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Commit81a2579

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Clarify nbtree array exhaustion comments.
Strictly speaking, we only need to make sure to leave the scan's arraykeys in their final positions (final for the current scan direction) tohandle SAOP array exhaustion because btgettuple might only return asubset of the items for the final page (final for the current scandirection), before the scan changes direction. While it's typical forso->currPos to be invalidated shortly after the scan's arrays are firstexhausted, and while so->currPos invalidation does obviate the need toleave the scan's arrays in any particular state, we can't rely on any ofthat actually happening when handling array exhaustion. Adjust commentsto make all of that a lot clearer.Oversight in commit5bf748b, which enhanced nbtree ScalarArrayOpexecution.
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‎src/backend/access/nbtree/nbtutils.c

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Original file line numberDiff line numberDiff line change
@@ -1424,14 +1424,19 @@ _bt_advance_array_keys_increment(IndexScanDesc scan, ScanDirection dir)
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}
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/*
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* The array keys are now exhausted. (There isn't actually a distinct
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* state that represents array exhaustion, since index scans don't always
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* end after btgettuple returns "false".)
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* The array keys are now exhausted.
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*
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* Restore the array keys to the state they were in immediately before we
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* were called. This ensures that the arrays only ever ratchet in the
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* current scan direction. Without this, scans would overlook matching
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* tuples if and when the scan's direction was subsequently reversed.
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* current scan direction.
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*
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* Without this, scans could overlook matching tuples when the scan
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* direction gets reversed just before btgettuple runs out of items to
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* return, but just after _bt_readpage prepares all the items from the
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* scan's final page in so->currPos. When we're on the final page it is
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* typical for so->currPos to get invalidated once btgettuple finally
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* returns false, which'll effectively invalidate the scan's array keys.
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* That hasn't happened yet, though -- and in general it may never happen.
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*/
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_bt_start_array_keys(scan,-dir);
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