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Fix GIN data page split ratio calculation.

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The code that tried to split a page at 75/25 ratio, when appending to the
end of an index, was buggy in two ways. First, there was a silly typo that
caused it to just fill the left page as full as possible. But the logic as
it was intended wasn't correct either, and would actually have given a ratio
closer to 60/40 than 75/25.

Gaetano Mendola spotted the typo. Backpatch to 9.4, where this code was added.
This commit is contained in:
Heikki Linnakangas 2014-09-12 11:23:59 +03:00
parent 1d352325b8
commit 774a78ffe4
1 changed files with 17 additions and 12 deletions
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29
src/backend/access/gin/gindatapage.c
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@ -621,9 +621,9 @@ dataPlaceToPageLeaf(GinBtree btree, Buffer buf, GinBtreeStack *stack,
/*
* Had to split.
*
* We already divided the segments between the left and the right
* page. The left page was filled as full as possible, and the rest
* overflowed to the right page. When building a new index, that's
* leafRepackItems already divided the segments between the left and
* the right page. It filled the left page as full as possible, and
* put the rest to the right page. When building a new index, that's
* good, because the table is scanned from beginning to end and there
* won't be any more insertions to the left page during the build.
* This packs the index as tight as possible. But otherwise, split
@ -631,9 +631,10 @@ dataPlaceToPageLeaf(GinBtree btree, Buffer buf, GinBtreeStack *stack,
* until they're balanced.
*
* As a further heuristic, when appending items to the end of the
* page, split 75/25, one the assumption that subsequent insertions
* will probably also go to the end. This packs the index somewhat
* tighter when appending to a table, which is very common.
* page, try make the left page 75% full, one the assumption that
* subsequent insertions will probably also go to the end. This packs
* the index somewhat tighter when appending to a table, which is very
* common.
*/
if (!btree->isBuild)
{
@ -645,14 +646,18 @@ dataPlaceToPageLeaf(GinBtree btree, Buffer buf, GinBtreeStack *stack,
if (lastleftinfo->action != GIN_SEGMENT_DELETE)
{
segsize = SizeOfGinPostingList(lastleftinfo->seg);
/*
* Note that we check that the right page doesn't become
* more full than the left page even when appending. It's
* possible that we added enough items to make both pages
* more than 75% full.
*/
if ((leaf->lsize - segsize) - (leaf->rsize + segsize) < 0)
break;
if (append)
{
if ((leaf->lsize - segsize) - (leaf->lsize - segsize) < BLCKSZ / 4)
break;
}
else
{
if ((leaf->lsize - segsize) - (leaf->rsize + segsize) < 0)
if ((leaf->lsize - segsize) < (BLCKSZ * 3) / 4)
break;
}