We have a query that looks something like:

select a, b, c, d, e, f

from tbla inner join tblb on tbla.....

This query runs in the expected 200-400 ms range.

If we run the same query but reduce the select to just:

select a

from tbla inner join tblb on tbla.....

then the query runs for 30 minutes or more.

The rest of the query is identical. Is there a logical explanation for how this can happen?

Thanks.