I have a mysql table with a lot of long fields. One of them is an URL that can be very long (is type TEXT).

I had the issue
Cannot create a functional index on an expression that returns a BLOB or TEXT. Please consider using CAST.

I tried doing this:

create INDEX lowercase_url_index on users ( (CAST(LEFT(lower(url) ,
255) as CHAR(255))) ) USING BTREE;

But this index gets fully ignored when querying on the field in to lower

I tried creating an index on a text field to lower case but gets ignored