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.