Postgres Refresh Materialized View Locks
REFRESH MATERIALIZED VIEW CONCURRENTLY takes an EXCLUSIVE lock, so SELECTs may still run. Per the manual.

There seems to be an oversight with REFRESH MATERIALIZED VIEW (without CONCURRENTLY) as it's not listed there. A search of the source code shows that ExecRefreshMatView take an ACCESS EXCLUSIVE lock, as you might expect, so no other queries may run on the view.

I've submitted a docs patch to list it.