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Commit08e1eed

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Fix performance problem when building a lossy tidbitmap.
As pointed out by Sergey Koposov, repeated invocations of tbm_lossify canmake building a large tidbitmap into an O(N^2) operation. To fix, makesure we remove more than the minimum amount of information per call, andadd a fallback path to behave sanely if we're unable to fit the bitmapwithin the requested amount of memory.This has been wrong since the tidbitmap code was written, so back-patchto all supported branches.
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‎src/backend/nodes/tidbitmap.c

Lines changed: 19 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -953,8 +953,11 @@ tbm_lossify(TIDBitmap *tbm)
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/*
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* XXX Really stupid implementation: this just lossifies pages in
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* essentially random order. We should be paying some attention to the
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* number of bits set in each page, instead. Also it might be a good idea
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* to lossify more than the minimum number of pages during each call.
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* number of bits set in each page, instead.
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*
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* Since we are called as soon as nentries exceeds maxentries, we should
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* push nentries down to significantly less than maxentries, or else we'll
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* just end up doing this again very soon. We shoot for maxentries/2.
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*/
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Assert(!tbm->iterating);
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Assert(tbm->status==TBM_HASH);
@@ -975,7 +978,7 @@ tbm_lossify(TIDBitmap *tbm)
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/* This does the dirty work ... */
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tbm_mark_page_lossy(tbm,page->blockno);
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if (tbm->nentries <=tbm->maxentries)
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if (tbm->nentries <=tbm->maxentries /2)
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{
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/* we have done enough */
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hash_seq_term(&status);
@@ -988,6 +991,19 @@ tbm_lossify(TIDBitmap *tbm)
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* not care whether we visit lossy chunks or not.
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*/
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}
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/*
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* With a big bitmap and small work_mem, it's possible that we cannot
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* get under maxentries. Again, if that happens, we'd end up uselessly
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* calling tbm_lossify over and over. To prevent this from becoming a
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* performance sink, force maxentries up to at least double the current
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* number of entries. (In essence, we're admitting inability to fit
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* within work_mem when we do this.) Note that this test will not fire
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* if we broke out of the loop early; and if we didn't, the current
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* number of entries is simply not reducible any further.
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*/
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if (tbm->nentries>tbm->maxentries /2)
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tbm->maxentries=Min(tbm->nentries, (INT_MAX-1) /2)*2;
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}
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/*

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