Allow GIN's extractQuery method to signal that nothing can satisfy the query.

In this case extractQuery should returns -1 as nentries. This changes
prototype of extractQuery method to use int32* instead of uint32* for
nentries argument.
Based on that gincostestimate may see two corner cases: nothing will be found
or seqscan should be used.

Per proposal at http://archives.postgresql.org/pgsql-hackers/2007-01/msg01581.php

PS tsearch_core patch should be sightly modified to support changes, but I'm
waiting a verdict about reviewing of tsearch_core patch.

src/backend/access/gin/ginarrayproc.c

@@ -8,7 +8,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/access/gin/ginarrayproc.c,v 1.8 2007/01/05 22:19:21 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/access/gin/ginarrayproc.c,v 1.9 2007/01/31 15:09:45 teodor Exp $
  *-------------------------------------------------------------------------
  */
 #include "postgres.h"
@@ -38,7 +38,7 @@ Datum
 ginarrayextract(PG_FUNCTION_ARGS)
 {
 	ArrayType  *array;
-	uint32	   *nentries = (uint32 *) PG_GETARG_POINTER(1);
+	int32	   *nentries = (int32 *) PG_GETARG_POINTER(1);
 	Datum	   *entries = NULL;
 	int16		elmlen;
 	bool		elmbyval;
@@ -60,6 +60,21 @@ ginarrayextract(PG_FUNCTION_ARGS)
 					  elmlen, elmbyval, elmalign,
 					  &entries, NULL, (int *) nentries);
 
+	if ( *nentries == 0 && PG_NARGS() == 3 )
+	{
+		switch( PG_GETARG_UINT16(2) )
+		{
+			case GinOverlapStrategy:
+					*nentries = -1; /* nobody can be found */
+					break;
+			case GinContainsStrategy:
+			case GinContainedStrategy:
+			case GinEqualStrategy:
+			default:	/* require fullscan: GIN can't find void arrays */
+				break;
+		}
+	}
+
 	/* we should not free array, entries[i] points into it */
 	PG_RETURN_POINTER(entries);
 }

src/backend/access/gin/ginbulk.c

@@ -8,7 +8,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *			$PostgreSQL: pgsql/src/backend/access/gin/ginbulk.c,v 1.7 2007/01/05 22:19:21 momjian Exp $
+ *			$PostgreSQL: pgsql/src/backend/access/gin/ginbulk.c,v 1.8 2007/01/31 15:09:45 teodor Exp $
  *-------------------------------------------------------------------------
  */
 
@@ -191,13 +191,13 @@ ginChooseElem(BuildAccumulator *accum, ItemPointer heapptr, Datum *entries, uint
  * next middle on left part and middle of right part.
  */
 void
-ginInsertRecordBA(BuildAccumulator *accum, ItemPointer heapptr, Datum *entries, uint32 nentry)
+ginInsertRecordBA(BuildAccumulator *accum, ItemPointer heapptr, Datum *entries, int32 nentry)
 {
 	uint32		i,
 				nbit = 0,
 				offset;
 
-	if (nentry == 0)
+	if (nentry <= 0)
 		return;
 
 	i = nentry - 1;

src/backend/access/gin/ginget.c

@@ -8,7 +8,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *			$PostgreSQL: pgsql/src/backend/access/gin/ginget.c,v 1.5 2007/01/05 22:19:21 momjian Exp $
+ *			$PostgreSQL: pgsql/src/backend/access/gin/ginget.c,v 1.6 2007/01/31 15:09:45 teodor Exp $
  *-------------------------------------------------------------------------
  */
 
@@ -420,6 +420,7 @@ scanGetItem(IndexScanDesc scan, ItemPointerData *item)
 }
 
 #define GinIsNewKey(s)		( ((GinScanOpaque) scan->opaque)->keys == NULL )
+#define GinIsVoidRes(s)		( ((GinScanOpaque) scan->opaque)->isVoidRes == true )
 
 Datum
 gingetmulti(PG_FUNCTION_ARGS)
@@ -432,10 +433,13 @@ gingetmulti(PG_FUNCTION_ARGS)
 	if (GinIsNewKey(scan))
 		newScanKey(scan);
 
-	startScan(scan);
-
 	*returned_tids = 0;
 
+	if (GinIsVoidRes(scan))
+		PG_RETURN_BOOL(false);
+
+	startScan(scan);
+
 	do
 	{
 		if (scanGetItem(scan, tids + *returned_tids))
@@ -462,6 +466,9 @@ gingettuple(PG_FUNCTION_ARGS)
 	if (GinIsNewKey(scan))
 		newScanKey(scan);
 
+	if (GinIsVoidRes(scan))
+		PG_RETURN_BOOL(false);
+
 	startScan(scan);
 	res = scanGetItem(scan, &scan->xs_ctup.t_self);
 	stopScan(scan);

src/backend/access/gin/gininsert.c

@@ -8,7 +8,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *			$PostgreSQL: pgsql/src/backend/access/gin/gininsert.c,v 1.6 2007/01/05 22:19:21 momjian Exp $
+ *			$PostgreSQL: pgsql/src/backend/access/gin/gininsert.c,v 1.7 2007/01/31 15:09:45 teodor Exp $
  *-------------------------------------------------------------------------
  */
 
@@ -203,7 +203,7 @@ static uint32
 ginHeapTupleBulkInsert(GinBuildState *buildstate, Datum value, ItemPointer heapptr)
 {
 	Datum	   *entries;
-	uint32		nentries;
+	int32		nentries;
 	MemoryContext oldCtx;
 
 	oldCtx = MemoryContextSwitchTo(buildstate->funcCtx);
@@ -356,7 +356,7 @@ static uint32
 ginHeapTupleInsert(Relation index, GinState *ginstate, Datum value, ItemPointer item)
 {
 	Datum	   *entries;
-	uint32		i,
+	int32		i,
 				nentries;
 
 	entries = extractEntriesSU(ginstate, value, &nentries);

src/backend/access/gin/ginscan.c

@@ -8,7 +8,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *			$PostgreSQL: pgsql/src/backend/access/gin/ginscan.c,v 1.8 2007/01/05 22:19:21 momjian Exp $
+ *			$PostgreSQL: pgsql/src/backend/access/gin/ginscan.c,v 1.9 2007/01/31 15:09:45 teodor Exp $
  *-------------------------------------------------------------------------
  */
 
@@ -145,10 +145,12 @@ newScanKey(IndexScanDesc scan)
 				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
 				 errmsg("GIN indexes do not support whole-index scans")));
 
+	so->isVoidRes = false;
+
 	for (i = 0; i < scan->numberOfKeys; i++)
 	{
 		Datum	   *entryValues;
-		uint32		nEntryValues;
+		int32		nEntryValues;
 
 		if (scankey[i].sk_flags & SK_ISNULL)
 			elog(ERROR, "Gin doesn't support NULL as scan key");
@@ -162,6 +164,15 @@ newScanKey(IndexScanDesc scan)
 									   UInt16GetDatum(scankey[i].sk_strategy)
 															  )
 			);
+		if ( nEntryValues < 0 )
+		{
+			/*
+			 * extractQueryFn signals that nothing will be found,
+			 * so we can just set isVoidRes flag...
+			 */
+			so->isVoidRes = true;	
+			break;
+		}
 		if (entryValues == NULL || nEntryValues == 0)
 			/* full scan... */
 			continue;
@@ -173,7 +184,7 @@ newScanKey(IndexScanDesc scan)
 
 	so->nkeys = nkeys;
 
-	if (so->nkeys == 0)
+	if (so->nkeys == 0 && !so->isVoidRes)
 		ereport(ERROR,
 				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
 				 errmsg("GIN index does not support search with void query")));

src/backend/access/gin/ginutil.c

@@ -8,7 +8,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *			$PostgreSQL: pgsql/src/backend/access/gin/ginutil.c,v 1.9 2007/01/05 22:19:21 momjian Exp $
+ *			$PostgreSQL: pgsql/src/backend/access/gin/ginutil.c,v 1.10 2007/01/31 15:09:45 teodor Exp $
  *-------------------------------------------------------------------------
  */
 
@@ -148,7 +148,7 @@ cmpEntries(const Datum *a, const Datum *b, cmpEntriesData *arg)
 }
 
 Datum *
-extractEntriesS(GinState *ginstate, Datum value, uint32 *nentries,
+extractEntriesS(GinState *ginstate, Datum value, int32 *nentries,
 				bool *needUnique)
 {
 	Datum	   *entries;
@@ -178,7 +178,7 @@ extractEntriesS(GinState *ginstate, Datum value, uint32 *nentries,
 
 
 Datum *
-extractEntriesSU(GinState *ginstate, Datum value, uint32 *nentries)
+extractEntriesSU(GinState *ginstate, Datum value, int32 *nentries)
 {
 	bool		needUnique;
 	Datum	   *entries = extractEntriesS(ginstate, value, nentries,