Re: --EXTERNAL--Re: PSQL does not remove obvious useless joins

On 07/01/2016 01:28 PM, Sfiligoi, Igor wrote:
> 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

Those examples use explicit joins, so you might try that in your view
definition.

>
> I was hoping this use case would fit in.
>
> Any suggestions?
>
> Igor
>
> -----Original Message-----
> From: Merlin Moncure [mailto:mmoncure(at)gmail(dot)com]
> Sent: Friday, July 01, 2016 12:42 PM
> To: Sfiligoi, Igor <Igor(dot)Sfiligoi(at)ga(dot)com>
> Cc: pgsql-general(at)postgresql(dot)org
> Subject: --EXTERNAL--Re: [GENERAL] PSQL does not remove obvious useless joins
>
> On Fri, Jul 1, 2016 at 12:17 PM, Sfiligoi, Igor <Igor(dot)Sfiligoi(at)ga(dot)com> 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
>
>

--
Adrian Klaver
adrian(dot)klaver(at)aklaver(dot)com