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Update visibilitymap.c header comments.

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Recent work on index-only scans left this somewhat out of date.
This commit is contained in:
Robert Haas 2011-10-29 14:45:39 -04:00
parent 7609239f3e
commit fae54e4a16
1 changed files with 17 additions and 16 deletions
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33
src/backend/access/heap/visibilitymap.c
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@ -27,22 +27,28 @@
* the sense that we make sure that whenever a bit is set, we know the
* condition is true, but if a bit is not set, it might or might not be true.
*
* There's no explicit WAL logging in the functions in this file. The callers
* Clearing a visibility map bit is not separately WAL-logged. The callers
* must make sure that whenever a bit is cleared, the bit is cleared on WAL
* replay of the updating operation as well. Setting bits during recovery
* isn't necessary for correctness.
* replay of the updating operation as well.
*
* Currently, the visibility map is only used as a hint, to speed up VACUUM.
* A corrupted visibility map won't cause data corruption, although it can
* make VACUUM skip pages that need vacuuming, until the next anti-wraparound
* vacuum. The visibility map is not used for anti-wraparound vacuums, because
* When we *set* a visibility map during VACUUM, we must write WAL. This may
* seem counterintuitive, since the bit is basically a hint: if it is clear,
* it may still be the case that every tuple on the page is visible to all
* transactions; we just don't know that for certain. The difficulty is that
* there are two bits which are typically set together: the PD_ALL_VISIBLE bit
* on the page itself, and the visibility map bit. If a crash occurs after the
* visibility map page makes it to disk and before the updated heap page makes
* it to disk, redo must set the bit on the heap page. Otherwise, the next
* insert, update, or delete on the heap page will fail to realize that the
* visibility map bit must be cleared, possibly causing index-only scans to
* return wrong answers.
*
* VACUUM will normally skip pages for which the visibility map bit is set;
* such pages can't contain any dead tuples and therefore don't need vacuuming.
* The visibility map is not used for anti-wraparound vacuums, because
* an anti-wraparound vacuum needs to freeze tuples and observe the latest xid
* present in the table, even on pages that don't have any dead tuples.
*
* Although the visibility map is just a hint at the moment, the PD_ALL_VISIBLE
* flag on heap pages *must* be correct, because it is used to skip visibility
* checking.
*
* LOCKING
*
* In heapam.c, whenever a page is modified so that not all tuples on the
@ -72,11 +78,6 @@
* But when a bit is cleared, we don't have to do that because it's always
* safe to clear a bit in the map from correctness point of view.
*
* TODO
*
* It would be nice to use the visibility map to skip visibility checks in
* index scans.
*
*-------------------------------------------------------------------------
*/
#include "postgres.h"