Common Subquery Elimination in 8.1.7 causes incorrect results -

From: Reardon, Bruce (CALBBAY) <Bruce.Reardon_at_comalco.riotinto.com.au>
Date: Thu, 05 Apr 2001 20:28:25 -0700
Message-ID: <F001.002E3768.20010405195522@fatcity.com>

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
San Diego, California        -- Public Internet access / Mailing Lists
--------------------------------------------------------------------
To REMOVE yourself from this mailing list, send an E-Mail message
to: ListGuru_at_fatcity.com (note EXACT spelling of 'ListGuru') and in
the message BODY, include a line containing: UNSUB ORACLE-L
(or the name of mailing list you want to be removed from).  You may
also send the HELP command for other information (like subscribing).

Received on Thu Apr 05 2001 - 22:28:25 CDT