Fix problems with cached tuple descriptors disappearing while still in use

by creating a reference-count mechanism, similar to what we did a long time
ago for catcache entries.  The back branches have an ugly solution involving
lots of extra copies, but this way is more efficient.  Reference counting is
only applied to tupdescs that are actually in caches --- there seems no need
to use it for tupdescs that are generated in the executor, since they'll go
away during plan shutdown by virtue of being in the per-query memory context.
Neil Conway and Tom Lane

2 files changed, 14 insertions, 13 deletions

diff --git a/src/include/executor/executor.h b/src/include/executor/executor.h
index a29e1394974..d926d8b5c5c 100644
--- a/src/include/executor/executor.h
+++ b/src/include/executor/executor.h
@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2006, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/executor/executor.h,v 1.126 2006/03/05 15:58:56 momjian Exp $
+ * $PostgreSQL: pgsql/src/include/executor/executor.h,v 1.127 2006/06/16 18:42:23 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -249,8 +249,7 @@ extern ExprContext *MakePerTupleExprContext(EState *estate);
 	} while (0)
 
 extern void ExecAssignExprContext(EState *estate, PlanState *planstate);
-extern void ExecAssignResultType(PlanState *planstate,
-					 TupleDesc tupDesc, bool shouldFree);
+extern void ExecAssignResultType(PlanState *planstate, TupleDesc tupDesc);
 extern void ExecAssignResultTypeFromTL(PlanState *planstate);
 extern TupleDesc ExecGetResultType(PlanState *planstate);
 extern ProjectionInfo *ExecBuildProjectionInfo(List *targetList,
@@ -259,8 +258,7 @@ extern ProjectionInfo *ExecBuildProjectionInfo(List *targetList,
 extern void ExecAssignProjectionInfo(PlanState *planstate);
 extern void ExecFreeExprContext(PlanState *planstate);
 extern TupleDesc ExecGetScanType(ScanState *scanstate);
-extern void ExecAssignScanType(ScanState *scanstate,
-				   TupleDesc tupDesc, bool shouldFree);
+extern void ExecAssignScanType(ScanState *scanstate, TupleDesc tupDesc);
 extern void ExecAssignScanTypeFromOuterPlan(ScanState *scanstate);
 
 extern bool ExecRelationIsTargetRelation(EState *estate, Index scanrelid);
diff --git a/src/include/executor/tuptable.h b/src/include/executor/tuptable.h
index 1b2eb883ecf..b506651fbbd 100644
--- a/src/include/executor/tuptable.h
+++ b/src/include/executor/tuptable.h
@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2006, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/executor/tuptable.h,v 1.30 2006/03/05 15:58:56 momjian Exp $
+ * $PostgreSQL: pgsql/src/include/executor/tuptable.h,v 1.31 2006/06/16 18:42:23 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -15,6 +15,7 @@
 #define TUPTABLE_H
 
 #include "access/htup.h"
+#include "access/tupdesc.h"
 
 
 /*----------
@@ -56,13 +57,17 @@
  * TRUE, tts_shouldFree FALSE, tts_tuple NULL, tts_buffer InvalidBuffer,
  * and tts_nvalid zero.
  *
+ * The tupleDescriptor is simply referenced, not copied, by the TupleTableSlot
+ * code.  The caller of ExecSetSlotDescriptor() is responsible for providing
+ * a descriptor that will live as long as the slot does.  (Typically, both
+ * slots and descriptors are in per-query memory and are freed by memory
+ * context deallocation at query end; so it's not worth providing any extra
+ * mechanism to do more.  However, the slot will increment the tupdesc
+ * reference count if a reference-counted tupdesc is supplied.)
+ *
  * When tts_shouldFree is true, the physical tuple is "owned" by the slot
  * and should be freed when the slot's reference to the tuple is dropped.
  *
- * tts_shouldFreeDesc is similar to tts_shouldFree: if it's true, then the
- * tupleDescriptor is "owned" by the TupleTableSlot and should be
- * freed when the slot's reference to the descriptor is dropped.
- *
  * If tts_buffer is not InvalidBuffer, then the slot is holding a pin
  * on the indicated buffer page; drop the pin when we release the
  * slot's reference to that buffer.  (tts_shouldFree should always be
@@ -87,7 +92,6 @@ typedef struct TupleTableSlot
 	NodeTag		type;			/* vestigial ... allows IsA tests */
 	bool		tts_isempty;	/* true = slot is empty */
 	bool		tts_shouldFree; /* should pfree tuple? */
-	bool		tts_shouldFreeDesc;		/* should pfree descriptor? */
 	bool		tts_slow;		/* saved state for slot_deform_tuple */
 	HeapTuple	tts_tuple;		/* physical tuple, or NULL if none */
 	TupleDesc	tts_tupleDescriptor;	/* slot's tuple descriptor */
@@ -124,8 +128,7 @@ extern void ExecDropTupleTable(TupleTable table, bool shouldFree);
 extern TupleTableSlot *MakeSingleTupleTableSlot(TupleDesc tupdesc);
 extern void ExecDropSingleTupleTableSlot(TupleTableSlot *slot);
 extern TupleTableSlot *ExecAllocTableSlot(TupleTable table);
-extern void ExecSetSlotDescriptor(TupleTableSlot *slot,
-					  TupleDesc tupdesc, bool shouldFree);
+extern void ExecSetSlotDescriptor(TupleTableSlot *slot, TupleDesc tupdesc);
 extern TupleTableSlot *ExecStoreTuple(HeapTuple tuple,
 			   TupleTableSlot *slot,
 			   Buffer buffer,