Document that get_attstatsslot/free_attstatsslot only need to be passed

valid type information if they are asked to fetch the values part of a
pg_statistic slot; these arguments are unneeded if fetching only the
numbers part.  Use this to save a catcache lookup in btcostestimate,
which is looking like a bit of a hotspot in recent profiling.  Not a
big savings, but since it's essentially free, might as well do it.

src/backend/utils/adt/selfuncs.c

@@ -15,7 +15,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/utils/adt/selfuncs.c,v 1.189 2005/09/24 22:54:38 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/utils/adt/selfuncs.c,v 1.190 2005/10/11 17:27:14 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -4466,15 +4466,10 @@ btcostestimate(PG_FUNCTION_ARGS)
 
 	if (HeapTupleIsValid(tuple))
 	{
-		Oid			typid;
-		int32		typmod;
 		float4	   *numbers;
 		int			nnumbers;
 
-		/* XXX this code would break with different storage type */
-		get_atttypetypmod(relid, colnum, &typid, &typmod);
-
-		if (get_attstatsslot(tuple, typid, typmod,
+		if (get_attstatsslot(tuple, InvalidOid, 0,
 							 STATISTIC_KIND_CORRELATION,
 							 index->ordering[0],
 							 NULL, NULL, &numbers, &nnumbers))
@@ -4489,7 +4484,7 @@ btcostestimate(PG_FUNCTION_ARGS)
 			else
 				*indexCorrelation = varCorrelation;
 
-			free_attstatsslot(typid, NULL, 0, numbers, nnumbers);
+			free_attstatsslot(InvalidOid, NULL, 0, numbers, nnumbers);
 		}
 		ReleaseSysCache(tuple);
 	}

src/backend/utils/cache/lsyscache.c

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/utils/cache/lsyscache.c,v 1.127 2005/08/12 21:49:47 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/utils/cache/lsyscache.c,v 1.128 2005/10/11 17:27:14 tgl Exp $
  *
  * NOTES
  *	  Eventually, the index information should go through here, too.
@@ -1841,8 +1841,8 @@ get_attavgwidth(Oid relid, AttrNumber attnum)
  * entry, and we don't want to repeat the cache lookup unnecessarily.
  *
  * statstuple: pg_statistics tuple to be examined.
- * atttype: type OID of attribute.
- * atttypmod: typmod of attribute.
+ * atttype: type OID of attribute (can be InvalidOid if values == NULL).
+ * atttypmod: typmod of attribute (can be 0 if values == NULL).
  * reqkind: STAKIND code for desired statistics slot kind.
  * reqop: STAOP value wanted, or InvalidOid if don't care.
  * values, nvalues: if not NULL, the slot's stavalues are extracted.
@@ -1960,6 +1960,12 @@ get_attstatsslot(HeapTuple statstuple,
 	return true;
 }
 
+/*
+ * free_attstatsslot
+ *		Free data allocated by get_attstatsslot
+ *
+ * atttype need be valid only if values != NULL.
+ */
 void
 free_attstatsslot(Oid atttype,
 				  Datum *values, int nvalues,