RE: Question about use of DISTINCT
- From: "Powell, Mark" <mark.powell2@xxxxxx>
- To: oracle-l <oracle-l@xxxxxxxxxxxxx>
- Date: Mon, 17 Oct 2011 19:54:23 +0100
You would need to look at the explain plan for the specific query in question
to tell if the CBO recognized that the result set was distinct and did not
require a sort unique operation to filter out duplicates.
As a general rule unnecessary clauses should not appear in your SQL since at
best the clauses are filtered out by the optimizer and at worst cause extra
work to be performed.
-----Original Message-----
From: oracle-l-bounce@xxxxxxxxxxxxx [mailto:oracle-l-bounce@xxxxxxxxxxxxx] On
Behalf Of Sandra Becker
Sent: Monday, October 17, 2011 2:49 PM
To: oracle-l
Subject: Question about use of DISTINCT
IBM zSeries
SUSE 10
Oracle EE 10.2
A developer recently put some code into production that I did not get to see
until 3 days later when it was failing part way through and causing problems
with the other part. One of the things I noticed was the use of DISTINCT on
columns that are already unique. Because of the predicates and the way the
primary keys were defined, they will never use the primary key index or any
other unique index on the tables.
Question: Is using the DISTINCT on a column that is already unique causing
unnecessary overhead or is it just superfluous and the database is smart enough
to ignore it? I've run a few explain plans and the use of the DISTINCT does
not change the explain plan.
--
Sandy
Transzap, Inc.
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