Index usage question - The CBO is able to use the index for a numeric column but not if that same da

 The test case is below. Why is Oracle using the index on the numeric column but not the character column? What types of things should I look for to determine why this is?

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 drop table mi_entities_temp;

create table mi_entities_temp as select enty_key, to_char(enty_key) char_enty_key from mi_entities;

create index n_mi_enty_tmp_ek on mi_entities_temp(enty_key);

create index n_mi_enty_tmp_cek on mi_entities_temp(char_enty_key);

EXEC DBMS_STATS.gather_table_stats(user, 'MI_ENTITIES_TEMP');

explain plan for

SELECT MI_PAGING_OUTER_QUERY.*