Optimizer question

query on 11.2.0.4 DB

Table T has 1000000 rows, column A has all values Null, column B has 10 different values , no Nulls.

Index on T(A,B) is defined, num_distinct for index is 10.

select * from T where A=?                             , where ? is not Null uses column statistics, expects 1 row, makes index scan

select * from T where A=? and B=?                , both not Null uses index statistics, expects 100000 rows, makes table full scan.

Why for the second query optimizer switched from using column statistics to index statistics ? It is some bug ? How to get correct plan for the second query ?

Best Regards,
Petr

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