Sorry... the example was incomplete. All the fields are defined as not-null. So it is guaranteed to always match the join. And PostgreSQL release notes claim that PGSQL can do at least partial join removal: https://wiki.postgresql.org/wiki/What's_new_in_PostgreSQL_9.0#Join_Removal I was hoping this use case would fit in. Any suggestions? Igor -----Original Message----- From: Merlin Moncure [mailto:mmoncure@xxxxxxxxx] Sent: Friday, July 01, 2016 12:42 PM To: Sfiligoi, Igor <Igor.Sfiligoi@xxxxxx> Cc: pgsql-general@xxxxxxxxxxxxxx Subject: --EXTERNAL--Re: PSQL does not remove obvious useless joins On Fri, Jul 1, 2016 at 12:17 PM, Sfiligoi, Igor <Igor.Sfiligoi@xxxxxx> wrote: > Hello. > > We have a view that is very generic, and we noticed that PostgreSQL is > not very good at removing useless joins, which makes our queries very slow. > > We could change our code to avoid the view and write ad-hoc queries to > the underlying tables, but would prefer not to, if there is a way around it. > > (BTW: We are currently using psql 9.4) > > Here is a simplified implementation: > > # create table a (id int primary key, name varchar(128)); > > # create table b (id int primary key, name varchar(128)); > > # create table c (id int primary key, a_id int references a(id), b1_id > int references b(id), b2_id int references b(id), b3_id int references > b(id)); > > # create view v as select c.id, c.a_id, c.b1_id, c.b2_id , c.b3_id, > a.name a_name, b1.name b1_name, b2.name b2_name, b3.name b3_name from > c, a, b b1, b b2, b b3 where c.a_id=a.id and c.b1_id=b1.id and > c.b2_id=b2.id and c.b3_id=b3.id; > > When I try to get just info from tables c and b1: > > # select id, b1_name from v > > it still does all the joins (see below). > > I would expect just one join (due to the request of columns from the > two tables), > > since all joins are on foreign constrains referencing primary keys, > > there are no filters on the other tables, so it is guaranteed that the > useless joins will always return exactly one answer. I think what you're asking for is a lot more complex than it sounds, and incorrect. The precise state of the data influences how many records come back (in this case, either 1 or 0), for example if b3_id is null you get zero rows. More to the point, you *instructed* the server to make the join. There are strategies to make joins 'optional' at run time with respect to a query, but they are more complicated than simply withdrawing columns from the select list. Stepping back a bit, the query needs to be planned before peeking at the data in the tables. The planner is able to make assumptions against a statistical picture of the data but shouldn't be expected to actually inspect precise result data in order to generate a better plan. merlin -- Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
