Performance Issues

I have the following doubts.

1. Does postgres create an index on every primary key?  Usually, queries 
are performed against a table on the primary key, so, an index on it 
will be very useful.

2. If 'm executing a complex query and it takes 10 seconds to return the 
results -- it takes 10 seconds to execute the next time also.  I'm 
wondering if there's any kind of caching that can be enabled -- so, the 
next time it takes <10 seconds to return the results.

Thanks
Dhanaraj