Height Balanced Histogram on a unique varchar2 column

What benefit the CBO can gain from a unique varchar2 column having height balanced histogram (11.2.0.3.0) ?

drop table t1 purge;

create table t1

        (col1 number

,col2 varchar2(50)

,flag varchar2(2));

insert into t1

     select

            rownum

           ,dbms_random.string('s',20)

           ,case when rownum = 1

                 then 'Y1'

                 when rownum = 2

                 then 'Y2'

                 when mod(rownum,2) = 0

                 then 'N1'

                 else 'N2'

             end

      from   dual

      connect by rownum <= 1e5;

commit;

create unique index ind_t1 on t1(col2);

BEGIN       dbms_stats.gather_table_stats

        (user

,'T1'

,method_opt => 'FOR ALL COLUMNS SIZE SKEWONLY'

,estimate_percent => DBMS_STATS.AUTO_SAMPLE_SIZE

,cascade => true

,no_invalidate => FALSE);

 END; /

select column_name

      ,histogram

from

      user_tab_col_statistics

where table_name = 'T1';

COLUMN_NAME                    HISTOGRAM

------------------------------ ---------------

COL1                           NONE

COL2                           HEIGHT BALANCED

FLAG                           FREQUENCY

Thanks

-- 

Houri Mohamed

Oracle DBA-Developer-Performance & Tuning

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