From: | Manfred Koizar <mkoi-pg(at)aon(dot)at> |
---|---|
To: | "Peter Darley" <pdarley(at)kinesis-cem(dot)com> |
Cc: | "Richard Huxton" <dev(at)archonet(dot)com>, "Pgsql-Performance" <pgsql-performance(at)postgresql(dot)org> |
Subject: | Re: Possibly slow query |
Date: | 2005-01-31 11:06:01 |
Message-ID: | j44sv0hjeg0539tqriuppfo7o4och2dslc@email.aon.at |
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Lists: | pgsql-performance |
On Wed, 26 Jan 2005 07:16:25 -0800, "Peter Darley"
<pdarley(at)kinesis-cem(dot)com> wrote:
>SELECT User_ID
>FROM Assignments A NATURAL LEFT JOIN (SELECT * FROM Assignment_Settings
>WHERE Setting='Status') ASet
>WHERE A.User_ID IS NOT NULL
> AND ASet.Assignment_ID IS NULL
>GROUP BY User_ID;
"ASet.Assignment_ID IS NULL" and "value IS NULL" as you had in your
original post don't necessarily result in the same set of rows.
SELECT DISTINCT a.User_ID
FROM Assignments a
LEFT JOIN Assignment_Settings s
ON (a.Assignment_ID=s.Assignment_ID
AND s.Setting='Status')
WHERE a.User_ID IS NOT NULL
AND s.Value IS NULL;
Note how the join condition can contain subexpressions that only depend
on columns from one table.
BTW,
|neo=# \d assignment_settings
| [...]
| setting | character varying(250) | not null
| [...]
|Indexes:
| [...]
| "assignment_settings_assignment_id_setting" unique, btree (assignment_id, setting)
storing the setting names in their own table and referencing them by id
might speed up some queries (and slow down others). Certainly worth a
try ...
Servus
Manfred
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