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Re: partitioning question 1

From: "Joshua D(dot) Drake" <jd(at)commandprompt(dot)com>
To: Ben <midfield(at)gmail(dot)com>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: partitioning question 1
Date: 2010-10-28 19:44:12
Message-ID: 1288295052.22359.35.camel@jd-desktop (view raw, whole thread or download thread mbox)
Lists: pgsql-performance
On Thu, 2010-10-28 at 12:25 -0700, Ben wrote:

> i think we are talking about two different things here: the constraints on the table, and the where-clause constraints in a query which may or may not trigger constraint exclusion.  i understand that table constraints have to be constants -- it doesn't make much sense otherwise.  what i am wondering about is, will constraint exclusion be triggered for queries where the column that is being partitioned on is being constrained things that are not static constants, for instance, in a join.  (i'm pretty sure the answer is no, because i think constraint exclusion happens before real query planning.)  a concrete example :
> create table foo (i integer not null, j float not null);
> create table foo_1 (check ( i >= 0 and i < 10) ) inherits (foo);
> create table foo_2 (check ( i >= 10 and i < 20) ) inherits (foo);
> create table foo_3 (check ( i >= 20 and i < 30) ) inherits (foo);
> etc..
> create table bar (i integer not null, k float not null);
> my understanding is that a query like
> select * from foo, bar using (i);
> can't use constraint exclusion, even if the histogram of i-values on table bar says they only live in the range 0-9, and so the query will touch all of the tables.  i think this is not favorable compared to a single foo table with a well-maintained btree index on i.

My tests show you are incorrect:

part_test=# explain analyze select * from foo join bar using (i) where
 Nested Loop  (cost=34.26..106.76 rows=200 width=20) (actual
time=0.004..0.004 rows=0 loops=1)
   ->  Append  (cost=0.00..68.50 rows=20 width=12) (actual
time=0.004..0.004 rows=0 loops=1)
         ->  Seq Scan on foo  (cost=0.00..34.25 rows=10 width=12)
(actual time=0.001..0.001 rows=0 loops=1)
               Filter: (i = 9)
         ->  Seq Scan on foo_1 foo  (cost=0.00..34.25 rows=10 width=12)
(actual time=0.000..0.000 rows=0 loops=1)
               Filter: (i = 9)
   ->  Materialize  (cost=34.26..34.36 rows=10 width=12) (never
         ->  Seq Scan on bar  (cost=0.00..34.25 rows=10 width=12) (never
               Filter: (i = 9)
 Total runtime: 0.032 ms
(10 rows)

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