Support data-modifying commands (INSERT/UPDATE/DELETE) in WITH.

This patch implements data-modifying WITH queries according to the
semantics that the updates all happen with the same command counter value,
and in an unspecified order.  Therefore one WITH clause can't see the
effects of another, nor can the outer query see the effects other than
through the RETURNING values.  And attempts to do conflicting updates will
have unpredictable results.  We'll need to document all that.

This commit just fixes the code; documentation updates are waiting on
author.

Marko Tiikkaja and Hitoshi Harada

src/include/nodes/execnodes.h

@@ -346,7 +346,7 @@ typedef struct EState
 	/* If query can insert/delete tuples, the command ID to mark them with */
 	CommandId	es_output_cid;
 
-	/* Info about target table for insert/update/delete queries: */
+	/* Info about target table(s) for insert/update/delete queries: */
 	ResultRelInfo *es_result_relations; /* array of ResultRelInfos */
 	int			es_num_result_relations;		/* length of array */
 	ResultRelInfo *es_result_relation_info;		/* currently active array elt */
@@ -378,6 +378,8 @@ typedef struct EState
 
 	List	   *es_subplanstates;		/* List of PlanState for SubPlans */
 
+	List	   *es_auxmodifytables;	/* List of secondary ModifyTableStates */
+
 	/*
 	 * this ExprContext is for per-output-tuple operations, such as constraint
 	 * checks and index-value computations.  It will be reset for each output
@@ -1041,10 +1043,13 @@ typedef struct ResultState
 typedef struct ModifyTableState
 {
 	PlanState	ps;				/* its first field is NodeTag */
-	CmdType		operation;
+	CmdType		operation;		/* INSERT, UPDATE, or DELETE */
+	bool		canSetTag;		/* do we set the command tag/es_processed? */
+	bool		mt_done;		/* are we done? */
 	PlanState **mt_plans;		/* subplans (one per target rel) */
 	int			mt_nplans;		/* number of plans in the array */
 	int			mt_whichplan;	/* which one is being executed (0..n-1) */
+	ResultRelInfo *resultRelInfo;	/* per-subplan target relations */
 	List	  **mt_arowmarks;	/* per-subplan ExecAuxRowMark lists */
 	EPQState	mt_epqstate;	/* for evaluating EvalPlanQual rechecks */
 	bool		fireBSTriggers; /* do we need to fire stmt triggers? */