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Update comment in heapgetpage() regarding PD_ALL_VISIBLE vs. Hot Standby.

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Pavan Deolasee, slightly modified by me
This commit is contained in:
Robert Haas 2012-12-14 15:44:38 -05:00
parent fdb67eb2b6
commit 75758a6ff0
1 changed files with 16 additions and 3 deletions
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19
src/backend/access/heap/heapam.c
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@ -260,10 +260,23 @@ heapgetpage(HeapScanDesc scan, BlockNumber page)
/*
* If the all-visible flag indicates that all tuples on the page are
* visible to everyone, we can skip the per-tuple visibility tests. But
* not in hot standby mode. A tuple that's already visible to all
* visible to everyone, we can skip the per-tuple visibility tests.
*
* Note: In hot standby, a tuple that's already visible to all
* transactions in the master might still be invisible to a read-only
* transaction in the standby.
* transaction in the standby. We partly handle this problem by tracking
* the minimum xmin of visible tuples as the cut-off XID while marking a
* page all-visible on master and WAL log that along with the visibility
* map SET operation. In hot standby, we wait for (or abort) all
* transactions that can potentially may not see one or more tuples on the
* page. That's how index-only scans work fine in hot standby. A crucial
* difference between index-only scans and heap scans is that the
* index-only scan completely relies on the visibility map where as heap
* scan looks at the page-level PD_ALL_VISIBLE flag. We are not sure if the
* page-level flag can be trusted in the same way, because it might get
* propagated somehow without being explicitly WAL-logged, e.g. via a full
* page write. Until we can prove that beyond doubt, let's check each
* tuple for visibility the hard way.
*/
all_visible = PageIsAllVisible(dp) && !snapshot->takenDuringRecovery;