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Commit236d569

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Fix btree mark/restore bug.
Commit2ed5b87 introduced a bug inmark/restore, in an attempt to optimize repeated restores to thesame page. This caused an assertion failure during a merge joinwhich fed directly from an index scan, although the impact wouldnot be limited to that case. Revert the bad chunk of code fromthat commit.While investigating this bug it was discovered that a particular"paranoia" set of the mark position field would not prevent badbehavior; it would just make it harder to diagnose. Change thatinto an assertion, which will draw attention to any future problemin that area more directly.Backpatch to 9.5, where the bug was introduced.Bug #14169 reported by Shinta Koyanagi.Preliminary analysis by Tom Lane identified which commit causedthe bug.
1 parent43d3fbe commit236d569

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2 files changed

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‎src/backend/access/nbtree/nbtree.c

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Original file line numberDiff line numberDiff line change
@@ -592,25 +592,6 @@ btrestrpos(PG_FUNCTION_ARGS)
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*/
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so->currPos.itemIndex=so->markItemIndex;
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}
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elseif (so->currPos.currPage==so->markPos.currPage)
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{
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/*
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* so->markItemIndex < 0 but mark and current positions are on the
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* same page. This would be an unusual case, where the scan moved to
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* a new index page after the mark, restored, and later restored again
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* without moving off the marked page. It is not clear that this code
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* can currently be reached, but it seems better to make this function
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* robust for this case than to Assert() or elog() that it can't
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* happen.
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*
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* We neither want to set so->markItemIndex >= 0 (because that could
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* cause a later move to a new page to redo the memcpy() executions)
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* nor re-execute the memcpy() functions for a restore within the same
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* page. The previous restore to this page already set everything
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* except markPos as it should be.
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*/
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so->currPos.itemIndex=so->markPos.itemIndex;
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}
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else
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{
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/*

‎src/backend/access/nbtree/nbtsearch.c

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -1000,7 +1000,7 @@ _bt_first(IndexScanDesc scan, ScanDirection dir)
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so->currPos.moreRight= false;
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}
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so->numKilled=0;/* just paranoia */
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so->markItemIndex=-1;/* ditto */
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Assert(so->markItemIndex==-1);
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/* position to the precise item on the page */
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offnum=_bt_binsrch(rel,buf,keysCount,scankeys,nextkey);

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