Again, log in as a superuser.

Run this query:

SELECT datname, usename, query
       FROM pg_stat_activity
       WHERE waiting = true;

You will get a list of queries that are waiting on other backends. The following query will run on PostgreSQL versions older than 9.2:

SELECT datname, usename, current_query
       FROM pg_stat_activity
       WHERE waiting = true;

The pg_stat_activity system view has a Boolean field named waiting. This field indicates that a certain backend is waiting on a system lock.

The preceding query uses it to filter out only those queries that are waiting.

Some more explanations about the preceding queries are appropriate here.