Improve distinct query performance

Question

Any idea of how we can improve this query execution ? (maybe with some pre-aggregation)?

SELECT p.segment, country, count(distinct userid)
from pixel_data_opt p
WHERE country in ('US') 
  and segment is not null
GROUP BY p.segment, country;

I tried the below but it didn't help -

select  segment, country,sum(cnt)
from 
  (SELECT p.segment, country,  userid,count(*) as cnt
   from pixel_data_opt p
   WHERE country in ('US') 
     and segment is not null
   GROUP BY p.segment, country,userid
  )
group by 1,2;

Answer 1

There's nothing wrong with your first query - though, it could have been where country = 'US' - but optimizer (as far as Oracle is concerned) is smart enough to figure it out.

Is the country column indexed? If not, do that.

Also, gather statistics on the table.

It would probably help if you posted some more info, eg number of rows involved, explain plan as it shows figures that mean something.

Answer 2

For this query:

SELECT p.segment, country, count(distinct userid)
FROM pixel_data_opt p
WHERE country in ('US') AND
      segment is not null
GROUP BY p.segment, country;

You want an index on the table. There are several approaches. One reasonable choice is: pixel_data_opt(country, segment, userid) .

I would suggest rewriting the query as:

SELECT p.segment, 'US' as country, count(distinct userid)
FROM pixel_data_opt p
WHERE country in ('US') AND
      segment is not null
GROUP BY p.segment;

and using the above index.