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Fix and improve SnapshotType comments.

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The comment for SNAPSHOT_SELF was unfortunately explaining
SNAPSHOT_DIRTY, as reported by Sergei. Also expand a few comments, and
include a few more comments from heapam_visibility.c, so they're in an
AM independent place.

Reported-By: Sergei Kornilov
Author: Andres Freund
Discussion: https://postgr.es/m/9152241558192351@sas1-d856b3d759c7.qloud-c.yandex.net
This commit is contained in:
Andres Freund 2019-05-19 16:17:18 -07:00
parent ae35e1c9d7
commit 78d6a9cbd3
1 changed files with 24 additions and 5 deletions
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29
src/include/utils/snapshot.h
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@ -50,13 +50,15 @@ typedef enum SnapshotType
SNAPSHOT_MVCC = 0,
/*-------------------------------------------------------------------------
* A tuple is visible iff the tuple is valid including effects of open
* transactions.
* A tuple is visible iff the tuple is valid "for itself".
*
* Here, we consider the effects of:
* - all committed and in-progress transactions (as of the current instant)
* - all committed transactions (as of the current instant)
* - previous commands of this transaction
* - changes made by the current command
*
* Does _not_ include:
* - in-progress transactions (as of the current instant)
* -------------------------------------------------------------------------
*/
SNAPSHOT_SELF,
@ -79,6 +81,22 @@ typedef enum SnapshotType
* - all committed and in-progress transactions (as of the current instant)
* - previous commands of this transaction
* - changes made by the current command
*
* This is essentially like SNAPSHOT_SELF as far as effects of the current
* transaction and committed/aborted xacts are concerned. However, it
* also includes the effects of other xacts still in progress.
*
* A special hack is that when a snapshot of this type is used to
* determine tuple visibility, the passed-in snapshot struct is used as an
* output argument to return the xids of concurrent xacts that affected
* the tuple. snapshot->xmin is set to the tuple's xmin if that is
* another transaction that's still in progress; or to
* InvalidTransactionId if the tuple's xmin is committed good, committed
* dead, or my own xact. Similarly for snapshot->xmax and the tuple's
* xmax. If the tuple was inserted speculatively, meaning that the
* inserter might still back down on the insertion without aborting the
* whole transaction, the associated token is also returned in
* snapshot->speculativeToken. See also InitDirtySnapshot().
* -------------------------------------------------------------------------
*/
SNAPSHOT_DIRTY,
@ -92,9 +110,10 @@ typedef enum SnapshotType
/*
* A tuple is visible iff the tuple might be visible to some transaction;
* false if it's surely dead to everyone, ie, vacuumable.
* false if it's surely dead to everyone, i.e., vacuumable.
*
* Snapshot.xmin must have been set up with the xmin horizon to use.
* For visibility checks snapshot->min must have been set up with the xmin
* horizon to use.
*/
SNAPSHOT_NON_VACUUMABLE
} SnapshotType;