From: | Mike Mascari <mascarm(at)mascari(dot)com> |
---|---|
To: | Stephan Szabo <sszabo(at)megazone(dot)bigpanda(dot)com> |
Cc: | mike(at)linkify(dot)com, pgsql-general(at)postgresql(dot)org |
Subject: | Re: correlated delete with "in" and "left outer join" |
Date: | 2004-02-27 06:45:50 |
Message-ID: | 403EE79E.6010001@mascari.com |
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Lists: | pgsql-general |
Stephan Szabo wrote:
> On Thu, 26 Feb 2004 mike(at)linkify(dot)com wrote:
>
>
>>I'm using postgresl 7.3.2 and have a query that executes very slowly.
>>
>>There are 2 tables: Item and LogEvent. ItemID (an int4) is the primary key
>>of Item, and is also a field in LogEvent. Some ItemIDs in LogEvent do not
>>correspond to ItemIDs in Item, and periodically we need to purge the
>>non-matching ItemIDs from LogEvent.
>>
>>The query is:
>>
>>delete from LogEvent where EventType != 'i' and ItemID in
>>(select distinct e.ItemID from LogEvent e left outer join Item i
>>on e.ItemID = i.ItemID where e.EventType != 'i' and i.ItemID is null);
>>
>>I understand that using "in" is not very efficient.
>>
>>Is there some other way to write this query without the "in"?
>
>
> Perhaps
> delete from LogEvent where EventType != 'i' and not exists
> (select * from Item i where i.ItemID=LogEvent.ItemID);
Maybe I'm not reading his subquery correctly, but the left outer
join will produce a row from LogEvent regardless of whether or not a
matching row exists in Item, correct? So doesn't it reduce to:
DELETE FROM LogEvent WHERE EventType <> 'i';
???
Mike Mascari
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