Bad performance in a OR query

Hi,

I'm working on the usage of MaxDB in the RT System 
(http://bestpractical.com/rt/).
I'm using MaxDB 7.0.5.19.
I obtain really bad performances on some queries.

This is the part of a query which is not well using indexes:
SELECT main.id
FROM Groups main, ACL ACL_1
WHERE ((ACL_1.PrincipalId = main.id AND ACL_1.PrincipalType = 'Group')
OR (main.Domain = 'RT::Queue-Role' AND main.Instance = 3  AND main.Type 
= ACL_1.PrincipalType))

This query is not using my indexes, the explain commands gives to me:
'';'ACL_1';'';'TABLE SCAN';'         1'
'';'MAIN';'';'TABLE SCAN';'       580'
'';'';'';'     RESULT IS COPIED   , COSTVALUE IS';'    316698'

But the following query gives a similar result (if we don't concider 
duplicates values):
SELECT main.id
FROM Groups main, ACL ACL_1
WHERE ACL_1.PrincipalId = main.id AND ACL_1.PrincipalType = 'Group'
UNION
SELECT main.id
FROM Groups main, ACL ACL_1
WHERE main.Domain = 'RT::Queue-Role' AND main.Instance = 3  AND 
main.Type = ACL_1.PrincipalType

and this query uses my indexes and is much faster. For this query the 
execution plan is:
'';'ACL_1';'';'TABLE SCAN';'         1'
'';'MAIN';'ID';'JOIN VIA KEY COLUMN';'       580'
'';'ACL_1';'';'TABLE SCAN';'         1'
'';'MAIN';'GROUPS1';'JOIN VIA RANGE OF MULTIPLE INDEXED COL.';'       580'
'';'';'DOMAIN';'     (USED INDEX COLUMN)';''
'';'';'INSTANCE';'     (USED INDEX COLUMN)';''
'';'';'';'     RESULT IS COPIED   , COSTVALUE IS';'        19'


The optimization is not well working when I'm using the query with "OR".
Notes that the real query is more complex. This is just a part to show 
that indexes are not used with combination of conditions with a "OR".

Is this possible to configure MaxDB to correctly use indexes in the case ?

Thanks a lot for your help.

Michael Fernandez
DMS GmbH.


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