Re: strange query plan with LIMIT

2011/6/8  <anthony.shipman@symstream.com>:
> 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;

if you use FOR statement, there should be a problem in using a
implicit cursor - try to set a GUC cursor_tuple_fraction to 1.0.

Regards

Pavel Stehule


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