Message-ID: <001d01c29169$79fd1d20$4528a8c0@cblan.mblox.com>
From: "Peter Galbavy" <peter.galbavy@knowtion.net>
To: <pgsql-sql@postgresql.org>
Subject: help optimise this ?
Date: Thu, 21 Nov 2002 14:22:51 -0000
Organization: Knowtion Ltd.
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I have a table of image 'instances' where the columns include:

md5 char(32),                    -- the 'original' image md5 key
file_md5 char(32) primary key,   -- the md5 of each version of an image
image_width int,
image_length int

I want to then find either the largest (max) or smallest (min) version of an
image that falls within some range of sizes:

e.g.

select file_md5 from image_instance
where image_width =
        (select min(image_width) from image_instance where md5 =
'546b94e94851a56ee721f3b755f58462')
    and image_length =
        (select min(image_length) from image_instance where md5 =
'546b94e94851a56ee721f3b755f58462')
    and md5 = '546b94e94851a56ee721f3b755f58462'
    and image_width between 0 and 160
    and image_length between 0 and 160;

Now, having to do three selects on 'md5' to limit the search seems a little
unoptimal to me. Note that the test tables are small and I have no other
indexes apart from the 'primary key' constraint yet - this is not my primary
concern at this point, I would just like cleaner SQL.

All I want back is (for some definition) the 'file_md5' that best matches my
min/max criteria.

I have not - and will leave for now - the case where a cropped image results
in a scale change between width and length such that the min/max test
returns a different set of rows for each dimension. Argh.

And help given is greatly appreciated.

rgds,
--
Peter