Re: Order-by and indexes

On Wed, 29 Jun 2011 15:48:56 +0200, Odd Hogstad <odd.hogstad@smartm.no> wrote:

> SELECT * FROM "data" WHERE "data"."fk" = 238496 ORDER BY "data"."id" DESC
> LIMIT 1
>
> This query is quite slow. If I do a explain on it, it seems that it uses an
> Index Scan Backward.
>
> If I omit the order by on the query:
>
> SELECT * FROM "data" WHERE "data"."fk" = 238496 LIMIT 1
>
> It is very fast. And the explain says that it uses Index scan. This is also
> very fast if there aren't any matches. But I've read that I'm not guaranteed
> to get the correct match If I do not use a order by, postgres just returns
> its fastest possible match. Is this right? But will not the fastest possible
> match always be the first match in the index? Is there another way to make
> the order by query go faster?

Unfortunately (and AFAIK), you don't have any other solution as you want the
*latest* row; may be often clustering this table in this order would help a
bit.
Perhaps creating fragmented indexes could also help (1 >= data.fk < 50001, and so on)

JY
--
He asked me if I knew what time it was -- I said yes, but not right now.
        -- Steven Wright