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Improve comment on why we need ctid->(cmin,cmax) mapping.

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Combocids are only part of the problem. Explain the problem in more detail.

Reviewed-by: Andres Freund
Discussion: https://www.postgresql.org/message-id/1ba2899e-77f8-7866-79e5-f3b7d1251a3e@iki.fi
This commit is contained in:
Heikki Linnakangas
2023-06-30 18:30:32 +03:00
parent 054ff3b33a
commit a0dd4c95b9
1 changed files with 9 additions and 4 deletions
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13
src/backend/replication/logical/snapbuild.c
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@ -41,10 +41,15 @@
* transactions we need Snapshots that see intermediate versions of the * transactions we need Snapshots that see intermediate versions of the
* catalog in a transaction. During normal operation this is achieved by using * catalog in a transaction. During normal operation this is achieved by using
* CommandIds/cmin/cmax. The problem with that however is that for space * CommandIds/cmin/cmax. The problem with that however is that for space
* efficiency reasons only one value of that is stored * efficiency reasons, the cmin and cmax are not included in WAL records. We
* (cf. combocid.c). Since combo CIDs are only available in memory we log * cannot read the cmin/cmax from the tuple itself, either, because it is
* additional information which allows us to get the original (cmin, cmax) * reset on crash recovery. Even if we could, we could not decode combocids
* pair during visibility checks. Check the reorderbuffer.c's comment above * which are only tracked in the original backend's memory. To work around
* that, heapam writes an extra WAL record (XLOG_HEAP2_NEW_CID) every time a
* catalog row is modified, which includes the cmin and cmax of the
* tuple. During decoding, we insert the ctid->(cmin,cmax) mappings into the
* reorder buffer, and use them at visibility checks instead of the cmin/cmax
* on the tuple itself. Check the reorderbuffer.c's comment above
* ResolveCminCmaxDuringDecoding() for details. * ResolveCminCmaxDuringDecoding() for details.
* *
* To facilitate all this we need our own visibility routine, as the normal * To facilitate all this we need our own visibility routine, as the normal