Include result relation info in direct modify ForeignScan nodes.

FDWs that can perform an UPDATE/DELETE remotely using the "direct modify"
set of APIs need to access the ResultRelInfo of the target table. That's
currently available in EState.es_result_relation_info, but the next
commit will remove that field.

This commit adds a new resultRelation field in ForeignScan, to store the
target relation's RT index, and the corresponding ResultRelInfo in
ForeignScanState. The FDW's PlanDirectModify callback is expected to set
'resultRelation' along with 'operation'. The core code doesn't need them
for anything, they are for the convenience of FDW's Begin- and
IterateDirectModify callbacks.

Authors: Amit Langote, Etsuro Fujita
Discussion: https://www.postgresql.org/message-id/CA%2BHiwqGEmiib8FLiHMhKB%2BCH5dRgHSLc5N5wnvc4kym%2BZYpQEQ%40mail.gmail.com

src/backend/executor/nodeForeignscan.c

@@ -215,6 +215,13 @@ ExecInitForeignScan(ForeignScan *node, EState *estate, int eflags)
 	scanstate->fdwroutine = fdwroutine;
 	scanstate->fdw_state = NULL;
 
+	/*
+	 * For the FDW's convenience, look up the modification target relation's.
+	 * ResultRelInfo.
+	 */
+	if (node->resultRelation > 0)
+		scanstate->resultRelInfo = estate->es_result_relations[node->resultRelation - 1];
+
 	/* Initialize any outer plan. */
 	if (outerPlan(node))
 		outerPlanState(scanstate) =

src/backend/nodes/copyfuncs.c

@@ -758,6 +758,7 @@ _copyForeignScan(const ForeignScan *from)
 	COPY_NODE_FIELD(fdw_recheck_quals);
 	COPY_BITMAPSET_FIELD(fs_relids);
 	COPY_SCALAR_FIELD(fsSystemCol);
+	COPY_SCALAR_FIELD(resultRelation);
 
 	return newnode;
 }

src/backend/nodes/outfuncs.c

@@ -695,6 +695,7 @@ _outForeignScan(StringInfo str, const ForeignScan *node)
 	WRITE_NODE_FIELD(fdw_recheck_quals);
 	WRITE_BITMAPSET_FIELD(fs_relids);
 	WRITE_BOOL_FIELD(fsSystemCol);
+	WRITE_INT_FIELD(resultRelation);
 }
 
 static void

src/backend/nodes/readfuncs.c

@@ -2014,6 +2014,7 @@ _readForeignScan(void)
 	READ_NODE_FIELD(fdw_recheck_quals);
 	READ_BITMAPSET_FIELD(fs_relids);
 	READ_BOOL_FIELD(fsSystemCol);
+	READ_INT_FIELD(resultRelation);
 
 	READ_DONE();
 }

src/backend/optimizer/plan/createplan.c

@@ -5530,7 +5530,11 @@ make_foreignscan(List *qptlist,
 	plan->lefttree = outer_plan;
 	plan->righttree = NULL;
 	node->scan.scanrelid = scanrelid;
+
+	/* these may be overridden by the FDW's PlanDirectModify callback. */
 	node->operation = CMD_SELECT;
+	node->resultRelation = 0;
+
 	/* fs_server will be filled in by create_foreignscan_plan */
 	node->fs_server = InvalidOid;
 	node->fdw_exprs = fdw_exprs;

src/backend/optimizer/plan/setrefs.c

@@ -1310,6 +1310,10 @@ set_foreignscan_references(PlannerInfo *root,
 	}
 
 	fscan->fs_relids = offset_relid_set(fscan->fs_relids, rtoffset);
+
+	/* Adjust resultRelation if it's valid */
+	if (fscan->resultRelation > 0)
+		fscan->resultRelation += rtoffset;
 }
 
 /*

src/include/nodes/execnodes.h

@@ -1777,6 +1777,7 @@ typedef struct ForeignScanState
 	ScanState	ss;				/* its first field is NodeTag */
 	ExprState  *fdw_recheck_quals;	/* original quals not in ss.ps.qual */
 	Size		pscan_len;		/* size of parallel coordination information */
+	ResultRelInfo *resultRelInfo;	/* result rel info, if UPDATE or DELETE */
 	/* use struct pointer to avoid including fdwapi.h here */
 	struct FdwRoutine *fdwroutine;
 	void	   *fdw_state;		/* foreign-data wrapper can keep state here */

src/include/nodes/plannodes.h

@@ -599,12 +599,20 @@ typedef struct WorkTableScan
  * When the plan node represents a foreign join, scan.scanrelid is zero and
  * fs_relids must be consulted to identify the join relation.  (fs_relids
  * is valid for simple scans as well, but will always match scan.scanrelid.)
+ *
+ * If the FDW's PlanDirectModify() callback decides to repurpose a ForeignScan
+ * node to perform the UPDATE or DELETE operation directly in the remote
+ * server, it sets 'operation' and 'resultRelation' to identify the operation
+ * type and target relation.  Note that these fields are only set if the
+ * modification is performed *fully* remotely; otherwise, the modification is
+ * driven by a local ModifyTable node and 'operation' is left to CMD_SELECT.
  * ----------------
  */
 typedef struct ForeignScan
 {
 	Scan		scan;
 	CmdType		operation;		/* SELECT/INSERT/UPDATE/DELETE */
+	Index		resultRelation; /* direct modification target's RT index */
 	Oid			fs_server;		/* OID of foreign server */
 	List	   *fdw_exprs;		/* expressions that FDW may evaluate */
 	List	   *fdw_private;	/* private data for FDW */