Index on varchar is not used when column compares to literal integer

[Panayotis Matsinopoulos]

Hi,

I have a select on a table as follows:

select x from t where x = 13;

I already have an index on x and x is varchar(255).

However, explain shows that my query does not use index. And the query runs very slow.

When I replace the query with

select x from t where x = '13';

it runs lightning fast and index is used.

Why is that? Is there a global mysql configuration to convert integer literals to varchars in such cases and automatically use the index?

Thanks in advance
Panayotis