From: anthony.shipman@symstream.com
To: pgsql-performance@postgresql.org
Cc: Claudio Freire <klaussfreire@gmail.com>
Subject: Re: strange query plan with LIMIT
Date: Wed, 8 Jun 2011 18:34:07 +1000
Message-ID: <201106081834.07263.anthony.shipman@symstream.com> (raw)
In-Reply-To: <BANLkTim7DL+n0yuZp+Mpeq0Vozp1qJ4M0A@mail.gmail.com>
References: <201106071802.08272.anthony.shipman@symstream.com>
<BANLkTi=+6EYeiFwKv5Eq8AAMt_nCZ+8W6g@mail.gmail.com>
<BANLkTim7DL+n0yuZp+Mpeq0Vozp1qJ4M0A@mail.gmail.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;
--
Anthony Shipman | Life is the interval
Anthony.Shipman@symstream.com | between pay days.
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To: pgsql-performance@postgresql.org
Cc: anthony.shipman@symstream.com, klaussfreire@gmail.com
Subject: Re: strange query plan with LIMIT
In-Reply-To: <201106081834.07263.anthony.shipman@symstream.com>
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