Introduce custom path and scan providers.

This allows extension modules to define their own methods for
scanning a relation, and get the core code to use them.  It's
unclear as yet how much use this capability will find, but we
won't find out if we never commit it.

KaiGai Kohei, reviewed at various times and in various levels
of detail by Shigeru Hanada, Tom Lane, Andres Freund, Álvaro
Herrera, and myself.

src/backend/executor/nodeCustom.c

@@ -0,0 +1,126 @@
+/* ------------------------------------------------------------------------
+ *
+ * nodeCustom.c
+ *		Routines to handle execution of custom scan node
+ *
+ * Portions Copyright (c) 1996-2014, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * ------------------------------------------------------------------------
+ */
+#include "postgres.h"
+
+#include "executor/executor.h"
+#include "executor/nodeCustom.h"
+#include "nodes/execnodes.h"
+#include "nodes/plannodes.h"
+#include "parser/parsetree.h"
+#include "utils/hsearch.h"
+#include "utils/memutils.h"
+#include "utils/rel.h"
+
+CustomScanState *
+ExecInitCustomScan(CustomScan *cscan, EState *estate, int eflags)
+{
+	CustomScanState    *css;
+	Relation			scan_rel;
+
+	/* populate a CustomScanState according to the CustomScan */
+	css = (CustomScanState *) cscan->methods->CreateCustomScanState(cscan);
+	Assert(IsA(css, CustomScanState));
+
+	/* fill up fields of ScanState */
+	css->ss.ps.plan = &cscan->scan.plan;
+	css->ss.ps.state = estate;
+
+	/* create expression context for node */
+	ExecAssignExprContext(estate, &css->ss.ps);
+
+	/* initialize child expressions */
+	css->ss.ps.targetlist = (List *)
+		ExecInitExpr((Expr *) cscan->scan.plan.targetlist,
+					 (PlanState *) css);
+	css->ss.ps.qual = (List *)
+		ExecInitExpr((Expr *) cscan->scan.plan.qual,
+					 (PlanState *) css);
+
+	/* tuple table initialization */
+	ExecInitScanTupleSlot(estate, &css->ss);
+	ExecInitResultTupleSlot(estate, &css->ss.ps);
+
+	/* initialize scan relation */
+	scan_rel = ExecOpenScanRelation(estate, cscan->scan.scanrelid, eflags);
+	css->ss.ss_currentRelation = scan_rel;
+	css->ss.ss_currentScanDesc = NULL;	/* set by provider */
+	ExecAssignScanType(&css->ss, RelationGetDescr(scan_rel));
+
+	css->ss.ps.ps_TupFromTlist = false;
+
+	/*
+	 * Initialize result tuple type and projection info.
+	 */
+	ExecAssignResultTypeFromTL(&css->ss.ps);
+	ExecAssignScanProjectionInfo(&css->ss);
+
+	/*
+	 * The callback of custom-scan provider applies the final initialization
+	 * of the custom-scan-state node according to its logic.
+	 */
+	css->methods->BeginCustomScan(css, estate, eflags);
+
+	return css;
+}
+
+TupleTableSlot *
+ExecCustomScan(CustomScanState *node)
+{
+	Assert(node->methods->ExecCustomScan != NULL);
+	return node->methods->ExecCustomScan(node);
+}
+
+void
+ExecEndCustomScan(CustomScanState *node)
+{
+	Assert(node->methods->EndCustomScan != NULL);
+	node->methods->EndCustomScan(node);
+
+	/* Free the exprcontext */
+	ExecFreeExprContext(&node->ss.ps);
+
+	/* Clean out the tuple table */
+	ExecClearTuple(node->ss.ps.ps_ResultTupleSlot);
+	if (node->ss.ss_ScanTupleSlot)
+		ExecClearTuple(node->ss.ss_ScanTupleSlot);
+
+	/* Close the heap relation */
+	ExecCloseScanRelation(node->ss.ss_currentRelation);
+}
+
+void
+ExecReScanCustomScan(CustomScanState *node)
+{
+	Assert(node->methods->ReScanCustomScan != NULL);
+	node->methods->ReScanCustomScan(node);
+}
+
+void
+ExecCustomMarkPos(CustomScanState *node)
+{
+	if (!node->methods->MarkPosCustomScan)
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 errmsg("custom-scan \"%s\" does not support MarkPos",
+						node->methods->CustomName)));
+	node->methods->MarkPosCustomScan(node);
+}
+
+void
+ExecCustomRestrPos(CustomScanState *node)
+{
+	if (!node->methods->RestrPosCustomScan)
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 errmsg("custom-scan \"%s\" does not support MarkPos",
+						node->methods->CustomName)));
+	node->methods->RestrPosCustomScan(node);
+}