Re: Query with LIMIT clause

Always reply to list. It is also preferred to bottom-post.

Indexes are never simple answers and there isn't enough info to really give good advice here.

You should try different versions and estimate performance (read and write). My guess is that a compound index (2 columns) would work well though if you are going to filter/sort on initial date frequently (by itself) then it likely wants its own index anyway and having two separate indexes would be better.

David J.

On Sep 9, 2012, at 20:56, JORGE MALDONADO <jorgemal1960(at)gmail(dot)com> wrote:

> Firstly, who should I reply to, you or the list?
> Your solution is working pretty fine, I appreciate your advice. Now, I am sure that an index is a good idea in order to make the query fast. What would be a good criteria to define an index? Will an index for final date and another for initial date is the choice, or one index composed of both initial and final date?
>
> Regards,
> Jorge Maldonado
>
> On Sun, Sep 9, 2012 at 12:45 PM, David Johnston <polobo(at)yahoo(dot)com> wrote:
> From: pgsql-sql-owner(at)postgresql(dot)org [mailto:pgsql-sql-owner(at)postgresql(dot)org]
> On Behalf Of JORGE MALDONADO
> Sent: Sunday, September 09, 2012 1:26 PM
> To: pgsql-sql(at)postgresql(dot)org
> Subject: [SQL] Query with LIMIT clause
>
> I have the following records that I get from a query, the fields are date
> type in day/month/year format:
>
> -------------------------------------
> Initial Final
> Date Date
> -------------------------------------
> 27/08/2012 04/09/2012
> 29/08/2012 09/09/2012
> 28/08/2012 09/09/2012
> 30/08/2012 09/09/2012
> 30/08/2012 09/09/2012
> 27/08/2012 09/09/2012
> 31/08/2012 09/09/2012
> 28/08/2012 10/09/2012
> 05/09/2012 16/09/2012
>
> As you can see, this result is ordered by Final Date. What I need is to get
> the last 5 records only. If I use the LIMIT 5 clause I get the first 5. I
> have tried an ORDEY BY DESC but the result is the same. I will very much
> appreciate your comments.
>
> >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>.
>
> First, you should really order by both columns, not just "final date". The
> ties on 9/9/12 are unordered.
>
> Second, you will indeed need to reverse the sort order and then take the
> first 5 records; figuring out and limiting on the last 5 isn't worth the
> effort.
>
> SELECT initial_date, final_date
> FROM date_source
> ORDER BY final_date DESC, initial_date DESC
> LIMIT 5
>
> You can put the above into a sub-query and re-order if the final result is
> needed in ascending order.
>
> If this doesn't seem to work you will want to provide the exact
> query/queries you are trying so that someone may spot what you are doing
> wrong.
>
> Dave
>
>
>