Install a lookaside cache to speed up repeated lookups of the same operator

by short-circuiting schema search path and ambiguous-operator resolution
computations.  Remarkably, this buys as much as 45% speedup of repetitive
simple queries that involve operators that are not an exact match to the
input datatypes.  It should be marginally faster even for exact-match
cases, though I've not had success in proving an improvement in benchmark
tests.  Per report from Guillame Smet and subsequent discussion.

src/backend/catalog/namespace.c

@@ -13,7 +13,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/catalog/namespace.c,v 1.102 2007/11/25 02:09:46 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/catalog/namespace.c,v 1.103 2007/11/28 18:47:56 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -3006,6 +3006,40 @@ fetch_search_path(bool includeImplicit)
 	return result;
 }
 
+/*
+ * Fetch the active search path into a caller-allocated array of OIDs.
+ * Returns the number of path entries.  (If this is more than sarray_len,
+ * then the data didn't fit and is not all stored.)
+ *
+ * The returned list always includes the implicitly-prepended namespaces,
+ * but never includes the temp namespace.  (This is suitable for existing
+ * users, which would want to ignore the temp namespace anyway.)  This
+ * definition allows us to not worry about initializing the temp namespace.
+ */
+int
+fetch_search_path_array(Oid *sarray, int sarray_len)
+{
+	int			count = 0;
+	ListCell   *l;
+
+	recomputeNamespacePath();
+
+	foreach(l, activeSearchPath)
+	{
+		Oid			namespaceId = lfirst_oid(l);
+
+		if (namespaceId == myTempNamespace)
+			continue;			/* do not include temp namespace */
+
+		if (count < sarray_len)
+			sarray[count] = namespaceId;
+		count++;
+	}
+
+	return count;
+}
+
+
 /*
  * Export the FooIsVisible functions as SQL-callable functions.
  */

src/include/catalog/namespace.h

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2007, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/catalog/namespace.h,v 1.51 2007/11/15 22:25:17 momjian Exp $
+ * $PostgreSQL: pgsql/src/include/catalog/namespace.h,v 1.52 2007/11/28 18:47:56 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -115,5 +115,6 @@ extern void AtEOSubXact_Namespace(bool isCommit, SubTransactionId mySubid,
 extern char *namespace_search_path;
 
 extern List *fetch_search_path(bool includeImplicit);
+extern int	fetch_search_path_array(Oid *sarray, int sarray_len);
 
 #endif   /* NAMESPACE_H */