Add infrastructure to support EphemeralNamedRelation references.

A QueryEnvironment concept is added, which allows new types of
objects to be passed into queries from parsing on through
execution.  At this point, the only thing implemented is a
collection of EphemeralNamedRelation objects -- relations which
can be referenced by name in queries, but do not exist in the
catalogs.  The only type of ENR implemented is NamedTuplestore, but
provision is made to add more types fairly easily.

An ENR can carry its own TupleDesc or reference a relation in the
catalogs by relid.

Although these features can be used without SPI, convenience
functions are added to SPI so that ENRs can easily be used by code
run through SPI.

The initial use of all this is going to be transition tables in
AFTER triggers, but that will be added to each PL as a separate
commit.

An incidental effect of this patch is to produce a more informative
error message if an attempt is made to modify the contents of a CTE
from a referencing DML statement.  No tests previously covered that
possibility, so one is added.

Kevin Grittner and Thomas Munro
Reviewed by Heikki Linnakangas, David Fetter, and Thomas Munro
with valuable comments and suggestions from many others

src/backend/executor/nodeNamedtuplestorescan.c

@@ -0,0 +1,198 @@
+/*-------------------------------------------------------------------------
+ *
+ * nodeNamedtuplestorescan.c
+ *	  routines to handle NamedTuplestoreScan nodes.
+ *
+ * Portions Copyright (c) 1996-2017, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ *
+ * IDENTIFICATION
+ *	  src/backend/executor/nodeNamedtuplestorescan.c
+ *
+ *-------------------------------------------------------------------------
+ */
+
+#include "postgres.h"
+
+#include "executor/execdebug.h"
+#include "executor/nodeNamedtuplestorescan.h"
+#include "miscadmin.h"
+#include "utils/queryenvironment.h"
+
+static TupleTableSlot *NamedTuplestoreScanNext(NamedTuplestoreScanState *node);
+
+/* ----------------------------------------------------------------
+ *		NamedTuplestoreScanNext
+ *
+ *		This is a workhorse for ExecNamedTuplestoreScan
+ * ----------------------------------------------------------------
+ */
+static TupleTableSlot *
+NamedTuplestoreScanNext(NamedTuplestoreScanState *node)
+{
+	TupleTableSlot *slot;
+
+	/* We intentionally do not support backward scan. */
+	Assert(ScanDirectionIsForward(node->ss.ps.state->es_direction));
+
+	/*
+	 * Get the next tuple from tuplestore. Return NULL if no more tuples.
+	 */
+	slot = node->ss.ss_ScanTupleSlot;
+	(void) tuplestore_gettupleslot(node->relation, true, false, slot);
+	return slot;
+}
+
+/*
+ * NamedTuplestoreScanRecheck -- access method routine to recheck a tuple in
+ * EvalPlanQual
+ */
+static bool
+NamedTuplestoreScanRecheck(NamedTuplestoreScanState *node, TupleTableSlot *slot)
+{
+	/* nothing to check */
+	return true;
+}
+
+/* ----------------------------------------------------------------
+ *		ExecNamedTuplestoreScan(node)
+ *
+ *		Scans the CTE sequentially and returns the next qualifying tuple.
+ *		We call the ExecScan() routine and pass it the appropriate
+ *		access method functions.
+ * ----------------------------------------------------------------
+ */
+TupleTableSlot *
+ExecNamedTuplestoreScan(NamedTuplestoreScanState *node)
+{
+	return ExecScan(&node->ss,
+					(ExecScanAccessMtd) NamedTuplestoreScanNext,
+					(ExecScanRecheckMtd) NamedTuplestoreScanRecheck);
+}
+
+
+/* ----------------------------------------------------------------
+ *		ExecInitNamedTuplestoreScan
+ * ----------------------------------------------------------------
+ */
+NamedTuplestoreScanState *
+ExecInitNamedTuplestoreScan(NamedTuplestoreScan *node, EState *estate, int eflags)
+{
+	NamedTuplestoreScanState *scanstate;
+	EphemeralNamedRelation enr;
+
+	/* check for unsupported flags */
+	Assert(!(eflags & (EXEC_FLAG_BACKWARD | EXEC_FLAG_MARK)));
+
+	/*
+	 * NamedTuplestoreScan should not have any children.
+	 */
+	Assert(outerPlan(node) == NULL);
+	Assert(innerPlan(node) == NULL);
+
+	/*
+	 * create new NamedTuplestoreScanState for node
+	 */
+	scanstate = makeNode(NamedTuplestoreScanState);
+	scanstate->ss.ps.plan = (Plan *) node;
+	scanstate->ss.ps.state = estate;
+
+	enr = get_ENR(estate->es_queryEnv, node->enrname);
+	if (!enr)
+		elog(ERROR, "executor could not find named tuplestore \"%s\"",
+			 node->enrname);
+
+	Assert(enr->reldata);
+	scanstate->relation = (Tuplestorestate *) enr->reldata;
+	scanstate->tupdesc = ENRMetadataGetTupDesc(&(enr->md));
+	scanstate->readptr =
+		tuplestore_alloc_read_pointer(scanstate->relation, 0);
+
+	/*
+	 * The new read pointer copies its position from read pointer 0, which
+	 * could be anywhere, so explicitly rewind it.
+	 */
+	tuplestore_rescan(scanstate->relation);
+
+	/*
+	 * XXX: Should we add a function to free that read pointer when done?
+	 * This was attempted, but it did not improve performance or memory usage
+	 * in any tested cases.
+	 */
+
+	/*
+	 * Miscellaneous initialization
+	 *
+	 * create expression context for node
+	 */
+	ExecAssignExprContext(estate, &scanstate->ss.ps);
+
+	/*
+	 * initialize child expressions
+	 */
+	scanstate->ss.ps.qual =
+		ExecInitQual(node->scan.plan.qual, (PlanState *) scanstate);
+
+	/*
+	 * tuple table initialization
+	 */
+	ExecInitResultTupleSlot(estate, &scanstate->ss.ps);
+	ExecInitScanTupleSlot(estate, &scanstate->ss);
+
+	/*
+	 * The scan tuple type is specified for the tuplestore.
+	 */
+	ExecAssignScanType(&scanstate->ss, scanstate->tupdesc);
+
+	/*
+	 * Initialize result tuple type and projection info.
+	 */
+	ExecAssignResultTypeFromTL(&scanstate->ss.ps);
+	ExecAssignScanProjectionInfo(&scanstate->ss);
+
+	return scanstate;
+}
+
+/* ----------------------------------------------------------------
+ *		ExecEndNamedTuplestoreScan
+ *
+ *		frees any storage allocated through C routines.
+ * ----------------------------------------------------------------
+ */
+void
+ExecEndNamedTuplestoreScan(NamedTuplestoreScanState *node)
+{
+	/*
+	 * Free exprcontext
+	 */
+	ExecFreeExprContext(&node->ss.ps);
+
+	/*
+	 * clean out the tuple table
+	 */
+	ExecClearTuple(node->ss.ps.ps_ResultTupleSlot);
+	ExecClearTuple(node->ss.ss_ScanTupleSlot);
+}
+
+/* ----------------------------------------------------------------
+ *		ExecReScanNamedTuplestoreScan
+ *
+ *		Rescans the relation.
+ * ----------------------------------------------------------------
+ */
+void
+ExecReScanNamedTuplestoreScan(NamedTuplestoreScanState *node)
+{
+	Tuplestorestate *tuplestorestate = node->relation;
+
+	ExecClearTuple(node->ss.ps.ps_ResultTupleSlot);
+
+	ExecScanReScan(&node->ss);
+
+	/*
+	 * Rewind my own pointer.
+	 */
+	tuplestore_select_read_pointer(tuplestorestate, node->readptr);
+	tuplestore_rescan(tuplestorestate);
+}