Message-Id: <6.2.1.2.0.20050615100524.04f9d6a8@localhost>
Date: Wed, 15 Jun 2005 10:46:56 +0800
To: pgsql-performance@postgresql.org
From: K C Lau <kclau60@netvigator.com>
Subject: SELECT LIMIT 1 VIEW Performance Issue
Mime-Version: 1.0
Content-Type: text/plain; charset="us-ascii"; format=flowed

Hi All,

I previously posted the following as a sequel to my SELECT DISTINCT 
Performance Issue question. We would most appreciate any clue or 
suggestions on how to overcome this show-stopping issue. We are using 8.0.3 
on Windows.

Is it a known limitation when using a view with SELECT ... LIMIT 1?

Would the forthcoming performance enhancement with MAX help when used 
within a view, as in:

create or replace view VCurPlayer as select * from Player a
where a.AtDate = (select Max(b.AtDate) from Player b where a.PlayerID = 
b.PlayerID);

select PlayerID,AtDate from VCurPlayer where PlayerID='22220';

Thanks and regards,
KC.

---------

At 19:45 05/06/06, PFC wrote:

>>Previously, we have also tried to use LIMIT 1 instead of DISTINCT, but
>>the performance was no better:
>>select PlayerID,AtDate from Player where PlayerID='22220' order by
>>PlayerID desc, AtDate desc LIMIT 1
>
>         The DISTINCT query will pull out all the rows and keep only one, 
> so the
>one with LIMIT should be faster. Can you post explain analyze of the LIMIT 
>query ?

Actually the problem with LIMIT 1 query is when we use views with the LIMIT 
1 construct. The direct SQL is ok:

esdt=> explain analyze select PlayerID,AtDate from Player where 
PlayerID='22220'
  order by PlayerID desc, AtDate desc LIMIT 1;

  Limit  (cost=0.00..1.37 rows=1 width=23) (actual time=0.000..0.000 rows=1 
loops=1)
    ->  Index Scan Backward using pk_player on player  (cost=0.00..16074.23 
rows=11770 width=23) (actual time=0.000..0.000 rows=1 loops=1)
          Index Cond: ((playerid)::text = '22220'::text)
  Total runtime: 0.000 ms

esdt=> create or replace view VCurPlayer3 as select * from Player a
where AtDate = (select b.AtDate from Player b where a.PlayerID = b.PlayerID
order by b.PlayerID desc, b.AtDate desc LIMIT 1);

esdt=> explain analyze select PlayerID,AtDate,version from VCurPlayer3 
where PlayerID='22220';
  Index Scan using pk_player on player a  (cost=0.00..33072.78 rows=59 
width=27)
(actual time=235.000..235.000 rows=1 loops=1)
    Index Cond: ((playerid)::text = '22220'::text)
    Filter: ((atdate)::text = ((subplan))::text)
    SubPlan
      ->  Limit  (cost=0.00..1.44 rows=1 width=23) (actual 
time=0.117..0.117 rows=1 loops=1743)
            ->  Index Scan Backward using pk_player on player 
b  (cost=0.00..14023.67 rows=9727 width=23) (actual time=0.108..0.108 
rows=1 loops=1743)
                  Index Cond: (($0)::text = (playerid)::text)
  Total runtime: 235.000 ms

The problem appears to be in the loops=1743 scanning all 1743 data records 
for that player.

Regards, KC.