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, 6 insertions, 0 deletions

diff --git a/src/backend/executor/execScan.c b/src/backend/executor/execScan.c
index 3f0d8093875..fa475014f13 100644
--- a/src/backend/executor/execScan.c
+++ b/src/backend/executor/execScan.c
@@ -251,6 +251,12 @@ ExecAssignScanProjectionInfo(ScanState *node)
 	/* Vars in an index-only scan's tlist should be INDEX_VAR */
 	if (IsA(scan, IndexOnlyScan))
 		varno = INDEX_VAR;
+	/* Also foreign or custom scan on pseudo relation should be INDEX_VAR */
+	else if (scan->scanrelid == 0)
+	{
+		Assert(IsA(scan, ForeignScan) || IsA(scan, CustomScan));
+		varno = INDEX_VAR;
+	}
 	else
 		varno = scan->scanrelid;