From: | Robert Haas <robertmhaas(at)gmail(dot)com> |
---|---|
To: | Andres Freund <andres(at)anarazel(dot)de> |
Cc: | "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: Quals not pushed down into lateral |
Date: | 2017-04-13 20:34:12 |
Message-ID: | CA+TgmoZk=ubtVi9Ym6i6AOYyLObiM2NvvpKjtFG_WA+g=2H9=g@mail.gmail.com |
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On Thu, Mar 16, 2017 at 4:45 AM, Andres Freund <andres(at)anarazel(dot)de> wrote:
> During citus development we noticed that restrictions aren't pushed down
> into lateral subqueries, even if they semantically could. For example,
> in this dumbed down example:
>
> postgres[31776][1]=# CREATE TABLE t_2(id serial primary key);
> postgres[31776][1]=# CREATE TABLE t_1(id serial primary key);
>
> Comparing:
>
> postgres[31776][1]=# EXPLAIN SELECT * FROM t_1 JOIN (SELECT * FROM t_2 GROUP BY id) s ON (t_1.id = s.id) WHERE t_1.id = 3;
> postgres[31776][1]=# EXPLAIN SELECT * FROM t_1, LATERAL (SELECT * FROM t_2 WHERE t_1.id = t_2.id GROUP BY id) s WHERE t_1.id = 3;
Interesting. That does seem like we are missing a trick.
Not exactly related, but I think we need to improve optimization
around CTEs, too. AFAICT, what we've got right now, almost everybody
hates.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
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