Re: How to eliminate extra "NOT EXISTS"-query here?

Andreas Joseph Krogh <andreas@visena.com> writes:
> -- This works, but I'd rather not do the extra EXISTS
> select * from test t
> WHERE (NOT ARRAY ['x', 'y', 'z', 't']::varchar[] <@ (select array_agg(s.v) from
> stuffs WHERE s.test_id = t.id)
> OR NOT EXISTS (
> select * from stuff s where s.test_id = t.id
> )
>  )
> ;

> So, I want to return all entries in test not having any of ARRAY ['x', 'y', 
> 'z', 't'] referenced in the table stuff, and I'd like to have test.id="d" 
> returned as well, but in order to do that I need to execute the “or not 
> exists”-query. Is it possible to avoid that?

Probably not directly, but perhaps you could improve the performance of
this query by converting the sub-selects into a left join:

select * from test t
  left join
    (select s.test_id, array_agg(s.v) as arr from stuffs group by s.test_id) ss
  on ss.test_id = t.id
WHERE (NOT ARRAY ['x', 'y', 'z', 't']::varchar[] <@ ss.arr)
      OR ss.test_id IS NULL;

Another possibility is

...
WHERE (ARRAY ['x', 'y', 'z', 't']::varchar[] <@ ss.arr) IS NOT TRUE

but I don't think that's more readable really, and it will save little.

In either case, this would result in computing array_agg once for
each group of test_id values in "stuffs", while your original computes
a similar aggregate for each row in "test".  So whether this is better
depends on the relative sizes of the tables, although my proposal
avoids random access to "stuffs" so it will have some advantage.

regards, tom lane

Excellent, thanks!

--

Andreas Joseph Krogh

CTO / Partner - Visena AS

Mobile: +47 909 56 963

andreas@visena.com

www.visena.com