What might be blocking my Postgres ALTER?

gistfile1.sql

select distinct st.pid, 
       current_timestamp - st.query_start as locked_time,
       st.state,
       st.usename,
       st.client_hostname,
       st.application_name
from pg_locks l
inner join pg_stat_activity st on l.pid = st.pid
where st.datname = 'analytics_master'
order by current_timestamp - st.query_start desc;