From: Tom Lane <tgl@sss.pgh.pa.us>
To: Jean-Luc Lachance <jllachan@nsd.ca>
Cc: pgsql-sql@postgresql.org
Subject: Re: Performance on update from join
Date: Wed, 08 May 2002 17:00:30 -0400
Message-ID: <12514.1020891630@sss.pgh.pa.us> (raw)
In-Reply-To: <3CD94F3C.4B116582@nsd.ca>
References: <20020507142203.4114.qmail@uwdvg001.cms.usa.net>
<200205081145.37254.mallah@trade-india.com>
<3CD94F3C.4B116582@nsd.ca>
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
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To: pgsql-sql@postgresql.org
Cc: tgl@sss.pgh.pa.us, jllachan@nsd.ca
Subject: Re: Performance on update from join
In-Reply-To: <12514.1020891630@sss.pgh.pa.us>
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