You are just talking to the right topic . In WWW.DBASUPPORT.COM , there is a detail explanation in this area.Usuallly We do:
//Explicitly Disabling an Index
select name from emp where emp_type||'' = 'A'
//Explicitly Disabling an Index
select name from emp where emp_type||'' = 'A'
usually the performance hike is quite limited if you disable the index . on the other side , indexing the column might bring you a lot of performance benefits .for number column you can make change like
COL1+0=10 //original is COL1=10