Efficiency in defining PK with two fields‬

[itaym02]

The DB is MySql but I am not sure it is relevant.

I want to define a PK for my table, A key which consists of two fields.
Field A has a cardinality of 30000
Field B has a cardinality of 15000

Which of them should come first in the PK assuming all my searches are using both fields in the WHERE clause.

Thanks