How is "UNION ALL " work in ORACLE ?

From: <yewpc_at_my-dejanews.com>
Date: Fri, 29 Jan 1999 08:56:24 GMT
Message-ID: <78rt3m$1bh$1@nnrp1.dejanews.com>

Hi,
I don't understand how actually Oracle execute a SQL statement that has UNION ALL condition on it.
Let say I have the following SQL :

Select a, b, c
from tb_txn
where txn_code = 'OLN'
UNION ALL
Select a, b, c
from tb_txn_hist
where txn_code = 'OLN';

Is it Oracle will execute the 1st select and put the result into a buffer first then execute the 2nd select and put the result into the same buffer then only fetch all the select records out ?

Is there a way to force it to execute the 1st and 2nd select parallelly ?

What I want is to improve the performance. Currently the txn_code is indexed and it may have up to 300 rows of same the value out of 50 millions records.

Anyone have any idea or suggestion ?

Thank you very much .

-----------== Posted via Deja News, The Discussion Network ==---------- http://www.dejanews.com/ Search, Read, Discuss, or Start Your Own Received on Fri Jan 29 1999 - 02:56:24 CST