Question

I’m trying to make my query run as quickly as possible but i’m struggling to get it under 5 seconds.

I think it’s because i’m referencing two linked databases

Here’s my query

select column2, column3, column4 
  from table1@dev 
 where column1 in (
          select distinct column2 
            from table2@dev 
           where column3 > 0
                  ) 
order by column1

Is there a way to optimise this query any more?

I’ve tried using join but it seems to make the query run longer

Thanks in advance

EDIT

From further investigation the DRIVING_SITE makes it run very quick like this

select /*+ DRIVING_SITE(table1) */ t1.column2, t1.column3, t1.column4
from table1@dev t1, table2@dev t2
WHERE t2.column3 > 0

But as soon as I add the distinct column2 in it makes it run really slow

Answer 1

First, no need for distinct. The query can be written as:

select * 
  from table1@dev 
 where column1 in (
          select column2 
            from table2@dev 
           where column3 > 0
                  ) 
order by column1

Second, there are (at least) two more ways to write it. Either with JOIN:

select t1.* 
  from table1@dev t1
  join table2@dev t2
 where t2.column2 = t1.column1
   and t2.column3 > 0 
group by
       t1.id, t1.column1, ... 

order by t1.column1

or (my preference) with EXISTS:

select t1.* 
  from table1@dev t1 
 where exists
       ( select *
           from table2@dev 
          where t2.column2 = t1.column1
            and t2.column3 > 0
                  ) 
order by column1

---

In any case, you should check the execution plans for all of them.

I would expect performance to be best if you have an index on table1.column1 and for table2, either an index on column2 or a composite index on (column3, column2)