Question

I’ve got a problem where my count(*) will return the number of rows before distinct rows are filtered.

This is a simplified version of my query. Note that I’ll extract a lot of other data from the tables, so group by won’t return the same result, as I’d have to group by maybe 10 columns. The way it works is that m is a map mapping q, c and kl, so there can be several references to q.id. I only want one.

SELECT distinct on (q.id) count(*) over() as full_count
from q, c, kl, m 
where c.id = m.chapter_id
    and q.id = m.question_id
    and q.active = 1
    and c.class_id = m.class_id
    and kl.id = m.class_id
order by q.id asc

If I run this i get full_count = 11210 while it only returns 9137 rows. If I run it without the distinct on (q.id), distinct on (q.id) is indeed the number of rows.

So it seems that the count function doesn’t have access to the filtered rows. How can I solve this? Do I need to rethink my approach?

Answer 1

Can you use a subquery:

select qid, count(*) over () as full_count
from (SELECT distinct q.id
      from q, c, kl, m 
      where c.id = m.chapter_id
            and q.id = m.question_id
            and q.active = 1
            and c.class_id = m.class_id
            and kl.id = m.class_id
     ) t
order by q.id asc

But the group by is the right approach. The key word distinct in selectis really just syntactic sugar for doing a group by on all non-aggregate-functioned columns.