When the join optimizer calculates the order tables should be
joined, fanout is of great importance. The fanout for a table
't_y' in a join plan consistig of (t_1,...,t_x,t_y,..) is the
average number of rows from 't_y' that will match each partial
row combination from (t_1,...,t_x) and all predicates applicable
to 't_y'. It is important to keep the fanout as low as possible
because it directly influences how many row combinations will be
joined with the next table in the join order.

Before this worklog, MySQL only considered:
 * the fanout that is directly determined by the chosen access
   method.
 * a very basic heuristic for table/index scans: 
   - If 'ref' access is applicable, the fanout for scans is 75%
     of the number of rows in the table.
   - If 'range' access is applicable, the fanout for scans 
     equals the number of rows estimated by 'range' access 
     relative to number of rows in the table 
     ("rows_estimate_range_access/rows_in_table").

This worklog changes this by making the join optimizer consider
fanout estimates for any access method and for all conditions
(except subqueries and outer join ON predicates), including
non-indexed columns, when calculating partial plans. By taking
this into account, query conditions can be evaluated earlier
during query execution which in turn can reduce the number of
inspected rows.

The entry point for the new calculation of fanout estimates 
beyond that of the access method is the function 
calculate_condition_filter()