From: anthony.shipman@symstream.com
To: pgsql-performance@postgresql.org
Subject: Re: strange query plan with LIMIT
Date: Wed, 8 Jun 2011 18:34:07 +1000
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Cc: Claudio Freire <klaussfreire@gmail.com>
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On Wednesday 08 June 2011 17:39, Claudio Freire wrote:
> Of course optimally executing a plan with limit is a lot different
> than one without.

I imagined that limit just cuts out a slice of the query results. 
If it can find 80000 rows in 0.5 seconds then I would have thought that 
returning just the first 100 of them should be just as easy.

>
> Just... why are you sorting by diag_id?
>
> I believe you would be better off sorting by timestamp than diag_id,
> but I don't know what the query is supposed to do.

The timestamp is only almost monotonic. I need to scan the table in slices and 
I use limit and offset to select the slice.

I've forced the query order with some pgsql like:

declare
    query   character varying;
    rec     record;
begin
    -- PG 8.3 doesn't have the 'using' syntax nor 'return query execute'

    execute 'create temporary table tt on commit drop as ' ||
        'select diag_id from tdiag ' || v_where;

    query = 'select * from tdiag where diag_id in (select * from tt) ' ||
            'order by diag_id ' || v_limit || ' ' || v_offset;

    for rec in execute query loop
        return next rec;
    end loop;
end;

-- 
Anthony Shipman                 | Life is the interval
Anthony.Shipman@symstream.com   | between pay days.