rule hint ignored?

Hi all,

We have the same query using the /*+ rule */ hint in DEV, TST, and PRD. In our TST environment in looks like the plan is being generated by the cost based optimizer instead.

Part of explain plan from TEST:

Execution Plan

    0      SELECT STATEMENT Optimizer=HINT: RULE (Cost=5207 Card=2031 B
           ytes=89364)
    1    0   SORT (UNIQUE) (Cost=5207 Card=2031 Bytes=89364)
    2    1     HASH JOIN (Cost=5190 Card=2031 Bytes=89364)
  .......................

Part of explain plan from DEV, PRD
Execution Plan

    0 SELECT STATEMENT Optimizer=HINT: RULE     1 0 SORT (UNIQUE)

    2    1     TABLE ACCESS (BY INDEX ROWID) OF 'ADDRESS'
    3    2       NESTED LOOPS
........................

The DEV and TST boxes were both created from copies of production a couple of months ago, and the data is kept partly in sync. The data differs in each instance by at most 5%, and stats are all up to date. Databases at 9206 and all three have pretty much the same init.ora (except open_cursors, sga, buffers etc)

Since this is the same exact query (cut an pasted), does anyone have any suggestions on what might be causing the TST system to ignore the rule hint? (BTW, the RBO does give the better results, just not sure why rule is ignored in one instance and used in the other two)

QUERY:
select /*+ rule */ distinct ent.id_number from entity ent ,address adr ,degrees deg where (ent.delete_ind = 'N' and adr.id_number = ent.id_number and deg.id_number = ent.id_number ) and ( ent.record_status_code = 'M' and adr.state_code = 'LA' and deg.school_code = 'JYM' )

parallel query and query rewrite are turned off for all three instances too

Thanks

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