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Document WAL rules related to PD_ALL_VISIBLE in README.

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Also improve comments.

Discussion: https://postgr.es/m/a50005c1c537f89bb359057fd70e66bb83bce969.camel@j-davis.com
Reviewed-by: Peter Geoghegan
This commit is contained in:
Jeff Davis
2022-11-12 08:37:50 -08:00
parent d6a3dbe14f
commit 97c61f70d1
3 changed files with 32 additions and 8 deletions
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17
src/backend/access/heap/visibilitymap.c
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@@ -223,13 +223,13 @@ visibilitymap_pin_ok(BlockNumber heapBlk, Buffer vmbuf)
* visibilitymap_set - set bit(s) on a previously pinned page
*
* recptr is the LSN of the XLOG record we're replaying, if we're in recovery,
* or InvalidXLogRecPtr in normal running. The page LSN is advanced to the
* or InvalidXLogRecPtr in normal running. The VM page LSN is advanced to the
* one provided; in normal running, we generate a new XLOG record and set the
* page LSN to that value. cutoff_xid is the largest xmin on the page being
* marked all-visible; it is needed for Hot Standby, and can be
* InvalidTransactionId if the page contains no tuples. It can also be set
* to InvalidTransactionId when a page that is already all-visible is being
* marked all-frozen.
* page LSN to that value (though the heap page's LSN may *not* be updated;
* see below). cutoff_xid is the largest xmin on the page being marked
* all-visible; it is needed for Hot Standby, and can be InvalidTransactionId
* if the page contains no tuples. It can also be set to InvalidTransactionId
* when a page that is already all-visible is being marked all-frozen.
*
* Caller is expected to set the heap page's PD_ALL_VISIBLE bit before calling
* this function. Except in recovery, caller should also pass the heap
@@ -289,6 +289,11 @@ visibilitymap_set(Relation rel, BlockNumber heapBlk, Buffer heapBuf,
/*
* If data checksums are enabled (or wal_log_hints=on), we
* need to protect the heap page from being torn.
*
* If not, then we must *not* update the heap page's LSN. In
* this case, the FPI for the heap page was omitted from the
* WAL record inserted above, so it would be incorrect to
* update the heap page's LSN.
*/
if (XLogHintBitIsNeeded())
{