Re: ORDER BY increases consistent gets?
- From: "Juan Cachito Reyes Pacheco" <jreyes@xxxxxxxxxxxxxxxx>
- To: <oracle-l@xxxxxxxxxxxxx>
- Date: Mon, 5 Apr 2004 09:34:29 -0400
To improve performance, try creating an index based on the columns included
in the order by clause (exactly with desc if it uses).
And if possible all the fields you query too, so Oracle will read only the
index.
Juan Carlos Reyes Pacheco
OCP
Database 9.2 Standard Edition
----- Original Message -----
From: "DENNIS WILLIAMS" <DWILLIAMS@xxxxxxxxxxxxx>
To: <oracle-l@xxxxxxxxxxxxx>
Sent: Saturday, April 03, 2004 6:20 PM
Subject: RE: ORDER BY increases consistent gets?
Thomas - Usually ORDER BY causes a sort to occur. A simple check would be to
run EXPLAIN PLAN on both versions of the query.
Dennis Williams
DBA
Lifetouch, Inc.
dwilliams@xxxxxxxxxxxxx
-----Original Message-----
From: oracle-l-bounce@xxxxxxxxxxxxx
[mailto:oracle-l-bounce@xxxxxxxxxxxxx]On Behalf Of Thomas A. La Porte
Sent: Saturday, April 03, 2004 12:25 PM
To: oracle-l@xxxxxxxxxxxxx
Subject: ORDER BY increases consistent gets?
Env: Oracle 8.1.7.4 on RedHat AS2.1 (2.4.9-e.35 kernel)
A developer brought me a query yesterday in which the
introduction of an ORDER BY clause alters the performance
substantially.
The query is as follows:
SELECT reserve_host, usage,
to_char(request_time, 'MM/DD/YYYY HH24:MI:SS')
FROM licadmin.licmgr_requests
WHERE request_time > to_date('3/14/2004', 'MM/DD/YYYY')
AND release_time < to_date('1970', 'YYYY')
ORDER BY request_time
Without the ORDER BY clause, the query requires about 100
consistent gets, while with the ORDER BY clause it takes over
50000 consistent gets. According to autotrace and tkprof the
execution plans are the same [see below].
--
Thomas A. La Porte
<mailto:tlaporte@xxxxxxxxx>
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