temp tables versus the global sql area

In the Momjian book it is stated:

Temporary tables are ideal for holding intermediate data used by 
the current SQL session. For example, suppose you need to do many 
SELECTs on the result of a complex query. An efficient strategy 
is to execute the complex query once, then store the result in a 
temporary table.


...However, my question is doesn't postgresql calculate the cost 
of complex queries and store there result in some sort of global 
sql area sorted by LRU or cost so that you can simply write 
complex queries and take care of automatic caching?