Re: Postgres_fdw join pushdown - wrong results with whole-row reference

On 2016/06/22 17:11, Amit Langote wrote:

I wonder whether such a whole-row-var would arise from the nullable side
of a join? I guess not.  Not that I'm saying we shouldn't account for that
case at all since any and every whole-row-var in the targetlist currently
gets that treatment, even those that are known non-nullable. Couldn't we
have prevented the latter somehow?  IOW, only generate the CASE WHEN when
a Var being deparsed is known nullable as the comment there says:

deparse.c:

1639 /*
1640  * In case the whole-row reference is under an outer join then it has
1641  * to go NULL whenver the rest of the row goes NULL. Deparsing a join
1642  * query would always involve multiple relations, thus qualify_col
1643  * would be true.
1644  */
1645 if (qualify_col)
1646 {
1647     appendStringInfoString(buf, "CASE WHEN");
1648     ADD_REL_QUALIFIER(buf, varno);
1649     appendStringInfo(buf, "* IS NOT NULL THEN ");
1650 }

I think we could address this in another way once we support deparsing subqueries; rewrite the remote query into something that wouldn't need the CASE WHEN conversion.  For example, we currently have:

postgres=# explain verbose select ft2 from ft1 left join ft2 on ft1.a = ft2.a;
                                                                QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------------------
 Foreign Scan  (cost=100.00..110.04 rows=1 width=32)
   Output: ft2.*
   Relations: (public.ft1) LEFT JOIN (public.ft2)
   Remote SQL: SELECT CASE WHEN r2.* IS NOT NULL THEN ROW(r2.a, r2.b) END FROM (public.t1 r1 LEFT JOIN public.t2 r2 ON (((r1.a = r2.a))))
(4 rows)

However, if we support deparsing subqueries, the remote query in the above example could be rewritten into something like this:

SELECT ss.c2 FROM t1 LEFT JOIN (SELECT t2.a, ROW(a, b) FROM t2) ss(c1, c2) ON (t1.a = ss.c1);

So we would no longer need "CASE WHEN r2.* IS NOT NULL THEN ROW(r2.a, r2.b) END" in the target list in the remote query.

For the CASE WHEN conversion for a system column other than ctid, we could also address this by replacing the whole-row reference in the IS NOT NULL condition in that conversion with the system column reference.