Support for custom slots in the custom executor nodes

Some custom table access method may have their tuple format and use custom
executor nodes for their custom scan types. The ability to set a custom slot
would save them from tuple format conversion. Other users of custom executor
nodes may also benefit.

Discussion: https://postgr.es/m/CAPpHfduJUU6ToecvTyRE_yjxTS80FyPpct4OHaLFk3OEheMTNA@mail.gmail.com
Author: Alexander Korotkov
Reviewed-by: Pavel Borisov

src/backend/executor/nodeCustom.c

@@ -29,6 +29,7 @@ CustomScanState *
 ExecInitCustomScan(CustomScan *cscan, EState *estate, int eflags)
 {
 	CustomScanState *css;
+	const TupleTableSlotOps *slotOps;
 	Relation	scan_rel = NULL;
 	Index		scanrelid = cscan->scan.scanrelid;
 	int			tlistvarno;
@@ -63,6 +64,14 @@ ExecInitCustomScan(CustomScan *cscan, EState *estate, int eflags)
 		css->ss.ss_currentRelation = scan_rel;
 	}
 
+	/*
+	 * Use a custom slot if specified in CustomScanState or use virtual slot
+	 * otherwise.
+	 */
+	slotOps = css->slotOps;
+	if (!slotOps)
+		slotOps = &TTSOpsVirtual;
+
 	/*
 	 * Determine the scan tuple type.  If the custom scan provider provided a
 	 * targetlist describing the scan tuples, use that; else use base
@@ -73,14 +82,14 @@ ExecInitCustomScan(CustomScan *cscan, EState *estate, int eflags)
 		TupleDesc	scan_tupdesc;
 
 		scan_tupdesc = ExecTypeFromTL(cscan->custom_scan_tlist);
-		ExecInitScanTupleSlot(estate, &css->ss, scan_tupdesc, &TTSOpsVirtual);
+		ExecInitScanTupleSlot(estate, &css->ss, scan_tupdesc, slotOps);
 		/* Node's targetlist will contain Vars with varno = INDEX_VAR */
 		tlistvarno = INDEX_VAR;
 	}
 	else
 	{
 		ExecInitScanTupleSlot(estate, &css->ss, RelationGetDescr(scan_rel),
-							  &TTSOpsVirtual);
+							  slotOps);
 		/* Node's targetlist will contain Vars with varno = scanrelid */
 		tlistvarno = scanrelid;
 	}

src/include/nodes/execnodes.h

@@ -1949,6 +1949,7 @@ typedef struct CustomScanState
 	List	   *custom_ps;		/* list of child PlanState nodes, if any */
 	Size		pscan_len;		/* size of parallel coordination information */
 	const struct CustomExecMethods *methods;
+	const struct TupleTableSlotOps *slotOps;
 } CustomScanState;
 
 /* ----------------------------------------------------------------