Re: PG won't use index on ORDER BY <expression>

On Thursday 09 August 2007 22:38:46 Andreas Joseph Krogh wrote:
> On Thursday 09 August 2007 22:00:54 Gregory Stark wrote:
> > "Andreas Joseph Krogh" <andreak(at)officenet(dot)no> writes:
> > > I create an index:
> > > CREATE INDEX person_lowerfullname_idx ON
> > > person((lower(COALESCE(firstname, '')) || lower(COALESCE(lastname,
> > > ''))) varchar_pattern_ops);
> >
> > Why are you declaring it using the varchar_pattern_ops?
> >
> > The default operator set is the one you want for handling ordering. The
> > pattern_ops operator set is for handling things like x LIKE 'foo%'
>
> Ooops, just fugured that out. But - it still doesn't use the index if I
> remove the "varchar_pattern_ops". I solved it by adding a function:
>
> CREATE OR REPLACE FUNCTION concat_lower(varchar, varchar) RETURNS varchar
> AS $$
> SELECT lower(coalesce($1, '')) || lower(coalesce($2, ''))
> $$ LANGUAGE SQL IMMUTABLE;
>
> And than creating an index:
> CREATE INDEX person_lowerfullname_idx ON person(concat_lower(firstname,
> lastname));
>
> Another question then: Why doesn't "varchar_pattern_ops" handle ordering?
> This means I need 2 indexes on the columns I want to match with LIKE and
> ORDER BY. Just doesn't seem right to need 2 "similar" indexes...

Hmm, one more question:
If I want to ORDER BY "created" too, the index is not used anymore:
-- This uses index:
EXPLAIN ANALYZE select firstname, lastname from person order by
concat_lower(firstname, lastname) ASC limit 10;
-- This doesn't
EXPLAIN ANALYZE select firstname, lastname from person order by
concat_lower(firstname, lastname) ASC, created DESC limit 10;

I figured out that it's because they have different sort-order (ASC/DESC). If
both are ASC, the index is used.

Is there a way around this?

--
Andreas Joseph Krogh <andreak(at)officenet(dot)no>
Senior Software Developer / Manager
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