Message-ID: <4AEAA684.60905@archonet.com>
Date: Fri, 30 Oct 2009 08:40:36 +0000
From: Richard Huxton <dev@archonet.com>
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To: Grant Masan <grant.massan@gmail.com>
CC: pgsql-sql@postgresql.org
Subject: Re: slow count(CASE) query
References: <c09b5d640910291127k6a49d3a8w91a85cf69640bb3@mail.gmail.com>
In-Reply-To: <c09b5d640910291127k6a49d3a8w91a85cf69640bb3@mail.gmail.com>
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Grant Masan wrote:
> 
> select '000100' as length, sum(ship1) as ship1 ,sum(ship2) as
> ship2,sum(ship3) as ship3,sum(ship4) as ship4,sum(ship5) as ship5 FROM
> (select
> count(CASE WHEN (type between 40 and 49)  THEN 1 ELSE NULL END) as ship1,
> count(CASE WHEN (type between 60 and 69) THEN 1 ELSE NULL END) as ship2,
> count(CASE WHEN (type between 70 and 79) THEN 1 ELSE NULL END) as ship3,
> count(CASE WHEN (type between 80 and 89)  THEN 1 ELSE NULL END) as ship4,
> count(CASE WHEN (type >90) THEN 1 ELSE NULL END) as ship5
> FROM school_proj_boat where length <100
> GROUP BY type
> ORDER BY type) as koo
> 
> UNION ALL
> 
> select '100200' as length, sum(ship1) as ship1 ,sum(ship2) as
...
> FROM school_proj_boat where length between 100 and 200
> GROUP BY type
> ORDER BY type) as koo
> 
> UNION ALL
...

First thing is to ditch the UNION ALLs. You're basically repeating the
same query.

Create a lookup table: length_codes (code, min_length, max_length)
Data: ('100100', 0, 99), ('100200', 100, 199), ...


SELECT length_code AS length, sum...
FROM (
  SELECT
    lc.code AS length_code,
    count(case)...
  FROM
    school_proj_boat spb, length_codes lc
  WHERE
    spb.length BETWEEN lc.min_length AND lc.max_length
  ) AS koo
;

It's easy to forget that you can join against a table using any
condition, it doesn't have to be equality. Here we use BETWEEN to
replace our UNIONs.

You'll want a unique constraint on length_codes.code and you should
really write a custom trigger to make sure none of the
min_length..max_length ranges overlap. In practice, you're probably only
setting this table up once so might not bother.

-- 
  Richard Huxton
  Archonet Ltd