2011/10/19 Tom Lane <[email protected]>:
> Robert Haas <[email protected]> writes:
>> On Sun, Oct 16, 2011 at 4:46 AM, Kohei KaiGai <[email protected]> wrote:
>>> I tried to reproduce the scenario with enough small from/join_collapse_limit
>>> (typically 1), but it allows to push down qualifiers into the least scan
>>> plan.
>
>> Hmm, you're right. LIMIT 1000000000 prevents qual pushdown, but
>> hitting from_collapse_limit/join_collapse_limit apparently doesn't. I
>> could have sworn I've seen this work the other way, but I guess not.
>
> No, the collapse_limit variables are entirely unrelated to subquery
> flattening, or to qual pushdown for that matter. They only restrict the
> number of join paths we consider. And we will attempt to push down
> quals into an unflattened subquery, too, if it looks safe. See
> subquery_is_pushdown_safe, qual_is_pushdown_safe, etc in allpaths.c.
>
I tried to observe the behavior with a bit modification of is_simple_subquery
that become to return 'false' always.
(It is a simulation if and when a view with security_barrier would be given.)
The expected behavior is to keep sub-query without flatten.
However, the externally provided qualifiers are correctly pushed down.
Do we need to focus on the code around above functions rather than
distribute_qual_to_rels, to prevent undesirable pushing-down across
security barrier?
postgres=# CREATE VIEW v1 AS SELECT * FROM t1 WHERE a > 100;
CREATE VIEW
postgres=# CREATE VIEW v2 AS SELECT * FROM t2 JOIN t3 ON x = s;
CREATE VIEW
postgres=# EXPLAIN SELECT * FROM v1 WHERE b = 'bbb';
QUERY PLAN
----------------------------------------------------
Seq Scan on t1 (cost=0.00..28.45 rows=2 width=36)
Filter: ((a > 100) AND (b = 'bbb'::text))
(2 rows)
postgres=# EXPLAIN SELECT * FROM v2 WHERE t = 'ttt';
QUERY PLAN
----------------------------------------------------------------
Hash Join (cost=25.45..52.73 rows=37 width=72)
Hash Cond: (t2.x = t3.s)
-> Seq Scan on t2 (cost=0.00..22.30 rows=1230 width=36)
-> Hash (cost=25.38..25.38 rows=6 width=36)
-> Seq Scan on t3 (cost=0.00..25.38 rows=6 width=36)
Filter: (t = 'ttt'::text)
(6 rows)
Thanks,
--
KaiGai Kohei <[email protected]>
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