Re: Multi-key index not beeing used - bug?

Thanks Tobias. The difference here though, is that in terms of your
database I am doing a query to select the most recent transaction for
EACH user at once, not just for one user. If I do a similar query to
yours to get the last transaction for a single user, my query is fast
like yours. It's when I'm doing a query to get the results for all
users at once that it is slow. If you try a query to get the most
recent transaction of all useres at once you will run into the same
problem I am having.

Graham.

Tobias Brox wrote:

>Look at this:
>
>NBET=> explain select * from account_transaction where users_id=123456 order by created desc limit 10;
> QUERY PLAN
>-------------------------------------------------------------------------------------------------------------------------------------------
> Limit (cost=0.00..27.40 rows=10 width=213)
> -> Index Scan Backward using account_transaction_on_user_and_timestamp on account_transaction (cost=0.00..1189.19 rows=434 width=213)
> Index Cond: (users_id = 123456)
>(3 rows)
>
>NBET=> explain select * from account_transaction where users_id=123456 order by created desc, id desc limit 10;
> QUERY PLAN
>------------------------------------------------------------------------------------------------------------------------------
> Limit (cost=1114.02..1114.04 rows=10 width=213)
> -> Sort (cost=1114.02..1115.10 rows=434 width=213)
> Sort Key: created, id
> -> Index Scan using account_transaction_by_users_id on account_transaction (cost=0.00..1095.01 rows=434 width=213)
> Index Cond: (users_id = 123456)
>(5 rows)
>
>In case the explains doesn't explain themself good enough: we have a
>transaction table with ID (primary key, serial), created (a timestamp)
>and a users_id. Some of the users have generated thousands of
>transactions, and the above query is a simplified version of the query
>used to show the users their last transactions. Since we have a large
>user base hammering our servers with this request, the speed is
>significant.
>
>We have indices on the users_id field and the (users_id, created)-tuple.
>
>The timestamp is set by the application and has a resolution of 1 second
>- so there may easily be several transactions sharing the same
>timestamp, but this is an exception not the rule. I suppose the
>developers needed to add the ID to the sort list to come around a bug,
>but still prefering to have the primary sorting by created to be able to
>use the index. One workaround here is to order only by id desc and
>create a new index on (users_id, id) - but I really don't like adding
>more indices to the transaction table.
>
>
>---------------------------(end of broadcast)---------------------------
>TIP 6: explain analyze is your friend
>
>

--
Graham Davis
Refractions Research Inc.
gdavis(at)refractions(dot)net