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

Nope, no difference how I express the joins:
create view v1 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 join a on (c.a_id=a.id) join b b1 on (c.b1_id=b1.id) join b b2 on (c.b2_id=b2.id) join b b3 on (c.b3_id=b3.id);

# explain select id, b1_name from v1;
QUERY PLAN
--------------------------------------------------------------------------------
Nested Loop (cost=1.02..5.47 rows=1 width=7)
Join Filter: (c.b3_id = b3.id)
-> Nested Loop (cost=1.02..4.34 rows=1 width=11)
Join Filter: (c.a_id = a.id)
-> Nested Loop (cost=1.02..3.25 rows=1 width=15)
Join Filter: (c.b2_id = b2.id)
-> Hash Join (cost=1.02..2.12 rows=1 width=19)
Hash Cond: (b1.id = c.b1_id)
-> Seq Scan on b b1 (cost=0.00..1.06 rows=6 width=7)
-> Hash (cost=1.01..1.01 rows=1 width=20)
-> Seq Scan on c (cost=0.00..1.01 rows=1 width=20)
-> Seq Scan on b b2 (cost=0.00..1.06 rows=6 width=4)
-> Seq Scan on a (cost=0.00..1.04 rows=4 width=4)
-> Seq Scan on b b3 (cost=0.00..1.06 rows=6 width=4)
(14 rows)

Igor

PS: Here are the updated table definitions:
create table a (id int not null primary key, name varchar(128));
create table b (id int not null primary key, name varchar(128));
create table c (id int not null primary key, a_id int not null references a(id), b1_id int not null references b(id), b2_id int not null references b(id), b3_id int not null references b(id));

-----Original Message-----
From: Adrian Klaver [mailto:adrian(dot)klaver(at)aklaver(dot)com]
Sent: Friday, July 01, 2016 1:38 PM
To: Sfiligoi, Igor <Igor(dot)Sfiligoi(at)ga(dot)com>; Merlin Moncure <mmoncure(at)gmail(dot)com>
Cc: pgsql-general(at)postgresql(dot)org
Subject: Re: --EXTERNAL--Re: [GENERAL] 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_Rem
> oval

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