do unneeded outer joins cost? - Mailing list pgsql-sql

for example:

select    t1.*
from      t1
left join t2 on( <expensive condition> );

since t2 is not used in the result nor is it used in determining the result, is the <expensive condition> to left join
t2used?
 

(the question goes as to how much generated sql needs to be cleaned to be efficient)