On Thu, 2003-02-06 at 07:59, Scott Morrison wrote:
>
> My current query:
> select a.* from sample a where (id,date) in (select a.id,max(date) from
> sample where date<='<<the date>>' and id=a.id) order by id;
max(date) will require a scan on the recordset in the subselect. You
might be better to use 'ORDER BY date DESC LIMIT 1' which will use an
index on date if one exists.
SELECT * FROM sample a WHERE (id, date)
IN (SELECT a.id, date FROM sample WHERE id=a.id ORDER BY date DESC
LIMIT 1)
When you had 20,000 records you would definitely be wanting an index on
date and an index on id.
I think that the LIMIT clause on subselects is only implemented in
recent PostgreSQL however - 7.2 on, I believe.
To examine the query plans that PostgreSQL comes up with, and help you
choose a better structure for your query, you should use "EXPLAIN <<the
query>>" .
Regards,
Andrew.
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