Fix postgres_fdw failure with whole-row Vars of type RECORD.

Commit 86dc90056 expects that FDWs can cope with whole-row Vars for
their tables, even if the Vars are marked with vartype RECORDOID.
Previously, whole-row Vars generated by the planner had vartype equal
to the relevant table's rowtype OID.  (The point behind this change is
to enable sharing of resjunk columns across inheritance child tables.)

It turns out that postgres_fdw fails to cope with this, though through
bad fortune none of its test cases exposed that.  Things mostly work,
but when we try to read back a value of such a Var, the expected
rowtype is not available to record_in().  Fortunately, it's not
difficult to hack up the tupdesc that controls this process to
substitute the foreign table's rowtype for RECORDOID.  Thus we can
solve the runtime problem while still sharing the resjunk column
with other tables.

Per report from Alexander Pyhalov.

Discussion: https://postgr.es/m/7817fb9ebd6661cdf9b67dec6e129a78@postgrespro.ru

1 files changed, 8 insertions, 0 deletions

diff --git a/contrib/postgres_fdw/sql/postgres_fdw.sql b/contrib/postgres_fdw/sql/postgres_fdw.sql
index 17dba77d7ec..79295e996dd 100644
--- a/contrib/postgres_fdw/sql/postgres_fdw.sql
+++ b/contrib/postgres_fdw/sql/postgres_fdw.sql
@@ -1255,6 +1255,14 @@ UPDATE ft2 AS target SET (c2) = (
         WHERE target.c1 = src.c1
 ) WHERE c1 > 1100;
 
+-- Test UPDATE involving a join that can be pushed down,
+-- but a SET clause that can't be
+EXPLAIN (VERBOSE, COSTS OFF)
+UPDATE ft2 d SET c2 = CASE WHEN random() >= 0 THEN d.c2 ELSE 0 END
+  FROM ft2 AS t WHERE d.c1 = t.c1 AND d.c1 > 1100;
+UPDATE ft2 d SET c2 = CASE WHEN random() >= 0 THEN d.c2 ELSE 0 END
+  FROM ft2 AS t WHERE d.c1 = t.c1 AND d.c1 > 1100;
+
 -- Test UPDATE/DELETE with WHERE or JOIN/ON conditions containing
 -- user-defined operators/functions
 ALTER SERVER loopback OPTIONS (DROP extensions);