Re: Top-k optimizations?

David Fetter wrote:
> Folks,
>
> As this came up in a work situation, I was wondering a little bit
> about the top-k issue. Right now, top-k is implemented (most easily,
> I think) via a SELECT with a LIMIT and no OFFSET. 3 questions arise
> from this.

I think the simplest LIMIT query doesn't make it easy to show ties;
but if you don't want the equivalent of MSSQL's "with ties" clause
I think LIMIT works well.

> 1. Are there currently any optimizations specific to top-k in
> PostgreSQL? If so, what are they?

Well, when I do queries like
"select * from customers order by dollarsspent desc limit 3"
it happily uses an index on dollarsspent.

> 3. What kinds of top-k optimizations might (eventually) be included
> in PostgreSQL?

I think a slightly related topic is whether syntactically it'd be
nice if postgresql had the SQL 2003 optional olap features to
specify ways of doing top-k queries as described here:
http://troels.arvin.dk/db/rdbms/#select-top-n

SELECT * FROM (
SELECT
RANK() OVER (ORDER BY age ASC) AS ranking,
person_id, person_name, age
FROM person
) AS foo
WHERE ranking <= 3

Seems IBM and Oracle support that syntax or something very similar.

Yeah, I know it's probably an orthogonal question to the
optimizations one, but might make porting nicer.