Josh Berkus wrote:
> where I have rarely seen a difference of more than 3:1. As I
> understand it, this is because NOT EXISTS can use optimized join
> algorithms to locate matching rows, whereas NOT IN must compare each
> row against every possible matching value in the subselect.
>
> It also makes a difference whether or not the referenced field(s) in
> the subselect is indexed. EXISTS will often use an index to compare
> the values in the master query to the sub-query. As far as I know, IN
> can use an index to retrieve the subquery values, but not to sort or
> compare them after they have been retreived into memory.
I wonder if "[NOT] IN (subselect)" could be improved with a hash table in
similar fashion to the hash aggregate solution Tom recently implemented?
Joe
