Fix misstatements added by Bruce.

diff --git a/src/backend/utils/time/tqual.c b/src/backend/utils/time/tqual.c
index 81c6ed863b980ba90533178fabe529af8b363a05..ac06d91dc485e5fe4428a6a69dd73d73d09ae45c 100644 (file)
--- a/src/backend/utils/time/tqual.c
+++ b/src/backend/utils/time/tqual.c
@@ -16,7 +16,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *       $Header: /cvsroot/pgsql/src/backend/utils/time/tqual.c,v 1.48 2002/01/16 23:09:09 momjian Exp $
+ *       $Header: /cvsroot/pgsql/src/backend/utils/time/tqual.c,v 1.49 2002/01/16 23:51:56 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -38,10 +38,10 @@ bool                ReferentialIntegritySnapshotOverride = false;
 
 /*
  * HeapTupleSatisfiesItself
+ *             True iff heap tuple is valid "for itself".
  *
- *     Visible tuples are those of:
- *
- *             transactions committed before our _command_ started (READ COMMITTED)
+ *     Here, we consider the effects of:
+ *             all committed transactions (as of the current instant)
  *             previous commands of this transaction
  *             changes made by the current command
  *
@@ -156,13 +156,15 @@ HeapTupleSatisfiesItself(HeapTupleHeader tuple)
 
 /*
  * HeapTupleSatisfiesNow
+ *             True iff heap tuple is valid "now".
  *
- *     Visible tuples are those of:
- *
- *             transactions committed before our _command_ started (READ COMMITTED)
+ *     Here, we consider the effects of:
+ *             all committed transactions (as of the current instant)
  *             previous commands of this transaction
  *
- *     Does _not_ include changes made by the current command
+ * Note we do _not_ include changes made by the current command.  This
+ * solves the "Halloween problem" wherein an UPDATE might try to re-update
+ * its own output tuples.
  *
  * Note:
  *             Assumes heap tuple is valid.
@@ -302,8 +304,7 @@ HeapTupleSatisfiesNow(HeapTupleHeader tuple)
 
 /*
  * HeapTupleSatisfiesToast
- *
- *     Valid if the heap tuple is valid for TOAST usage.
+ *             True iff heap tuple is valid as a TOAST row.
  *
  * This is a simplified version that only checks for VACUUM moving conditions.
  * It's appropriate for TOAST usage because TOAST really doesn't want to do
@@ -361,12 +362,8 @@ HeapTupleSatisfiesToast(HeapTupleHeader tuple)
 /*
  * HeapTupleSatisfiesUpdate
  *
- *     Same as HeapTupleSatisfiesNow, but returns more information needed
- *     by UPDATE.
- *
- * This applies the same checks as HeapTupleSatisfiesNow,
- * but returns a more detailed result code, since UPDATE needs to know
- * more than "is it visible?".
+ *     Same logic as HeapTupleSatisfiesNow, but returns a more detailed result
+ *     code, since UPDATE needs to know more than "is it visible?".
  */
 int
 HeapTupleSatisfiesUpdate(HeapTuple htuple)
@@ -484,13 +481,14 @@ HeapTupleSatisfiesUpdate(HeapTuple htuple)
        return HeapTupleUpdated;        /* updated by other */
 }
 
-/* HeapTupleSatisfiesDirty
- *
- *     Visible tuples are those of:
+/*
+ * HeapTupleSatisfiesDirty
+ *             True iff heap tuple is valid including effects of open transactions.
  *
- *             _any_ in-progress transaction
+ *     Here, we consider the effects of:
+ *             all committed and in-progress transactions (as of the current instant)
  *             previous commands of this transaction
- *             changes by the current command
+ *             changes made by the current command
  *
  * This is essentially like HeapTupleSatisfiesItself as far as effects of
  * the current transaction and committed/aborted xacts are concerned.
@@ -614,15 +612,15 @@ HeapTupleSatisfiesDirty(HeapTupleHeader tuple)
 
 /*
  * HeapTupleSatisfiesSnapshot
+ *             True iff heap tuple is valid for the given snapshot.
  *
- *     Visible tuples are those of:
- *
- *             transactions committed before our transaction started (SERIALIZABLE)
+ *     Here, we consider the effects of:
+ *             all transactions committed as of the time of the given snapshot
  *             previous commands of this transaction
  *
  *     Does _not_ include:
- *             transactions in-progress when our transaction started
- *             transactions committed after our transaction started
+ *             transactions shown as in-progress by the snapshot
+ *             transactions started after the snapshot was taken
  *             changes made by the current command
  *
  * This is the same as HeapTupleSatisfiesNow, except that transactions that
@@ -770,11 +768,9 @@ HeapTupleSatisfiesSnapshot(HeapTupleHeader tuple, Snapshot snapshot)
 /*
  * HeapTupleSatisfiesVacuum
  *
- *     Visible tuples are those of:
- *
- *             tuples visible by any running transaction
- *
- *     Used by VACUUM and related operations.
+ *     Determine the status of tuples for VACUUM purposes.  Here, what
+ *     we mainly want to know is if a tuple is potentially visible to *any*
+ *     running transaction.  If so, it can't be removed yet by VACUUM.
  *
  * OldestXmin is a cutoff XID (obtained from GetOldestXmin()). Tuples
  * deleted by XIDs >= OldestXmin are deemed "recently dead"; they might