A question on MySQL SELECT Algorithm/Function
To see how MySQL is doing something you want to look at the execution plan. More information on MySQL Execution plans at http://dev.mysql.com/doc/refman/5.5/en/execution-plan-information.html . You should think of SQL as a declarative language, you tell it to do something , and it writes a program to do that. The execution plan shows you the program it wrote.

SQL will be able to look up specific entries when there is an index on the column ( like an index from a book). However, a binary field with a split distribution is a classic example of a column that doesn't have good selectivity, in other words, it isn't going to narrow down the search very much.