Most of the time, pgmq uses clock_timestamp() when setting or comparing vt. This is the case for the following functions:

• read
• read_with_poll
• send
• send_batch

But these functions use now():

• pop
• set_vt

Is there a reason for the difference?

Depending on which is used, the behavior can be different when used inside a transaction, which might be unexpected. I ran into this while testing something and it surprised me until I looked at the code.

Here is an example showing what I'm talking about:
Inside a transaction, send a message to the queue. You can't pop the message, but you can read it.

postgres=# begin transaction;
BEGIN
postgres=*# select pgmq.send('my_queue', '{"foo": "bar1"}');
 send 
------
   38
(1 row)

postgres=*# select pgmq.pop('my_queue');
 pop 
-----
(0 rows)

postgres=*# select pgmq.read('my_queue', 30, 1);
                                             read                                             
----------------------------------------------------------------------------------------------
 (37,2,"2024-12-20 19:44:53.600822+00","2024-12-20 19:46:10.624711+00","{""foo"": ""bar1""}")
(1 row)

postgres=*# commit;
COMMIT

If there isn't a reason for them to be different, I think pop and set_vt should use clock_timestamp(), to be consistent with the other functions.