Allow FDWs and custom scan providers to replace joins with scans.

Foreign data wrappers can use this capability for so-called "join
pushdown"; that is, instead of executing two separate foreign scans
and then joining the results locally, they can generate a path which
performs the join on the remote server and then is scanned locally.
This commit does not extend postgres_fdw to take advantage of this
capability; it just provides the infrastructure.

Custom scan providers can use this in a similar way.  Previously,
it was only possible for a custom scan provider to scan a single
relation.  Now, it can scan an entire join tree, provided of course
that it knows how to produce the same results that the join would
have produced if executed normally.

KaiGai Kohei, reviewed by Shigeru Hanada, Ashutosh Bapat, and me.

1 files changed, 24 insertions, 10 deletions

diff --git a/src/backend/executor/nodeCustom.c b/src/backend/executor/nodeCustom.c
index b07932b32e5..db1b4f2ffa4 100644
--- a/src/backend/executor/nodeCustom.c
+++ b/src/backend/executor/nodeCustom.c
@@ -23,7 +23,7 @@ CustomScanState *
 ExecInitCustomScan(CustomScan *cscan, EState *estate, int eflags)
 {
 	CustomScanState    *css;
-	Relation			scan_rel;
+	Index				scan_relid = cscan->scan.scanrelid;
 
 	/* populate a CustomScanState according to the CustomScan */
 	css = (CustomScanState *) cscan->methods->CreateCustomScanState(cscan);
@@ -48,12 +48,26 @@ ExecInitCustomScan(CustomScan *cscan, EState *estate, int eflags)
 	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));
-
+	/*
+	 * open the base relation and acquire an appropriate lock on it;
+	 * also, get and assign the scan type
+	 */
+	if (scan_relid > 0)
+	{
+		Relation		scan_rel;
+
+		scan_rel = ExecOpenScanRelation(estate, scan_relid, eflags);
+		css->ss.ss_currentRelation = scan_rel;
+		css->ss.ss_currentScanDesc = NULL;	/* set by provider */
+		ExecAssignScanType(&css->ss, RelationGetDescr(scan_rel));
+	}
+	else
+	{
+		TupleDesc	ps_tupdesc;
+
+		ps_tupdesc = ExecCleanTypeFromTL(cscan->custom_ps_tlist, false);
+		ExecAssignScanType(&css->ss, ps_tupdesc);
+	}
 	css->ss.ps.ps_TupFromTlist = false;
 
 	/*
@@ -89,11 +103,11 @@ ExecEndCustomScan(CustomScanState *node)
 
 	/* Clean out the tuple table */
 	ExecClearTuple(node->ss.ps.ps_ResultTupleSlot);
-	if (node->ss.ss_ScanTupleSlot)
-		ExecClearTuple(node->ss.ss_ScanTupleSlot);
+	ExecClearTuple(node->ss.ss_ScanTupleSlot);
 
 	/* Close the heap relation */
-	ExecCloseScanRelation(node->ss.ss_currentRelation);
+	if (node->ss.ss_currentRelation)
+		ExecCloseScanRelation(node->ss.ss_currentRelation);
 }
 
 void