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Commit774a78f

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Fix GIN data page split ratio calculation.
The code that tried to split a page at 75/25 ratio, when appending to theend of an index, was buggy in two ways. First, there was a silly typo thatcaused it to just fill the left page as full as possible. But the logic asit was intended wasn't correct either, and would actually have given a ratiocloser to 60/40 than 75/25.Gaetano Mendola spotted the typo. Backpatch to 9.4, where this code was added.
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‎src/backend/access/gin/gindatapage.c

Lines changed: 17 additions & 12 deletions
Original file line numberDiff line numberDiff line change
@@ -621,19 +621,20 @@ dataPlaceToPageLeaf(GinBtree btree, Buffer buf, GinBtreeStack *stack,
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/*
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* Had to split.
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*
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*We already divided the segments between the left and the right
625-
* page.Theleft pagewas filledas full as possible, and the rest
626-
*overflowed to the right page. When building a new index, that's
624+
*leafRepackItems already divided the segments between the left and
625+
*the rightpage.It filled theleft page as full as possible, and
626+
*put the rest to the right page. When building a new index, that's
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* good, because the table is scanned from beginning to end and there
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* won't be any more insertions to the left page during the build.
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* This packs the index as tight as possible. But otherwise, split
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* 50/50, by moving segments from the left page to the right page
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* until they're balanced.
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*
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* As a further heuristic, when appending items to the end of the
634-
* page, split 75/25, one the assumption that subsequent insertions
635-
* will probably also go to the end. This packs the index somewhat
636-
* tighter when appending to a table, which is very common.
634+
* page, try make the left page 75% full, one the assumption that
635+
* subsequent insertions will probably also go to the end. This packs
636+
* the index somewhat tighter when appending to a table, which is very
637+
* common.
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*/
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if (!btree->isBuild)
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{
@@ -645,14 +646,18 @@ dataPlaceToPageLeaf(GinBtree btree, Buffer buf, GinBtreeStack *stack,
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if (lastleftinfo->action!=GIN_SEGMENT_DELETE)
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{
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segsize=SizeOfGinPostingList(lastleftinfo->seg);
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/*
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* Note that we check that the right page doesn't become
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* more full than the left page even when appending. It's
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* possible that we added enough items to make both pages
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* more than 75% full.
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*/
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if ((leaf->lsize-segsize)- (leaf->rsize+segsize)<0)
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break;
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if (append)
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{
650-
if ((leaf->lsize-segsize)- (leaf->lsize-segsize)<BLCKSZ /4)
651-
break;
652-
}
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else
654-
{
655-
if ((leaf->lsize-segsize)- (leaf->rsize+segsize)<0)
660+
if ((leaf->lsize-segsize)< (BLCKSZ*3) /4)
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break;
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}
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