemId(dp, lineoff);
for (;;)
{
/*
* Only continue scanning the page while we have lines left.
*
* Note that this protects us from accessing line pointers past
* PageGetMaxOffsetNumber(); both for forward scans when we resume the
* table scan, and for when we start scanning a new page.
*/
while (linesleft > 0)
{
if (ItemIdIsNormal(lpp))
@ -8556,10 +8570,8 @@ heap_xlog_vacuum(XLogReaderState *record)
ItemIdSetUnused(lp);
}
/*
* Update the page's hint bit about whether it has free pointers
*/
PageSetHasFreeLinePointers(page);
/* Attempt to truncate line pointer array now */
PageTruncateLinePointerArray(page);
PageSetLSN(page, lsn);
MarkBufferDirty(buffer);