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Re: correlated delete with "in" and "left outer join"

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 (view raw or flat)
Thread:
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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Next:From: Mike MascariDate: 2004-02-27 06:49:30
Subject: Re: correlated delete with "in" and "left outer join"
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Subject: Re: Slow queries in PL/PGSQL function

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