Correlated subqueries slow as hell?

My question has two parts:

1. MYTABLE has an SSN and an ID column, among others. ID is unique, SSN is not. If I want to see all the attributes for each row that contains a *unique* SSN, and for those SSNs which have multiple rows I don't care which row I see, how should I form the query? Is it possible to form this query without using a correlated subquery, or must I use something like:

select * from MYTABLE T where T.ID = (select max(ID) from MYTABLE X

        where X.SSN = T.SSN);

2) What are internal mechanics that Oracle uses to resolve correlated subqueries? Does it fetch all records from the table(s) in the main query and then step through them one -by-one, for each row evaluating the subquery, or does it do something more (less?) clever? I'm trying to figure out why the above query is so slow. When issued against a small, 100 rows, table, it returns the answer instantaneously. But when the query is used against a larger table, 30K rows, it hangs. This, despite the fact that MYTABLE is indexed on both ID and SSN. Ostensibly the query would return eventually, but I have only waited a half hour before giving up.

Any insights appreciated. Thanks.

-- 

Joe Panico
NeXTStep/OpenStep Developer
BLaCKSMITH Inc.

jpanico_at_netcom.com

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