Oracle Analyze Function-Based Index Ignored or Not Working

Question:  I have created a function-based index on a table column, yet when I check by execution plan, the function-based index is ignored by the optimizer.

Why does Oracle ignore a function-based index? 

How do I force Oracle to use a function-based index?

Answer: :  The optimizer is aware of all indexes at index creation time, but you still must analyze the function-based index, else the optimizer may ignore the index.  

The most common problem when a function-based index is ignored is when the DBA has forgotten to analyze the function-based index.  After creating a function-based index, you want to re-gather table statistics and get extended statistics on the function.

In this case we create a function-based index and then analyze using the method_opt argument set to FOR ALL HIDDEN COLUMNS.

 Create an FBI on emp_nbr column with NULL values

create index
     emp_null_emp_nbr_idx
on
    emp
    (nvl(ename,o));

To make the function-based index work, we must now analyze the index.  Depending on your release, these methods are appropriate:

EXEC DBMS_STATS.gather_index_stats('EMP', ‘emp_null_emp_nbr_idx’);

exec dbms_stats.gather_table_stats(
    ownname=>null,
    tabname=> 'EMP',
    estimate_percent=>null,
    cascade=>true,
    method_opt=> ‘FOR ALL HIDDEN COLUMNS SIZE 1′
);

In Oracle 11g and beyond, Oracle recommends analyzing “extended” statistics on function-based indexes:

begin
   dbms_stats.gather_table_stats (
      ownname    => 'SCOTT',
      tabname    => 'EMP',
      method_opt => 'for all columns size skewonly for columns (nvl(ename,o))'  
);
end;

Also see these details on when a function-based index is ignored.

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