IPC:ProcArrayGroupUpdate

Waiting for the group leader to clear the transaction ID at transaction end.

The "Group Update" Optimization

In older versions of PostgreSQL, every process finishing a transaction had to acquire a lock on the ProcArray individually to clear its Transaction ID. On very busy servers, hundreds of processes would "fight" for this lock simultaneously, leading to massive overhead. To fix this, PostgreSQL now uses Group Updates:

When multiple processes need to update the ProcArray at the same time, they join a "queue."
One process is designated as the Group Leader.
The Group Leader acquires the lock once and performs the updates for everyone in the queue.
The other processes (the followers) wait. Their wait event is recorded as ProcArrayGroupUpdate.