The fastest way to check if some records in a database table?

Question

I have a huge table to work with . I want to check if there are some records whose parent_id equals my passing value . currently what I implement this is by using \"select

Answer 1

There's no real difference between:

select 'y' 
  from dual 
 where exists (select 1 
                 from child_table 
                where parent_key = :somevalue)

and

select 'y' 
  from mytable 
 where parent_key = :somevalue 
   and rownum = 1;

... at least in Oracle10gR2 and up. Oracle's smart enough in that release to do a FAST DUAL operation where it zeroes out any real activity against it. The second query would be easier to port if that's ever a consideration.

The real performance differentiator is whether or not the parent_key column is indexed. If it's not, then you should run something like:

select 'y' 
  from dual 
 where exists (select 1 
                 from parent_able 
                where parent_key = :somevalue)