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, 20 insertions, 11 deletions

diff --git a/src/backend/executor/nodeForeignscan.c b/src/backend/executor/nodeForeignscan.c
index 7399053ae7d..fa553ace5d6 100644
--- a/src/backend/executor/nodeForeignscan.c
+++ b/src/backend/executor/nodeForeignscan.c
@@ -102,7 +102,7 @@ ForeignScanState *
 ExecInitForeignScan(ForeignScan *node, EState *estate, int eflags)
 {
 	ForeignScanState *scanstate;
-	Relation	currentRelation;
+	Index		scanrelid = node->scan.scanrelid;
 	FdwRoutine *fdwroutine;
 
 	/* check for unsupported flags */
@@ -141,16 +141,24 @@ ExecInitForeignScan(ForeignScan *node, EState *estate, int eflags)
 	ExecInitScanTupleSlot(estate, &scanstate->ss);
 
 	/*
-	 * open the base relation and acquire appropriate lock on it.
+	 * open the base relation and acquire an appropriate lock on it;
+	 * also, get and assign the scan type
 	 */
-	currentRelation = ExecOpenScanRelation(estate, node->scan.scanrelid, eflags);
-	scanstate->ss.ss_currentRelation = currentRelation;
+	if (scanrelid > 0)
+	{
+		Relation	currentRelation;
 
-	/*
-	 * get the scan type from the relation descriptor.  (XXX at some point we
-	 * might want to let the FDW editorialize on the scan tupdesc.)
-	 */
-	ExecAssignScanType(&scanstate->ss, RelationGetDescr(currentRelation));
+		currentRelation = ExecOpenScanRelation(estate, scanrelid, eflags);
+		scanstate->ss.ss_currentRelation = currentRelation;
+		ExecAssignScanType(&scanstate->ss, RelationGetDescr(currentRelation));
+	}
+	else
+	{
+		TupleDesc	ps_tupdesc;
+
+		ps_tupdesc = ExecCleanTypeFromTL(node->fdw_ps_tlist, false);
+		ExecAssignScanType(&scanstate->ss, ps_tupdesc);
+	}
 
 	/*
 	 * Initialize result tuple type and projection info.
@@ -161,7 +169,7 @@ ExecInitForeignScan(ForeignScan *node, EState *estate, int eflags)
 	/*
 	 * Acquire function pointers from the FDW's handler, and init fdw_state.
 	 */
-	fdwroutine = GetFdwRoutineForRelation(currentRelation, true);
+	fdwroutine = GetFdwRoutine(node->fdw_handler);
 	scanstate->fdwroutine = fdwroutine;
 	scanstate->fdw_state = NULL;
 
@@ -193,7 +201,8 @@ ExecEndForeignScan(ForeignScanState *node)
 	ExecClearTuple(node->ss.ss_ScanTupleSlot);
 
 	/* close the relation. */
-	ExecCloseScanRelation(node->ss.ss_currentRelation);
+	if (node->ss.ss_currentRelation)
+		ExecCloseScanRelation(node->ss.ss_currentRelation);
 }
 
 /* ----------------------------------------------------------------