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Commitd781138

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Add more comments to tqual.c for visibility functions.
1 parentcf97080 commitd781138

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1 file changed

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‎src/backend/utils/time/tqual.c

Lines changed: 48 additions & 21 deletions
Original file line numberDiff line numberDiff line change
@@ -16,7 +16,7 @@
1616
* Portions Copyright (c) 1994, Regents of the University of California
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*
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* IDENTIFICATION
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* $Header: /cvsroot/pgsql/src/backend/utils/time/tqual.c,v 1.47 2002/01/1620:29:02 tgl Exp $
19+
* $Header: /cvsroot/pgsql/src/backend/utils/time/tqual.c,v 1.48 2002/01/1623:09:09 momjian Exp $
2020
*
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*-------------------------------------------------------------------------
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*/
@@ -38,9 +38,12 @@ boolReferentialIntegritySnapshotOverride = false;
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3939
/*
4040
* HeapTupleSatisfiesItself
41-
*True iff heap tuple is valid for "itself."
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*"itself" means valid as of everything that's happened
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*in the current transaction, _including_ the current command.
41+
*
42+
*Visible tuples are those of:
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*
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*transactions committed before our _command_ started (READ COMMITTED)
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*previous commands of this transaction
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*changes made by the current command
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*
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* Note:
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*Assumes heap tuple is valid.
@@ -153,10 +156,13 @@ HeapTupleSatisfiesItself(HeapTupleHeader tuple)
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/*
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* HeapTupleSatisfiesNow
156-
*True iff heap tuple is valid "now."
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*"now" means valid including everything that's happened
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* in the current transaction _up to, but not including,_
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* the current command.
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*
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*Visible tuples are those of:
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*
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*transactions committed before our _command_ started (READ COMMITTED)
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*previous commands of this transaction
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*
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*Does _not_ include changes made by the current command
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*
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* Note:
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*Assumes heap tuple is valid.
@@ -296,11 +302,12 @@ HeapTupleSatisfiesNow(HeapTupleHeader tuple)
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297303
/*
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* HeapTupleSatisfiesToast
299-
*True iff heap tuple is valid for TOAST usage.
305+
*
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*Valid if the heap tuple is valid for TOAST usage.
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*
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* This is a simplified version that only checks for VACUUM moving conditions.
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* It's appropriate for TOAST usage because TOAST really doesn't want to do
303-
* its own time qual checks; if you can see the main-table row that contains
310+
* its own time qual checks; if you can see the maintable row that contains
304311
* a TOAST reference, you should be able to see the TOASTed value. However,
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* vacuuming a TOAST table is independent of the main table, and in case such
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* a vacuum fails partway through, we'd better do this much checking.
@@ -353,11 +360,13 @@ HeapTupleSatisfiesToast(HeapTupleHeader tuple)
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/*
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* HeapTupleSatisfiesUpdate
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*Check whether a tuple can be updated.
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*
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* This applies exactly the same checks as HeapTupleSatisfiesNow,
359-
* but returns a more-detailed result code, since UPDATE needs to know
360-
* more than "is it visible?"
364+
*Same as HeapTupleSatisfiesNow, but returns more information needed
365+
*by UPDATE.
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*
367+
* This applies the same checks as HeapTupleSatisfiesNow,
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* but returns a more detailed result code, since UPDATE needs to know
369+
* more than "is it visible?".
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*/
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int
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HeapTupleSatisfiesUpdate(HeapTuplehtuple)
@@ -475,9 +484,13 @@ HeapTupleSatisfiesUpdate(HeapTuple htuple)
475484
returnHeapTupleUpdated;/* updated by other */
476485
}
477486

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/*
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* HeapTupleSatisfiesDirty
480-
*True iff heap tuple is valid, including effects of concurrent xacts.
487+
/* HeapTupleSatisfiesDirty
488+
*
489+
*Visible tuples are those of:
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*
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*_any_ in-progress transaction
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*previous commands of this transaction
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*changes by the current command
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*
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* This is essentially like HeapTupleSatisfiesItself as far as effects of
483496
* the current transaction and committed/aborted xacts are concerned.
@@ -601,11 +614,20 @@ HeapTupleSatisfiesDirty(HeapTupleHeader tuple)
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602615
/*
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* HeapTupleSatisfiesSnapshot
604-
*True iff heap tuple is valid for the given snapshot.
617+
*
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*Visible tuples are those of:
619+
*
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*transactions committed before our transaction started (SERIALIZABLE)
621+
*previous commands of this transaction
622+
*
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*Does _not_ include:
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*transactions in-progress when our transaction started
625+
*transactions committed after our transaction started
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*changes made by the current command
605627
*
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* This is the same as HeapTupleSatisfiesNow, except that transactions that
607629
* were in progress or as yet unstarted when the snapshot was taken will
608-
* be treated as uncommitted, even if theyreallyhave committed by now.
630+
* be treated as uncommitted, even if they have committed by now.
609631
*
610632
* (Notice, however, that the tuple status hint bits will be updated on the
611633
* basis of the true state of the transaction, even if we then pretend we
@@ -746,8 +768,13 @@ HeapTupleSatisfiesSnapshot(HeapTupleHeader tuple, Snapshot snapshot)
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747769

748770
/*
749-
* HeapTupleSatisfiesVacuum - determine tuple status for VACUUM and related
750-
*operations
771+
* HeapTupleSatisfiesVacuum
772+
*
773+
*Visible tuples are those of:
774+
*
775+
*tuples visible by any running transaction
776+
*
777+
*Used by VACUUM and related operations.
751778
*
752779
* OldestXmin is a cutoff XID (obtained from GetOldestXmin()).Tuples
753780
* deleted by XIDs >= OldestXmin are deemed "recently dead"; they might

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