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/backend/executor/nodeWorktablescan.c

@@ -0,0 +1,194 @@
+/*-------------------------------------------------------------------------
+ *
+ * nodeWorktablescan.c
+ *	  routines to handle WorkTableScan nodes.
+ *
+ * Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ *
+ * IDENTIFICATION
+ *	  $PostgreSQL: pgsql/src/backend/executor/nodeWorktablescan.c,v 1.1 2008/10/04 21:56:53 tgl Exp $
+ *
+ *-------------------------------------------------------------------------
+ */
+
+#include "postgres.h"
+
+#include "executor/execdebug.h"
+#include "executor/nodeWorktablescan.h"
+
+static TupleTableSlot *WorkTableScanNext(WorkTableScanState *node);
+
+/* ----------------------------------------------------------------
+ *		WorkTableScanNext
+ *
+ *		This is a workhorse for ExecWorkTableScan
+ * ----------------------------------------------------------------
+ */
+static TupleTableSlot *
+WorkTableScanNext(WorkTableScanState *node)
+{
+	TupleTableSlot *slot;
+	EState	   *estate;
+	ScanDirection direction;
+	Tuplestorestate *tuplestorestate;
+
+	/*
+	 * get information from the estate and scan state
+	 */
+	estate = node->ss.ps.state;
+	direction = estate->es_direction;
+
+	tuplestorestate = node->rustate->working_table;
+
+	/*
+	 * Get the next tuple from tuplestore. Return NULL if no more tuples.
+	 */
+	slot = node->ss.ss_ScanTupleSlot;
+	(void) tuplestore_gettupleslot(tuplestorestate,
+								   ScanDirectionIsForward(direction),
+								   slot);
+	return slot;
+}
+
+/* ----------------------------------------------------------------
+ *		ExecWorkTableScan(node)
+ *
+ *		Scans the worktable sequentially and returns the next qualifying tuple.
+ *		It calls the ExecScan() routine and passes it the access method
+ *		which retrieves tuples sequentially.
+ * ----------------------------------------------------------------
+ */
+TupleTableSlot *
+ExecWorkTableScan(WorkTableScanState *node)
+{
+	/*
+	 * use WorkTableScanNext as access method
+	 */
+	return ExecScan(&node->ss, (ExecScanAccessMtd) WorkTableScanNext);
+}
+
+
+/* ----------------------------------------------------------------
+ *		ExecInitWorkTableScan
+ * ----------------------------------------------------------------
+ */
+WorkTableScanState *
+ExecInitWorkTableScan(WorkTableScan *node, EState *estate, int eflags)
+{
+	WorkTableScanState *scanstate;
+	ParamExecData *prmdata;
+
+	/* check for unsupported flags */
+	Assert(!(eflags & EXEC_FLAG_MARK));
+
+	/*
+	 * WorkTableScan should not have any children.
+	 */
+	Assert(outerPlan(node) == NULL);
+	Assert(innerPlan(node) == NULL);
+
+	/*
+	 * create new WorkTableScanState for node
+	 */
+	scanstate = makeNode(WorkTableScanState);
+	scanstate->ss.ps.plan = (Plan *) node;
+	scanstate->ss.ps.state = estate;
+
+	/*
+	 * Find the ancestor RecursiveUnion's state
+	 * via the Param slot reserved for it.
+	 */
+	prmdata = &(estate->es_param_exec_vals[node->wtParam]);
+	Assert(prmdata->execPlan == NULL);
+	Assert(!prmdata->isnull);
+	scanstate->rustate = (RecursiveUnionState *) DatumGetPointer(prmdata->value);
+	Assert(scanstate->rustate && IsA(scanstate->rustate, RecursiveUnionState));
+
+	/*
+	 * Miscellaneous initialization
+	 *
+	 * create expression context for node
+	 */
+	ExecAssignExprContext(estate, &scanstate->ss.ps);
+
+	/*
+	 * initialize child expressions
+	 */
+	scanstate->ss.ps.targetlist = (List *)
+		ExecInitExpr((Expr *) node->scan.plan.targetlist,
+					 (PlanState *) scanstate);
+	scanstate->ss.ps.qual = (List *)
+		ExecInitExpr((Expr *) node->scan.plan.qual,
+					 (PlanState *) scanstate);
+
+#define WORKTABLESCAN_NSLOTS 2
+
+	/*
+	 * tuple table initialization
+	 */
+	ExecInitResultTupleSlot(estate, &scanstate->ss.ps);
+	ExecInitScanTupleSlot(estate, &scanstate->ss);
+
+	/*
+	 * The scan tuple type (ie, the rowtype we expect to find in the work
+	 * table) is the same as the result rowtype of the ancestor RecursiveUnion
+	 * node.  Note this depends on the assumption that RecursiveUnion doesn't
+	 * allow projection.
+	 */
+	ExecAssignScanType(&scanstate->ss,
+					   ExecGetResultType(&scanstate->rustate->ps));
+
+	/*
+	 * Initialize result tuple type and projection info.
+	 */
+	ExecAssignResultTypeFromTL(&scanstate->ss.ps);
+	ExecAssignScanProjectionInfo(&scanstate->ss);
+
+	scanstate->ss.ps.ps_TupFromTlist = false;
+
+	return scanstate;
+}
+
+int
+ExecCountSlotsWorkTableScan(WorkTableScan *node)
+{
+	return ExecCountSlotsNode(outerPlan(node)) +
+		ExecCountSlotsNode(innerPlan(node)) +
+		WORKTABLESCAN_NSLOTS;
+}
+
+/* ----------------------------------------------------------------
+ *		ExecEndWorkTableScan
+ *
+ *		frees any storage allocated through C routines.
+ * ----------------------------------------------------------------
+ */
+void
+ExecEndWorkTableScan(WorkTableScanState *node)
+{
+	/*
+	 * Free exprcontext
+	 */
+	ExecFreeExprContext(&node->ss.ps);
+
+	/*
+	 * clean out the tuple table
+	 */
+	ExecClearTuple(node->ss.ps.ps_ResultTupleSlot);
+	ExecClearTuple(node->ss.ss_ScanTupleSlot);
+}
+
+/* ----------------------------------------------------------------
+ *		ExecWorkTableScanReScan
+ *
+ *		Rescans the relation.
+ * ----------------------------------------------------------------
+ */
+void
+ExecWorkTableScanReScan(WorkTableScanState *node, ExprContext *exprCtxt)
+{
+	ExecClearTuple(node->ss.ps.ps_ResultTupleSlot);
+	tuplestore_rescan(node->rustate->working_table);
+}