EXPLAIN EXTENDED warning.

Query optimizer searches for the constant tables and optimizes them away. This
means that fields of such tables are substituted for their values and on later
phases they are treated as constants. After this constant tables are removed
from the query execution plan. Nevertheless constant tables were shown in 
the EXPLAIN EXTENDED warning thus producing query that might be not an
equivalent of the original query.
        
Now the print_join function skips all tables that were optimized away from
printing to the EXPLAIN EXTENDED warning. If all tables were optimized away it
produces the 'FROM dual' clause.