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Commit3f44959

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Avoid unneeded nbtree backwards scan buffer locks.
Teach nbtree backwards scans to avoid relocking a just-read leaf page toread its current left sibling link when it isn't truly necessary. Thishappened inside _bt_readnextpage whenever _bt_readpage had alreadydetermined that there'll be no further matches to the left (or at leastnone for the current primitive index scan, for a scan with array keys).A new precheck inside _bt_readnextpage is all that we need to avoidthese useless lock acquisitions. Arguably, using a precheck like thiswas a missed opportunity for commit2ed5b87, which taught nbtree todrop leaf page pins early to avoid blocking cleanup by VACUUM. Forwardsscans already managed to avoid relocking the page like this.The optimization added by this commit is particularly helpful withbackwards scans that use array keys where the scan must perform multipleprimitive index scans. Such backwards scans will now avoid a uselessleaf page re-lock at the end of each primitive index scan.Note that this commit does not attempt to avoid needlessly re-locking aleaf page that was just read when the scan must follow the leaf page'sleft link. That more ambitious optimization could work by stashing theleft link when the page is first read by a backwards scan, allowing thesubsequent _bt_readnextpage call to optimistically skip re-reading theoriginal page just to get a new copy of its left link. For now we onlyaddress cases where we don't care about our original page's left link.Author: Peter Geoghegan <pg@bowt.ie>Reviewed-By: Matthias van de Meent <boekewurm+postgres@gmail.com>Discussion:https://postgr.es/m/CAH2-Wz=xgs7PojG=EUvhgadwENzu_mY_riNh-w9wFPsaS717ew@mail.gmail.com
1 parentf011e82 commit3f44959

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‎src/backend/access/nbtree/nbtsearch.c‎

Lines changed: 18 additions & 5 deletions
Original file line numberDiff line numberDiff line change
@@ -1916,16 +1916,20 @@ _bt_readpage(IndexScanDesc scan, ScanDirection dir, OffsetNumber offnum,
19161916
}
19171917
}
19181918
}
1919+
/* When !continuescan, there can't be any more matches, so stop */
19191920
if (!pstate.continuescan)
1920-
{
1921-
/* there can't be any more matches, so stop */
1922-
so->currPos.moreLeft= false;
19231921
break;
1924-
}
19251922

19261923
offnum=OffsetNumberPrev(offnum);
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}
19281925

1926+
/*
1927+
* We don't need to visit page to the left when no more matches will
1928+
* be found there
1929+
*/
1930+
if (!pstate.continuescan||P_LEFTMOST(opaque))
1931+
so->currPos.moreLeft= false;
1932+
19291933
Assert(itemIndex >=0);
19301934
so->currPos.firstItem=itemIndex;
19311935
so->currPos.lastItem=MaxTIDsPerBTreePage-1;
@@ -2240,6 +2244,15 @@ _bt_readnextpage(IndexScanDesc scan, BlockNumber blkno, ScanDirection dir)
22402244
so->currPos.currPage=blkno;
22412245
}
22422246

2247+
/* Done if we know that the left sibling link isn't of interest */
2248+
if (!so->currPos.moreLeft)
2249+
{
2250+
BTScanPosUnpinIfPinned(so->currPos);
2251+
_bt_parallel_done(scan);
2252+
BTScanPosInvalidate(so->currPos);
2253+
return false;
2254+
}
2255+
22432256
/*
22442257
* Walk left to the next page with data. This is much more complex
22452258
* than the walk-right case because of the possibility that the page
@@ -2260,7 +2273,7 @@ _bt_readnextpage(IndexScanDesc scan, BlockNumber blkno, ScanDirection dir)
22602273

22612274
for (;;)
22622275
{
2263-
/* Done if we knowthere are no matching keys to the left */
2276+
/* Done if we knowthat the left sibling link isn't of interest */
22642277
if (!so->currPos.moreLeft)
22652278
{
22662279
_bt_relbuf(rel,so->currPos.buf);

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