Re: OR Vs UNION

From: Mark Richard <mrichard_at_transurban.com.au>
Date: Mon, 23 Jun 2003 23:16:29 -0700
Message-ID: <F001.005B841E.20030623225951@fatcity.com>

In that simple example the OR is almost certainly faster...

UNION forces a sort operation to be performed (UNION ALL is better in this respect).
If the query is forced to perform a full table scan then the union option will perform two scans. There will be similar issues if a full index scan is performed. Of course a lot of data will be cached for the second pass but it must still be done.

Therefore, in your simple example (not knowing table volumes, indexing strategies, etc) I would suggest the OR option. Having said that, there are examples in complex queries where a union may perform better (and it may be easier to write as well).

Regards,

Mark.

                                                                                       
                                               
                      Saminathan_Seeran                                                
                                               
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                      24/06/2003 15:34                                                 
                                               
                      Please respond to                                                
                                               
                      ORACLE-L

Hi All,

Could someone explain to me which one is best in the following two queries w.r.t performance?

Thanks
Sami

Query1)

select distinct empployee_id from employees where department_id=10 or department_id=20
/

Query2)

select employee_id from employees where department_id=10 union
select employee_id from employees where department_id=20
/

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Received on Tue Jun 24 2003 - 01:16:29 CDT