where with function

[marco pilolli]

hi
i have 2 servers
ho 2 server
the first one use an old release (5.0.xx)
when i do an explian of a query lie

select a from table where b=funzionepersonalizzata(data,idutente)

the query use the field B index

in a new server with mysql 5.1.54 (ubuntu 11.04 server)

thw same query don't use index e read all the rows, spending around 7 sec for run

where is the difference?

the new server is old server dump (data+structure)

thanks a lot