Oracle-L: Join vs. Subselect

From: Carle, William T (Bill), ALCAS <wcarle_at_att.com>
Date: Mon, 11 Nov 2002 08:23:35 -0800
Message-ID: <F001.00500183.20021111082335@fatcity.com>

Hi,

Here is the situation. The application coded a query that looks like this:

select * from table1
where objid in (select objid from table2);

There is an index on objid in table 1 that isn't being used. An explain shows it is using this system view vw_nso_1 that is used to transform an IN subquery to a join. If you recode the query to:

select a.* from table1 a, table2 b
where a.objid = b.objid;

Then it will use the index. My question is: shouldn't it use the index in both cases. I know the join is a better way to code it and I have told the application that, but I would think that the first way would use an index anyway. Ideas?

Bill Carle
AT&T
Database Administrator
816-995-3922
wcarle_at_att.com

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Author: Carle, William T (Bill), ALCAS
INET: wcarle_at_att.com

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