Wrong results with cost-based optimizer?

Hi folx.

Because of performance problems in one of our applications we tried Oracle's cost-based optimizer. We did the following: fist, we generated statistics on all tables with

        ANALYZE TABLE <table> COMPUTE STATISTICS;

and then we switched the RDBMS (on a per session basis) explicitly to cost based optimization with

        ALTER SESSION SET OPTIMIZER_GOAL = FIRST_ROWS; After doing this, we observed that several SELECT statements (with outer joins only?) gave other results than before. For example, the statement

SELECT COUNT(*)
FROM person, address
WHERE person.address_id = address.id (+);

             ^ fk 1:c ^ pk
yields 33482 (# of all persons with an address) in case of cost based optimization and 57081 (# of all persons with or without an address) in case of rule based optimization. Only the latter result is correct, of course.

Is this a known bug? Is there any way to avoid such incorrect behavior of the cost-based optimizer? Or did we made something wrong?

--
Frank Seitz, WMD GmbH, Hamburg, +49-40-58958-175, seitz_at_wmd.de