Redesign query-snapshot timing so that volatile functions in READ COMMITTED

mode see a fresh snapshot for each command in the function, rather than
using the latest interactive command's snapshot.  Also, suppress fresh
snapshots as well as CommandCounterIncrement inside STABLE and IMMUTABLE
functions, instead using the snapshot taken for the most closely nested
regular query.  (This behavior is only sane for read-only functions, so
the patch also enforces that such functions contain only SELECT commands.)
As per my proposal of 6-Sep-2004; I note that I floated essentially the
same proposal on 19-Jun-2002, but that discussion tailed off without any
action.  Since 8.0 seems like the right place to be taking possibly
nontrivial backwards compatibility hits, let's get it done now.

1 files changed, 16 insertions, 31 deletions

diff --git a/src/backend/executor/execMain.c b/src/backend/executor/execMain.c
index b009e73e105..ea9dce019b1 100644
--- a/src/backend/executor/execMain.c
+++ b/src/backend/executor/execMain.c
@@ -26,7 +26,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/executor/execMain.c,v 1.237 2004/09/11 18:28:34 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/executor/execMain.c,v 1.238 2004/09/13 20:06:46 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -106,15 +106,6 @@ static void EvalPlanQualStop(evalPlanQual *epq);
  * field of the QueryDesc is filled in to describe the tuples that will be
  * returned, and the internal fields (estate and planstate) are set up.
  *
- * If useCurrentSnapshot is true, run the query with the latest available
- * snapshot, instead of the normal QuerySnapshot.  Also, if it's an update
- * or delete query, check that the rows to be updated or deleted would be
- * visible to the normal QuerySnapshot.  (This is a special-case behavior
- * needed for referential integrity updates in serializable transactions.
- * We must check all currently-committed rows, but we want to throw a
- * can't-serialize error if any rows that would need updates would not be
- * visible under the normal serializable snapshot.)
- *
  * If explainOnly is true, we are not actually intending to run the plan,
  * only to set up for EXPLAIN; so skip unwanted side-effects.
  *
@@ -123,7 +114,7 @@ static void EvalPlanQualStop(evalPlanQual *epq);
  * ----------------------------------------------------------------
  */
 void
-ExecutorStart(QueryDesc *queryDesc, bool useCurrentSnapshot, bool explainOnly)
+ExecutorStart(QueryDesc *queryDesc, bool explainOnly)
 {
 	EState	   *estate;
 	MemoryContext oldcontext;
@@ -156,28 +147,12 @@ ExecutorStart(QueryDesc *queryDesc, bool useCurrentSnapshot, bool explainOnly)
 		estate->es_param_exec_vals = (ParamExecData *)
 			palloc0(queryDesc->plantree->nParamExec * sizeof(ParamExecData));
 
-	estate->es_instrument = queryDesc->doInstrument;
-
 	/*
-	 * Make our own private copy of the current query snapshot data.
-	 *
-	 * This "freezes" our idea of which tuples are good and which are not for
-	 * the life of this query, even if it outlives the current command and
-	 * current snapshot.
+	 * Copy other important information into the EState
 	 */
-	if (useCurrentSnapshot)
-	{
-		/* RI update/delete query --- must use an up-to-date snapshot */
-		estate->es_snapshot = CopyCurrentSnapshot();
-		/* crosscheck updates/deletes against transaction snapshot */
-		estate->es_crosscheck_snapshot = CopyQuerySnapshot();
-	}
-	else
-	{
-		/* normal query --- use query snapshot, no crosscheck */
-		estate->es_snapshot = CopyQuerySnapshot();
-		estate->es_crosscheck_snapshot = InvalidSnapshot;
-	}
+	estate->es_snapshot = queryDesc->snapshot;
+	estate->es_crosscheck_snapshot = queryDesc->crosscheck_snapshot;
+	estate->es_instrument = queryDesc->doInstrument;
 
 	/*
 	 * Initialize the plan state tree
@@ -1454,6 +1429,11 @@ ExecDelete(TupleTableSlot *slot,
 
 	/*
 	 * delete the tuple
+	 *
+	 * Note: if es_crosscheck_snapshot isn't InvalidSnapshot, we check that
+	 * the row to be deleted is visible to that snapshot, and throw a can't-
+	 * serialize error if not.  This is a special-case behavior needed for
+	 * referential integrity updates in serializable transactions.
 	 */
 ldelete:;
 	result = heap_delete(resultRelationDesc, tupleid,
@@ -1591,6 +1571,11 @@ lreplace:;
 
 	/*
 	 * replace the heap tuple
+	 *
+	 * Note: if es_crosscheck_snapshot isn't InvalidSnapshot, we check that
+	 * the row to be updated is visible to that snapshot, and throw a can't-
+	 * serialize error if not.  This is a special-case behavior needed for
+	 * referential integrity updates in serializable transactions.
 	 */
 	result = heap_update(resultRelationDesc, tupleid, tuple,
 						 &ctid,