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Message Posted: Thu, 01 Aug 2002 @ 11:41:58 GMT


     
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Subj:   Incorrect Processing of NOT EXISTS?
 
From:   Doorey, Andrew ST

Has anyone had any problems with NOT EXISTS not working as expected? I've just produced explains for 3 methods of achieving the same result. In the following examples there is a NUSI on colA on big_table:

(1) select colA
     from small_table
     where colA not in
     (select colA  from big_table  )
     group by 1

(2) select colA
     from small_table  t1
     where not exists
     (select 1 from big_table t2  where t1.colA = t2.colA  )
     group by 1

(3)  select colA
     from small_table
     where colA not in
     (select t1.colA
      from small_table t1
      ,big_table t2
      where t1.colA = t2.colA
      )
      group by 1

I would have expected method 2 to have a similar explain to method 3. However methods 1 & 2 both read big_table into spool (which takes over an hour), whereas method 3 just reads the index (which takes about a minute).

We've recently applied a minor upgrade and are now on V2R.04.01.01.12 release 04.01.01.36. Does anyone know if this is correct and has anyone else had any similar problems? I don't want to write code using the 3rd method if I don't have to as it'll be harder to maintain.


Thanks,

Andy Doorey.



     
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