4 files changed, 53 insertions, 24 deletions
diff --git a/src/include/access/heapam.h b/src/include/access/heapam.h
index 9c040b0dfd..3221734a6f 100644
--- a/src/include/access/heapam.h
+++ b/src/include/access/heapam.h
@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2005, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/access/heapam.h,v 1.103 2005/08/01 20:31:13 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/access/heapam.h,v 1.104 2005/08/20 00:39:59 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -152,19 +152,23 @@ extern bool heap_release_fetch(Relation relation, Snapshot snapshot,
 				   HeapTuple tuple, Buffer *userbuf, bool keep_buf,
 				   PgStat_Info *pgstat_info);
 
-extern ItemPointer heap_get_latest_tid(Relation relation, Snapshot snapshot,
+extern void heap_get_latest_tid(Relation relation, Snapshot snapshot,
 					ItemPointer tid);
 extern void setLastTid(const ItemPointer tid);
 
 extern Oid	heap_insert(Relation relation, HeapTuple tup, CommandId cid,
 						bool use_wal, bool use_fsm);
-extern HTSU_Result heap_delete(Relation relation, ItemPointer tid, ItemPointer ctid,
-			CommandId cid, Snapshot crosscheck, bool wait);
-extern HTSU_Result heap_update(Relation relation, ItemPointer otid, HeapTuple tup,
-		ItemPointer ctid, CommandId cid, Snapshot crosscheck, bool wait);
-extern HTSU_Result heap_lock_tuple(Relation relation, HeapTuple tup,
-				 Buffer *userbuf, CommandId cid,
-				 LockTupleMode mode, bool nowait);
+extern HTSU_Result heap_delete(Relation relation, ItemPointer tid,
+							   ItemPointer ctid, TransactionId *update_xmax,
+							   CommandId cid, Snapshot crosscheck, bool wait);
+extern HTSU_Result heap_update(Relation relation, ItemPointer otid,
+							   HeapTuple newtup,
+							   ItemPointer ctid, TransactionId *update_xmax,
+							   CommandId cid, Snapshot crosscheck, bool wait);
+extern HTSU_Result heap_lock_tuple(Relation relation, HeapTuple tuple,
+								   Buffer *buffer, ItemPointer ctid,
+								   TransactionId *update_xmax, CommandId cid,
+								   LockTupleMode mode, bool nowait);
 
 extern Oid	simple_heap_insert(Relation relation, HeapTuple tup);
 extern void simple_heap_delete(Relation relation, ItemPointer tid);
diff --git a/src/include/access/htup.h b/src/include/access/htup.h
index e394afd313..abc4dce9b9 100644
--- a/src/include/access/htup.h
+++ b/src/include/access/htup.h
@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2005, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/access/htup.h,v 1.75 2005/06/08 15:50:27 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/access/htup.h,v 1.76 2005/08/20 00:39:59 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -80,6 +80,21 @@
  * However, with the advent of subtransactions, a tuple may need both Xmax
  * and Cmin simultaneously, so this is no longer possible.
  *
+ * A word about t_ctid: whenever a new tuple is stored on disk, its t_ctid
+ * is initialized with its own TID (location).  If the tuple is ever updated,
+ * its t_ctid is changed to point to the replacement version of the tuple.
+ * Thus, a tuple is the latest version of its row iff XMAX is invalid or
+ * t_ctid points to itself (in which case, if XMAX is valid, the tuple is
+ * either locked or deleted).  One can follow the chain of t_ctid links
+ * to find the newest version of the row.  Beware however that VACUUM might
+ * erase the pointed-to (newer) tuple before erasing the pointing (older)
+ * tuple.  Hence, when following a t_ctid link, it is necessary to check
+ * to see if the referenced slot is empty or contains an unrelated tuple.
+ * Check that the referenced tuple has XMIN equal to the referencing tuple's
+ * XMAX to verify that it is actually the descendant version and not an
+ * unrelated tuple stored into a slot recently freed by VACUUM.  If either
+ * check fails, one may assume that there is no live descendant version.
+ *
  * Following the fixed header fields, the nulls bitmap is stored (beginning
  * at t_bits).	The bitmap is *not* stored if t_infomask shows that there
  * are no nulls in the tuple.  If an OID field is present (as indicated by
@@ -334,18 +349,29 @@ do { \
 /*
  * HeapTupleData is an in-memory data structure that points to a tuple.
  *
- * This new HeapTuple for version >= 6.5 and this is why it was changed:
+ * There are several ways in which this data structure is used:
+ *
+ * * Pointer to a tuple in a disk buffer: t_data points directly into the
+ *	 buffer (which the code had better be holding a pin on, but this is not
+ *	 reflected in HeapTupleData itself).  t_datamcxt must be NULL.
+ *
+ * * Pointer to nothing: t_data and t_datamcxt are NULL.  This is used as
+ *	 a failure indication in some functions.
+ *
+ * * Part of a palloc'd tuple: the HeapTupleData itself and the tuple
+ *	 form a single palloc'd chunk.  t_data points to the memory location
+ *	 immediately following the HeapTupleData struct (at offset HEAPTUPLESIZE),
+ *	 and t_datamcxt is the containing context.  This is used as the output
+ *	 format of heap_form_tuple and related routines.
  *
- * 1. t_len moved off on-disk tuple data - ItemIdData is used to get len;
- * 2. t_ctid above is not self tuple TID now - it may point to
- *	  updated version of tuple (required by MVCC);
- * 3. someday someone let tuple to cross block boundaries -
- *	  he have to add something below...
+ * * Separately allocated tuple: t_data points to a palloc'd chunk that
+ *	 is not adjacent to the HeapTupleData, and t_datamcxt is the context
+ *	 containing that chunk.
  *
- * Change for 7.0:
- *	  Up to now t_data could be NULL, the memory location directly following
- *	  HeapTupleData, or pointing into a buffer. Now, it could also point to
- *	  a separate allocation that was done in the t_datamcxt memory context.
+ * t_len should always be valid, except in the pointer-to-nothing case.
+ * t_self and t_tableOid should be valid if the HeapTupleData points to
+ * a disk buffer, or if it represents a copy of a tuple on disk.  They
+ * should be explicitly set invalid in manufactured tuples.
  */
 typedef struct HeapTupleData
 {
diff --git a/src/include/executor/executor.h b/src/include/executor/executor.h
index 2e42894788..6064ff2f4f 100644
--- a/src/include/executor/executor.h
+++ b/src/include/executor/executor.h
@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2005, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/executor/executor.h,v 1.118 2005/04/16 20:07:35 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/executor/executor.h,v 1.119 2005/08/20 00:40:13 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -98,7 +98,7 @@ extern bool ExecContextForcesOids(PlanState *planstate, bool *hasoids);
 extern void ExecConstraints(ResultRelInfo *resultRelInfo,
 				TupleTableSlot *slot, EState *estate);
 extern TupleTableSlot *EvalPlanQual(EState *estate, Index rti,
-			 ItemPointer tid);
+									ItemPointer tid, TransactionId priorXmax);
 
 /*
  * prototypes from functions in execProcnode.c
diff --git a/src/include/utils/tqual.h b/src/include/utils/tqual.h
index f12ae2233f..fa530ed977 100644
--- a/src/include/utils/tqual.h
+++ b/src/include/utils/tqual.h
@@ -8,7 +8,7 @@
  * Portions Copyright (c) 1996-2005, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/utils/tqual.h,v 1.57 2005/05/19 21:35:48 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/utils/tqual.h,v 1.58 2005/08/20 00:40:32 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -42,7 +42,6 @@ typedef struct SnapshotData
 	TransactionId *xip;			/* array of xact IDs in progress */
 	/* note: all ids in xip[] satisfy xmin <= xip[i] < xmax */
 	CommandId	curcid;			/* in my xact, CID < curcid are visible */
-	ItemPointerData tid;		/* required for Dirty snapshot -:( */
 } SnapshotData;
 
 typedef SnapshotData *Snapshot;