Interpreting MessageQueueSend state

On Mon, 2023-11-13 at 17:06 -0800, Benjamin Leis wrote:

I'm looking at a pg_stat_activity dump where there are a group of active
threads stuck for a long time in MessageQueueSend
7:52.394089+00 | IPC | MessageQueueSend | active |
| 27406510 | -5892847938856697008 | SELECT ....
Does this mean they are blocked on clients reading data or does it
represent something blocking on the server side?

That's something on the server: a parallel worker process is waiting to
send data to the parallel leader process. Look for the "leader_pid" column
in the "pg_stat_activity" line above and see what that process is doing.

I wouldn't worry about a parellel worker in that state, as long as there
is no problem with the parallel leader.

Yours,
Laurenz Albe