I have applied the following comment to summarize the visibility rules.
I also added a URL about the Halloween problem.
--
Bruce Momjian <bruce@momjian.us> http://momjian.us
EnterpriseDB http://enterprisedb.com
+ If your life is a hard drive, Christ can be your backup. +
Index: src/backend/utils/time/tqual.c
===================================================================
RCS file: /cvsroot/pgsql/src/backend/utils/time/tqual.c,v
retrieving revision 1.111
diff -c -c -r1.111 tqual.c
*** src/backend/utils/time/tqual.c 1 Jan 2009 17:23:53 -0000 1.111
--- src/backend/utils/time/tqual.c 9 Mar 2009 12:39:09 -0000
***************
*** 26,31 ****
--- 26,50 ----
* subtransactions of our own main transaction and so there can't be any
* race condition.
*
+ * Summary of visibility functions:
+ *
+ * HeapTupleSatisfiesMVCC()
+ * visible to supplied snapshot, excludes current command
+ * HeapTupleSatisfiesNow()
+ * visible to instant snapshot, excludes current command
+ * HeapTupleSatisfiesUpdate()
+ * like HeapTupleSatisfiesNow(), but with user-supplied command
+ * counter and more complex result
+ * HeapTupleSatisfiesSelf()
+ * visible to instant snapshot and current command
+ * HeapTupleSatisfiesDirty()
+ * like HeapTupleSatisfiesSelf(), but includes open transactions
+ * HeapTupleSatisfiesVacuum()
+ * visible to any running transaction, used by VACUUM
+ * HeapTupleSatisfiesToast()
+ * visible unless part of interrupted vacuum, used for TOAST
+ * HeapTupleSatisfiesAny()
+ * all tuples are visible
*
* Portions Copyright (c) 1996-2009, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
***************
*** 277,283 ****
*
* 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.
--- 296,302 ----
*
* 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, http://en.wikipedia.org/wiki/Halloween_Problem.
*
* Note:
* Assumes heap tuple is valid.
