Re: Strange (?) Index behavior?

On Fri, 5 Nov 2004 10:32:43 -0500, Allen Landsidel <alandsidel(at)gmail(dot)com> wrote:
> On Fri, 05 Nov 2004 10:07:38 -0500, Rod Taylor <pg(at)rbt(dot)ca> wrote:
>
>
> > > It seems to me that a query saying "SELECT column FROM table WHERE
> > > column LIKE 'AA%';" should be just as fast or very close to the first
> > > case up above. However, explain tells me that this query is not using
> > > the index above, which is what's not making sense to me.
> >
> > It looks for an exact expression match, and doesn't know about values
> > which are equal.
> >
> > You can provide both clauses.
> >
> > WHERE column LIKE 'A%' and column LIKE 'AA%';
>
> I see. That's not really optimal either however as you can probably
> see already.. adding AB, AC, AD...AZ is likely to be pretty bogus and
> at the least is time consuming.

I see now that you mean to add that to the SELECT clause and not the
index, my mistake.

> Perhaps.. SELECT column FROM (SELECT column FROM table WHERE column
> LIKE 'A%') AS sq WHERE column LIKE 'AA%';
>
> The query planner thinks this will be pretty fast indeed, and does use
> the index I am after.

This was indeed pretty fast. About 7 seconds, as was modifying the
WHERE as suggested above.

-Allen