I've had this problem also (sounds like a bug to me).
My work around was a query like this:
select id from mytable order by id desc limit 1;
This query used the index, whereas the max(id) query did not.
-Randy
> -----Original Message-----
> From: Sean Harding [mailto:sharding@dogcow.org]
> Sent: Wednesday, March 28, 2001 8:42 PM
> To: pgsql-general@postgresql.org
> Subject: [GENERAL] full table scan on 'select max(value) from table'?
>
>
> I have a table, 'mesg_headers', which holds headers from email
> messages. Each message has a unique integer ID within the system,
> 'mesgid'. mesgid is the primary key for mesg_headers, so it has index
> mesg_headers_pkey. This index is used if I do 'select * from
> mesg_headers
> where mesgnum = whatever', but if I do 'select max(mesgnum) from
> mesg_headers', I get a full table scan, which takes a long
> time (there are
> currently over 370,000 rows). Explains:
>
> email=# explain select * from mesg_headers where mesgnum = 100;
> NOTICE: QUERY PLAN:
>
> Index Scan using mesg_headers_pkey on mesg_headers
> (cost=0.00..4.99 rows=1 width=92)
>
> EXPLAIN
> email=# explain select max(mesgnum) from mesg_headers;
> NOTICE: QUERY PLAN:
>
> Aggregate (cost=80319.44..80319.44 rows=1 width=4)
> -> Seq Scan on mesg_headers (cost=0.00..79392.55
> rows=370755 width=4)
>
> EXPLAIN
>
>
> So is there anything I can do about this, or will
> max(mesgnum) never use an
> index? I'm migrating this db from MySQL, where the same query
> returns almost
> instantanously, so some of my code makes the assumption that
> it's a cheap
> operation. I could work around it, but it would definitely be
> nicer to find
> a way to just make it use an index.
>
> Thanks.
>
> sean
>
> --
> Sean Harding sharding@dogcow.org | "I am the captain and I
> have been told
> http://www.dogcow.org/sean/ | that tomorrow we land
> and my ship has
> | been sold." --Dar Williams
>
> ---------------------------(end of
> broadcast)---------------------------
> TIP 5: Have you checked our extensive FAQ?
>
> http://www.postgresql.org/users-lounge/docs/faq.html
>
