Enforcing lowest transaction isolation level across concurrent sessions?
There is no danger of locking with your scenario.

The only lock that is taken on facttable is the ACCESS SHARE lock that prevents others from dropping or altering the table while it is being read. Since ACCESS SHARE doesn't conflict with itself, that is no problem (two people can read the same poster).

The only consideration is the I/O load: if many processes perform a sequential scan on the table, that could be substantial. You should try to benefit from PostgreSQL's synchronized sequential scans by starting all processes at the same time, so that they have a chance to synchronize their sequential scans.