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Commit9b1f8af

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Fix hypothetical bug in heap backward scans
Both heapgettup() and heapgettup_pagemode() incorrectly set the first pageto scan in a backward scan in which the number of pages to scan wasspecified by heap_setscanlimits(). The code incorrectly started the scanat the end of the relation when startBlk was 0, or otherwise atstartBlk - 1, neither of which is correct when only scanning a subset ofpages.The fix here checks if heap_setscanlimits() has changed the number ofpages to scan and if so we set the first page to scan as the final page inthe specified range during backward scans.Proper adjustment of this code was forgotten when heap_setscanlimits() wasadded in7516f52 back in 9.5. However, practice, nowhere in core codeperforms backward scans after having used heap_setscanlimits(), yet, it ispossible an extension uses the heap functions in this way, hencebackpatch.An upcoming patch does use heap_setscanlimits() with backward scans, sothis must be fixed before that can go in.Author: David RowleyDiscussion:https://postgr.es/m/CAApHDvpGc9h0_oVD2CtgBcxCS1N-qDYZSeBRnUh+0CWJA9cMaA@mail.gmail.comBackpatch-through: 9.5, all supported versions
1 parentc26a086 commit9b1f8af

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‎src/backend/access/heap/heapam.c

Lines changed: 16 additions & 4 deletions
Original file line numberDiff line numberDiff line change
@@ -538,8 +538,14 @@ heapgettup(HeapScanDesc scan,
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* forward scanners.
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*/
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scan->rs_syncscan= false;
541-
/* start from last page of the scan */
542-
if (scan->rs_startblock>0)
541+
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/*
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* Start from last page of the scan. Ensure we take into account
544+
* rs_numblocks if it's been adjusted by heap_setscanlimits().
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*/
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if (scan->rs_numblocks!=InvalidBlockNumber)
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page= (scan->rs_startblock+scan->rs_numblocks-1) %scan->rs_nblocks;
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elseif (scan->rs_startblock>0)
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page=scan->rs_startblock-1;
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else
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page=scan->rs_nblocks-1;
@@ -812,8 +818,14 @@ heapgettup_pagemode(HeapScanDesc scan,
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* forward scanners.
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*/
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scan->rs_syncscan= false;
815-
/* start from last page of the scan */
816-
if (scan->rs_startblock>0)
821+
822+
/*
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* Start from last page of the scan. Ensure we take into account
824+
* rs_numblocks if it's been adjusted by heap_setscanlimits().
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*/
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if (scan->rs_numblocks!=InvalidBlockNumber)
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page= (scan->rs_startblock+scan->rs_numblocks-1) %scan->rs_nblocks;
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elseif (scan->rs_startblock>0)
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page=scan->rs_startblock-1;
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else
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page=scan->rs_nblocks-1;

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