Re: Performance on update from join

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=1000000width=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_pkeyon 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