Fix mishandling of system columns in FDW queries.

postgres_fdw would send query conditions involving system columns to the
remote server, even though it makes no effort to ensure that system
columns other than CTID match what the remote side thinks.  tableoid,
in particular, probably won't match and might have some use in queries.
Hence, prevent sending conditions that include non-CTID system columns.

Also, create_foreignscan_plan neglected to check local restriction
conditions while determining whether to set fsSystemCol for a foreign
scan plan node.  This again would bollix the results for queries that
test a foreign table's tableoid.

Back-patch the first fix to 9.3 where postgres_fdw was introduced.
Back-patch the second to 9.2.  The code is probably broken in 9.1 as
well, but the patch doesn't apply cleanly there; given the weak state
of support for FDWs in 9.1, it doesn't seem worth fixing.

Etsuro Fujita, reviewed by Ashutosh Bapat, and somewhat modified by me

contrib/postgres_fdw/deparse.c

@@ -254,6 +254,18 @@ foreign_expr_walker(Node *node,
 					var->varlevelsup == 0)
 				{
 					/* Var belongs to foreign table */
+
+					/*
+					 * System columns other than ctid should not be sent to
+					 * the remote, since we don't make any effort to ensure
+					 * that local and remote values match (tableoid, in
+					 * particular, almost certainly doesn't match).
+					 */
+					if (var->varattno < 0 &&
+						var->varattno != SelfItemPointerAttributeNumber)
+						return false;
+
+					/* Else check the collation */
 					collation = var->varcollid;
 					state = OidIsValid(collation) ? FDW_COLLATE_SAFE : FDW_COLLATE_NONE;
 				}