From 11a68e4b53ffccf336a2faf5fa380acda28e880b Mon Sep 17 00:00:00 2001 From: Michael Paquier Date: Sun, 26 Jul 2020 16:32:11 +0900 Subject: [PATCH] Tweak behavior of pg_stat_activity.leader_pid MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit The initial implementation of leader_pid in pg_stat_activity added by b025f32 took the approach to strictly print what a PGPROC entry includes. In short, if a backend has been involved in parallel query at least once, leader_pid would remain set as long as the backend is alive. For a parallel group leader, this means that the field would always be set after it participated at least once in parallel query, and after more discussions this could be confusing if using for example a connection pooler. This commit changes the data printed so as leader_pid becomes always NULL for a parallel group leader, showing up a non-NULL value only for the parallel workers, and actually as long as a parallel query is running as workers are shut down once the query has completed. This does not change the definition of any catalog, so no catalog bump is needed. Per discussion with Justin Pryzby, Álvaro Herrera, Julien Rouhaud and me. Discussion: https://postgr.es/m/20200721035145.GB17300@paquier.xyz Backpatch-through: 13 --- doc/src/sgml/monitoring.sgml | 9 +++------ src/backend/utils/adt/pgstatfuncs.c | 8 +++++++- 2 files changed, 10 insertions(+), 7 deletions(-) diff --git a/doc/src/sgml/monitoring.sgml b/doc/src/sgml/monitoring.sgml index dc49177c78..7dcddf478a 100644 --- a/doc/src/sgml/monitoring.sgml +++ b/doc/src/sgml/monitoring.sgml @@ -687,12 +687,9 @@ postgres 27093 0.0 0.0 30096 2752 ? Ss 11:34 0:00 postgres: ser leader_pid integer - Process ID of the parallel group leader if this process is or - has been involved in parallel query, or null. This field is set - when a process wants to cooperate with parallel workers, and - remains set as long as the process exists. For a parallel group leader, - this field is set to its own process ID. For a parallel worker, - this field is set to the process ID of the parallel group leader. + Process ID of the parallel group leader, if this process is a + parallel query worker. NULL if this process is a + parallel group leader or does not participate in parallel query. diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c index 2aff739466..95738a4e34 100644 --- a/src/backend/utils/adt/pgstatfuncs.c +++ b/src/backend/utils/adt/pgstatfuncs.c @@ -735,7 +735,13 @@ pg_stat_get_activity(PG_FUNCTION_ARGS) wait_event = pgstat_get_wait_event(raw_wait_event); leader = proc->lockGroupLeader; - if (leader) + + /* + * Show the leader only for active parallel workers. This + * leaves the field as NULL for the leader of a parallel + * group. + */ + if (leader && leader->pid != beentry->st_procpid) { values[29] = Int32GetDatum(leader->pid); nulls[29] = false;