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Charles Hooper  
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 More options Jul 4 2007, 12:04 pm
Newsgroups: comp.databases.oracle.misc
From: Charles Hooper <hooperc2...@yahoo.com>
Date: Wed, 04 Jul 2007 09:04:56 -0700
Local: Wed, Jul 4 2007 12:04 pm
Subject: Re: NOT IN
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On Jul 4, 10:27 am, colmkav <colmj...@yahoo.co.uk> wrote:

> On 4 Jul, 15:26, Charles Hooper <hooperc2...@yahoo.com> wrote:
> > On Jul 4, 8:47 am, colmkav <colmj...@yahoo.co.uk> wrote:

> > > is it possible to write an SQL Oracle query where you want to check
> > > whether a combination of fields exist in another query

> > > eg I tried the following but got an error:

> > > select count(*) from tmp_risk_sum where book, strategy, exposuretypeid
> > > not in (select book, strategy, exposuretypeid from tmp_risk_sum_ORACLE)

> > SELECT
> >   COUNT(*)
> > FROM
> >   TMP_RISK_SUM
> > WHERE
> >   (BOOK,STRATEGY,EXPOSURETYPEID) NOT IN (
> >     SELECT
> >       BOOK,
> >       STRATEGY,
> >       EXPOSURETYPEID
> >     FROM
> >       TMP_RISK_SUM_ORACLE);

> > You may find that this syntax executes more efficiently:
> > SELECT
> >   COUNT(*)
> > FROM
> >   TMP_RISK_SUM S,
> >   (SELECT DISTINCT
> >     BOOK,
> >     STRATEGY,
> >     EXPOSURETYPEID
> >   FROM
> >     TMP_RISK_SUM_ORACLE) SO
> > WHERE
> >   S.BOOK=SO.BOOK(+)
> >   AND S.STRATEGY=SO.STRATEGY(+)
> >   AND S.EXPOSURETYPEID=SO.EXPOSURETYPEID(+)
> >   AND SO.BOOK IS NULL;

> > Charles Hooper
> > IT Manager/Oracle DBA
> > K&M Machine-Fabricating, Inc.

> Why does this work? Surely you would need a similar SO.STRATEGY IS
> NULL a nd EXPOSURETYPEID IS NULL

> Also why does this execute better

The NOT IN syntax can be very inefficient.  On Oracle 8.1.7.3, the NOT
IN syntax for a particular SQL statement may require 5 minutes to
execute, while the outer join syntax (the second approach that I
provided) might execute in 15 seconds or less.  Newer releases of
Oracle may automatically perform the transformation from the NOT IN
syntax to the outer join syntax - with the likely requirement that the
columns be defined as NOT NULL.

Why is the outer join typically more efficient?  Assume that Oracle
determines that a full tablescan of the TMP_RISK_SUM_ORACLE table is
required to verify that a given combination of BOOK, STRATEGY, and
EXPOSURETYPEID in a row in the TMP_RISK_SUM table does not exist in
the TMP_RISK_SUM_ORACLE table.  This full tablescan will be required
for _each_ row in the TMP_RISK_SUM table.  Such an operation, even if
indexes are involved on the TMP_RISK_SUM_ORACLE table, could be both
expensive and time consuming.  The outer join syntax does not suffer
the same performance penalty - Oracle could create a hash probe table,
for example, for the results of the inline view.  Odd things can
happen if any of the columns involved may naturally contain NULL
values.

Why is it that we only need to specify that SO.BOOK IS NULL and not
that also that SO.STRATEGY IS NULL and EXPOSURETYPEID IS NULL?  It
only takes one exception to make an ALWAYS rule false.  If
TMP_RISK_SUM_ORACLE.BOOK can never be naturally NULL, the only time
when it would be NULL in this case is when Oracle is performing an
outer join and the row does not exist in the TMP_RISK_SUM_ORACLE
table.

Just for the sake of curiosity, please post the output of the DBMS
Xplan for the two methods using the 'ALLSTATS LAST' parameter as shown
here:
  http://jonathanlewis.wordpress.com/2006/11/09/dbms_xplan-in-10g/

Charles Hooper
IT Manager/Oracle DBA
K&M Machine-Fabricating, Inc.


 
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