postgres_fdw: Fix incorrect NULL handling in join pushdown.

something.* IS NOT NULL means that every attribute of the row is not
NULL, not that the row itself is non-NULL (e.g. because it's coming
from below an outer join.  Use (somevar.*)::pg_catalog.text IS NOT
NULL instead.

Ashutosh Bapat, per a report by Rushabh Lathia.  Reviewed by
Amit Langote and Etsuro Fujita.  Schema-qualification added by me.

contrib/postgres_fdw/deparse.c

@@ -1599,9 +1599,9 @@ deparseColumnRef(StringInfo buf, int varno, int varattno, PlannerInfo *root,
 
 		if (qualify_col)
 		{
-			appendStringInfoString(buf, "CASE WHEN ");
+			appendStringInfoString(buf, "CASE WHEN (");
 			ADD_REL_QUALIFIER(buf, varno);
-			appendStringInfo(buf, "* IS NOT NULL THEN %u END", fetchval);
+			appendStringInfo(buf, "*)::pg_catalog.text IS NOT NULL THEN %u END", fetchval);
 		}
 		else
 			appendStringInfo(buf, "%u", fetchval);
@@ -1643,9 +1643,9 @@ deparseColumnRef(StringInfo buf, int varno, int varattno, PlannerInfo *root,
 		 */
 		if (qualify_col)
 		{
-			appendStringInfoString(buf, "CASE WHEN ");
+			appendStringInfoString(buf, "CASE WHEN (");
 			ADD_REL_QUALIFIER(buf, varno);
-			appendStringInfo(buf, "* IS NOT NULL THEN ");
+			appendStringInfo(buf, "*)::pg_catalog.text IS NOT NULL THEN ");
 		}
 
 		appendStringInfoString(buf, "ROW(");

contrib/postgres_fdw/sql/postgres_fdw.sql

@@ -543,6 +543,11 @@ SELECT ft4.c1, q.* FROM ft4 LEFT JOIN (SELECT 13, ft1.c1, ft2.c1 FROM ft1 RIGHT
 CREATE USER MAPPING FOR CURRENT_USER SERVER loopback;
 DROP USER MAPPING FOR PUBLIC SERVER loopback;
 
+-- join with nullable side with some columns with null values
+UPDATE ft5 SET c3 = null where c1 % 9 = 0;
+EXPLAIN VERBOSE SELECT ft5, ft5.c1, ft5.c2, ft5.c3, ft4.c1, ft4.c2 FROM ft5 left join ft4 on ft5.c1 = ft4.c1 WHERE ft4.c1 BETWEEN 10 and 30 ORDER BY ft5.c1, ft4.c1;
+SELECT ft5, ft5.c1, ft5.c2, ft5.c3, ft4.c1, ft4.c2 FROM ft5 left join ft4 on ft5.c1 = ft4.c1 WHERE ft4.c1 BETWEEN 10 and 30 ORDER BY ft5.c1, ft4.c1;
+
 -- ===================================================================
 -- parameterized queries
 -- ===================================================================