Postgres partially hang after inactivity

Hi.

I have a weird problem with PostgreSQL on a Windows machine. This is a
test installation used to beta test a webapplication running with
Tomcat, so most of the times it sits idle. Yesterday and today I had to
update some table definitions, so issued the 'alter table...' command
and the server hanged. The webapplication runs well so the queries are
working but seems that somehow the modifications are locked. I'm not
sure if writing works (haven't tested it, will do the next time).

I'm new to postgres so I don't know exactly what to do to track down
this problem, the log files doesn't seems to report any issue. Any
suggestion about where to look to understand what's wrong ?

As additional informations, the webapplication uses hibernate to access
the database, I'm not sure but it is possible that the sessions are not
always closed cleanly or closed at all. Postgres version is 8.3.7.

Regards,
Marco.