optimizer OR transformation

Hi

I have some queries with ORs in 8i which ahd this plan

   0 SELECT STATEMENT Optimizer=RULE    1 0 SORT (ORDER BY)

   2    1     TABLE ACCESS (BY INDEX ROWID) OF 'S_LST_OF_VAL'
   3    2       INDEX (RANGE SCAN) OF 'S_LST_OF_VAL_U2' (UNIQUE)

in 9i this changed to

   0 SELECT STATEMENT Optimizer=RULE    1 0 SORT (ORDER BY)

   2    1     CONCATENATION
   3    2       TABLE ACCESS (BY INDEX ROWID) OF 'S_LST_OF_VAL'
   4    3         INDEX (RANGE SCAN) OF 'S_LST_OF_VAL_U1' (UNIQUE)
   5    2       TABLE ACCESS (BY INDEX ROWID) OF 'S_LST_OF_VAL'
   6    5         INDEX (RANGE SCAN) OF 'S_LST_OF_VAL_U1' (UNIQUE)
   7    2       TABLE ACCESS (BY INDEX ROWID) OF 'S_LST_OF_VAL'
   8    7         INDEX (RANGE SCAN) OF 'S_LST_OF_VAL_U1' (UNIQUE)
   9    2       TABLE ACCESS (BY INDEX ROWID) OF 'S_LST_OF_VAL'
  10    9         INDEX (RANGE SCAN) OF 'S_LST_OF_VAL_U1' (UNIQUE)

I was wondering which parameter (hidden of course) affects this change of plans?

I have checked _or_expand_nvl_predicate and _no_or_expansion but doesnt seem they are affceting the behaviour.

Using RBO by the way.

Thanks

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