Implement SQL-standard WITH clauses, including WITH RECURSIVE.

There are some unimplemented aspects: recursive queries must use UNION ALL
(should allow UNION too), and we don't have SEARCH or CYCLE clauses.
These might or might not get done for 8.4, but even without them it's a
pretty useful feature.

There are also a couple of small loose ends and definitional quibbles,
which I'll send a memo about to pgsql-hackers shortly.  But let's land
the patch now so we can get on with other development.

Yoshiyuki Asaba, with lots of help from Tatsuo Ishii and Tom Lane

src/include/nodes/nodes.h

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/nodes/nodes.h,v 1.212 2008/09/09 18:58:08 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/nodes/nodes.h,v 1.213 2008/10/04 21:56:55 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -44,6 +44,7 @@ typedef enum NodeTag
 	T_Plan = 100,
 	T_Result,
 	T_Append,
+	T_RecursiveUnion,
 	T_BitmapAnd,
 	T_BitmapOr,
 	T_Scan,
@@ -55,6 +56,8 @@ typedef enum NodeTag
 	T_SubqueryScan,
 	T_FunctionScan,
 	T_ValuesScan,
+	T_CteScan,
+	T_WorkTableScan,
 	T_Join,
 	T_NestLoop,
 	T_MergeJoin,
@@ -78,6 +81,7 @@ typedef enum NodeTag
 	T_PlanState = 200,
 	T_ResultState,
 	T_AppendState,
+	T_RecursiveUnionState,
 	T_BitmapAndState,
 	T_BitmapOrState,
 	T_ScanState,
@@ -89,6 +93,8 @@ typedef enum NodeTag
 	T_SubqueryScanState,
 	T_FunctionScanState,
 	T_ValuesScanState,
+	T_CteScanState,
+	T_WorkTableScanState,
 	T_JoinState,
 	T_NestLoopState,
 	T_MergeJoinState,
@@ -352,6 +358,8 @@ typedef enum NodeTag
 	T_LockingClause,
 	T_RowMarkClause,
 	T_XmlSerialize,
+	T_WithClause,
+	T_CommonTableExpr,
 
 	/*
 	 * TAGS FOR RANDOM OTHER STUFF