It might be worthwhile to experiment with 2 new indexes:
Create UNIQUE index articles_created_topic_idx on articles(created,
topic);
Create UNIQUE index articles_topic_created_idx on articles(topic,
created);
Probably, one of the two should become your primary key.
That will give the optimizer some new choices for plans.
> -----Original Message-----
> From: pgsql-general-owner@postgresql.org
> [mailto:pgsql-general-owner@postgresql.org] On Behalf Of Jeff Rogers
> Sent: Friday, June 18, 2004 12:28 PM
> To: pgsql-general@postgresql.org
> Subject: [GENERAL] can't win
>
>
>
> I have a query that it seems is destined to be slow one way
> or another. I
> have a table of around 30k articles, categorized by topic and
> ordered by date:
>
> create table articles (
> topic varchar(50),
> created date,
> data text
> );
>
> create index articles_topic_idx on articles(topic);
> create index articles_created_idx on articles(created);
>
> If I want to get the 5 most recent articles in a topic, I get
> a nice query
> plan and a fast query:
> # explain select * from articles where topic = 'Example'
> order by created desc
> limit 5 ;
> QUERY PLAN
>
>
> --------------------------------------------------------------
> -----------------
> -----------------------------
> Limit (cost=0.00..646.71 rows=5 width=828)
> -> Index Scan Backward using articles_created_idx on articles
> (cost=0.00..85202.16 rows=659 width=828)
> Filter: (topic = 'Example'::character varying)
> (3 rows)
>
> # select * from articles where topic = 'Example' order by
> created desc limit 5
> ;
> [.....]
> Time: 18.42 ms
>
> However, if the topic happens to not exist, this query takes
> a very long time: # select * from articles where topic =
> 'NO-Example' order by created desc
> limit 5 ;
> [.....]
> Time: 1075.36 ms
>
> If I drop the date index or get more articles (so it doesn't
> do the backward
> scan on articles_created_idx), then the situation is
> reversed: getting the
> most recent articles for a topic that exists takes a fair
> amount of time,
> while getting a topic that does not exist is nearly instantaneous.
>
> Is there any way I can get the best of both worlds?
>
> -J
>
>
>
>
>
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