Re: Common Subquery Elimination in 8.1.7 causes incorrect

I would strongly suggest the:

optimizer_features_enable = 8.1.6

because it also cures another problem with 8.1.7 wherein a complex query with multiple OR and AND conditions will return the wrong number of rows if you are using CBO (and/or any statistics are on the resultant table).

larry

At 07:55 PM 4/5/2001 -0800, you wrote:
>Hi,
>
>There is a note on Metalink (137430.1 reproduced below) describing a generic
>817 optimiser problem that will result in incorrect results when using
>certain query types.
>
>We are currently running 81511 on NT but testing an upgrade to 81711 on NT
>at the moment.
>
>The note suggests 2 workarounds, and my question to others is
>
>Should we set "optimizer_features_enable = 8.1.6"
>-or-
>should we set "_eliminate_common_subexpr = false"
>
>Which method does the list suggest, and what are the implications of each?
>
>Thanks,
>Bruce Reardon
>mailto:bruce.reardon_at_comalco.riotinto.com.au
>
>
>
>Doc ID: Note:137430.1
>results
>Type: ALERT
>Status: PUBLISHED
> Content Type: TEXT/PLAIN
>Creation Date: 23-MAR-2001
>Last Revision Date: 28-MAR-2001
>
>
> Common Subquery Elimination in 8.1.7 causes incorrect results
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> Versions Affected
> ~~~~~~~~~~~~~~~~~
> Oracle Server releases 8.1.7.0 and 8.1.7.1
>
> Fixed in 8.1.7.2 and 9.0.0.0
>
> Platforms Affected
> ~~~~~~~~~~~~~~~~~~
> GENERIC
>
> Description
> ~~~~~~~~~~~
> The bug is that rows may be returned when none are expected with the query
>below.
> Other queries may return more or less rows than expected depending on the
>predicates used.
>
>
> select * from tab1
> where (tab1.col1 = 1 and exists (select * from tab2))
> or (tab1.col1 = 2 and exists (select * from tab3))"
>
>
>There are several variations on this.
>Eg:
>select /*+ ALL_ROWS */ * from tab1
> where (tab1.col1 = 1 and col1 in (select col1 from tab2))
> or (tab1.col1 = 2 and col1 in (select col1 from tab3))
>;
>
>
>
>
> Note: The use of the two parts of the WHERE clause separated by an OR
>operator,
> each clause containing a non-correlated subquery.
>
>
> The problem is that common subquery elimination treats un-correlated
>subqueries
> as identical even when they are not.
>
>
>
> Likelihood of Occurrence
> ~~~~~~~~~~~~~~~~~~~~~~~~
>
> Any time the above construct is used.
> This construct is common in queries used by some third party applications
>including
> People Soft.
>
> This problem only occurs on database versions 8.1.7.0 and 8.1.7.1.
> Earlier versions are not affected.
>
>
> Possible Symptoms
> ~~~~~~~~~~~~~~~~~
> No error is raised.
> Unless the user is aware of incorrect results the problem will be
>undetected.
>
>
>
>
> Workaround
> ~~~~~~~~~~
> As the problem is caused by a new optimizer feature introduced in 8.1.7,
> the workaround is to set a parameter in the init<SID>.ora to disable the
>feature.
>
>
>Either set:
>
>
> "optimizer_features_enable = 8.1.6"
> -or-
> "_eliminate_common_subexpr = false"
>
>
> Patches
> ~~~~~~~
> This is expected to be fixed in the 8.1.7.2 Patch Set.
> The problem is fixed in Oracle9i.
> No single one off patches are currently available.
>
>
>
>
> References
> ~~~~~~~~~~
> QUERY RETURNING ROWS IN 8.1.7 WHEN NO ROWS EXPECTED [BUG:1578644]
>--
>Please see the official ORACLE-L FAQ: http://www.orafaq.com
>--
>Author: Reardon, Bruce (CALBBAY)
> INET: Bruce.Reardon_at_comalco.riotinto.com.au
>
>Fat City Network Services -- (858) 538-5051 FAX: (858) 538-5051
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-- 
Please see the official ORACLE-L FAQ: http://www.orafaq.com
-- 
Author: Larry Strickland
  INET: stricklandl_at_voyager.spjc.cc.fl.us

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