RE: SQL DISTINCT alternate

From: Toepke, Kevin M <ktoepke_at_cms.cendant.com>
Date: Wed, 20 Jun 2001 11:29:19 -0700
Message-ID: <F001.00330AEE.20010620113234@fatcity.com>

As I see it, there are several questions here:

1)      Can I use DISTINCT to do a sort?
        Yes and no. In Oracle 7, a DISTINCT does an implicit sort
        In Oracle 8, it does a "SORT NOSORT" operation. Dupicates are
removed but the output is NOT guaranteed to be in sorted order.
2)      How do I sort the output?
        Use the "ORDER BY" clause as in:
                SELECT a, b, c FROM tab_1 ORDER BY a, b, c
3)      How do I get around the performance hit of a SORT operation?
        In Oracle 8.0 through 8.1.6 (fixed in 9i and 8.1.7) there is a bug

that causes the optimizer to choose a horrendous execution path if you are ordering by a column that is referenced in the WHERE clause.

        Workaround From:
                SELECT a, b, c FROM tab_1 WHERE a > 33 ORDER BY a, b, c:
        to:
                SELECT /*+ NO_MERGE(d) */ a, b, c FROM (
                        SELECT a, b, c FROM tab_1 WHERE a > 33
                ) ORDRE BY a, b, c;

Kevin

-----Original Message-----

Sent: Wednesday, June 20, 2001 1:50 PM
To: Multiple recipients of list ORACLE-L

Hi
In a sql stmt what will be the impact if we used DISTINCT clause and how we can overcomes on the performance bottleneck caused by DISTINCT. like ... SELECT DISTINCT A,B,C FROM TAB_1 I want to get same output without using distinct?Is this possible?If, YES then how?
Thanks in advance
-Seema

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Author: Seema Singh
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Author: Toepke, Kevin M
INET: ktoepke_at_cms.cendant.com

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