Por que a instrução select solicita bloqueios exclusivos?

No postgres (9.5) selectsolicita bloqueio exclusivo. Por quê?

play=# \x on
Expanded display is on.
play=#   SELECT * FROM pg_locks pl
  LEFT JOIN pg_stat_activity psa
    ON pl.pid = psa.pid
    where
    pl.mode = 'ExclusiveLock'
;


-[ RECORD 1 ]------+---------------------------------
locktype           | virtualxid
database           |
relation           |
page               |
tuple              |
virtualxid         | 7/16
transactionid      |
classid            |
objid              |
objsubid           |
virtualtransaction | 7/16
pid                | 17376
mode               | ExclusiveLock
granted            | t
fastpath           | t
datid              | 1700431
datname            | play
pid                | 17376
usesysid           | 10
usename            | postgres
application_name   | psql
client_addr        | 127.0.0.1
client_hostname    |
client_port        | 54308
backend_start      | 2018-05-07 15:12:30.682069+02
xact_start         | 2018-05-07 15:12:59.513311+02
query_start        | 2018-05-07 15:12:59.513311+02
state_change       | 2018-05-07 15:12:59.513313+02
wait_event_type    |
wait_event         |
state              | active
backend_xid        |
backend_xmin       | 7152428
query              | SELECT * FROM pg_locks pl       +
                   |   LEFT JOIN pg_stat_activity psa+
                   |     ON pl.pid = psa.pid         +
                   |     where                       +
                   |     pl.mode = 'ExclusiveLock'   +
                   | ;