with t1 as (
Select distinct val_2 
The from big_table 
Where val_3=185 
) 
Select avg (val_1) 
The from aux_table a 
Where val_3 in (select val_2 from t1); 

Aux_table and big_table are being big table, big_table table has index (val_2 val_3)
Aux_table on val_3 created index in the table, but check the execution plan and found no index, force index is also useless,,,,
Have any bosses know why