Re: Query with Max, Order by is very slow.......

On Wed, 7 Apr 2004, Hemapriya wrote:

> Hi,
>
> we have table having 23 million rows.
> This is the table structure.
> Table Request:
>
>   Column   |            Type             | Modifiers
> -----------+-----------------------------+-----------
>  origindb  | character(1)                | not null
>  uid       | integer                     | not null
>  rtype     | integer                     |
>  senderid  | integer                     |
>  destaddr  | character varying(15)       |
>  opid      | integer                     |
>  devmodel  | integer                     |
>  ikind     | integer                     |
>  itemid    | character varying(10)       |
>  tranfk    | integer                     |
>  enteredon | timestamp without time zone |
>  status    | integer                     |
>  accountid | integer                     |
> Indexes:
>     "request_pkey" primary key, btree (origindb, uid)
>
> I do max Query like this
>
> select max(uid) from request where originDB=1;
>
> it took around 20 min to return the result..  Since
> max, count functions do the full table scan, i tried
> the workaround given..
>
> select uid from request where originDB=1 order by uid
> desc limit 1;
>
> this query runs forever.. i tried even without where
> condition..no result..

You really want an index on origindb and uid - the order by ... desc
limit 1 workaround is only quick if there's an index on the order by
field, and and where clause is faster if it can use an index to speed up
the query. I would say you might want an index on both of them together,
a joint index.

--

Sam Barnett-Cormack
Software Developer                           |  Student of Physics & Maths
UK Mirror Service (http://www.mirror.ac.uk)  |  Lancaster University