Message-ID: <3CD99D0B.DA4EA415@nsd.ca>
Date: Wed, 08 May 2002 17:47:55 -0400
From: Jean-Luc Lachance <jllachan@nsd.ca>
MIME-Version: 1.0
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: pgsql-sql@postgresql.org
Subject: Re: Performance on update from join
References: <20020507142203.4114.qmail@uwdvg001.cms.usa.net>
	<200205081145.37254.mallah@trade-india.com>
	<3CD94F3C.4B116582@nsd.ca> <12514.1020891630@sss.pgh.pa.us>
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Tom,

table the "c" is not implicated in your test query.

Try:

create table a (a1 int primary key, a2 int, a3 int, a4 int);
create table b (b1 int, b2 int, b3 int, b4 int, primary key (b1, b2));
create table d (d1 int, d2 int, d3 int, d4 int, primary key (d1, d2));
explain update a set a4 = d.d2 from b,d where a.a2 = b.b1 and a.a3 =
b.b2 and
 b.b3 = d.d1 and b.b4 = d.d2 and a.a4 >= d.d3 and a.a4 <= d.d4;

Which is closer to what I have.

            +-----------------------------------
            |                                  /\
A1  A2  A3  A4     B1  B2  B3  B4    D1  D2  D3  D4
    |   |          |   |   |   |     |   |
    +--------------+   |   +---------+   |
        |              |       |         |
        +--------------+       +---------+


Tom Lane wrote:
> 
> Jean-Luc Lachance <jllachan@nsd.ca> writes:
> > I was exploring ways to improve the time required to update a large
> > table from the join of two others as in:
> > UPDATE a FROM b, c;
> > I found that whatever index I create, compound or not, PG insist on
> > performing the cartesian product first.
> 
> Surely not.
> 
> test=# create table a (f1 int primary key, f2 int);
> NOTICE:  CREATE TABLE / PRIMARY KEY will create implicit index 'a_pkey' for table 'a'
> CREATE
> test=# create table b (f1 int primary key, f2 int);
> NOTICE:  CREATE TABLE / PRIMARY KEY will create implicit index 'b_pkey' for table 'b'
> CREATE
> test=# create table c (f1 int primary key, f2 int);
> NOTICE:  CREATE TABLE / PRIMARY KEY will create implicit index 'c_pkey' for table 'c'
> CREATE
> 
> test=# explain update a set f2 = a.f2 + 1 from b,c where a.f1 = b.f1;
>                                    QUERY PLAN
> 
> ---------------------------------------------------------------------------------
>  Nested Loop  (cost=0.00..30121.50 rows=1000000 width=18)
>    ->  Merge Join  (cost=0.00..121.50 rows=1000 width=18)
>          Merge Cond: ("outer".f1 = "inner".f1)
>          ->  Index Scan using a_pkey on a  (cost=0.00..52.00 rows=1000 width=14)
>          ->  Index Scan using b_pkey on b  (cost=0.00..52.00 rows=1000 width=4)
>    ->  Seq Scan on c  (cost=0.00..20.00 rows=1000 width=0)
> (6 rows)
> 
> The target table doesn't have any special status in the planner's
> consideration of possible join paths.  So whatever problem you are
> having, it's not that.  How about providing a more complete description
> of your tables and query?
> 
>                         regards, tom lane