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Commit21b0055

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Tweak behavior of pg_stat_activity.leader_pid
The initial implementation of leader_pid in pg_stat_activity added byb025f32 took the approach to strictly print what a PGPROC entryincludes. In short, if a backend has been involved in parallel query atleast 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 beset after it participated at least once in parallel query, and aftermore discussions this could be confusing if using for example aconnection pooler.This commit changes the data printed so as leader_pid becomes alwaysNULL for a parallel group leader, showing up a non-NULL value only forthe parallel workers, and actually as long as a parallel query isrunning as workers are shut down once the query has completed.This does not change the definition of any catalog, so no catalog bumpis needed. Per discussion with Justin Pryzby, Álvaro Herrera, JulienRouhaud and me.Discussion:https://postgr.es/m/20200721035145.GB17300@paquier.xyzBackpatch-through: 13
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‎doc/src/sgml/monitoring.sgml

Lines changed: 3 additions & 6 deletions
Original file line numberDiff line numberDiff line change
@@ -687,12 +687,9 @@ postgres 27093 0.0 0.0 30096 2752 ? Ss 11:34 0:00 postgres: ser
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<structfield>leader_pid</structfield> <type>integer</type>
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</para>
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<para>
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Process ID of the parallel group leader if this process is or
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has been involved in parallel query, or null. This field is set
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when a process wants to cooperate with parallel workers, and
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remains set as long as the process exists. For a parallel group leader,
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this field is set to its own process ID. For a parallel worker,
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this field is set to the process ID of the parallel group leader.
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Process ID of the parallel group leader, if this process is a
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parallel query worker. <literal>NULL</literal> if this process is a
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parallel group leader or does not participate in parallel query.
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</para></entry>
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</row>
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‎src/backend/utils/adt/pgstatfuncs.c

Lines changed: 7 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -735,7 +735,13 @@ pg_stat_get_activity(PG_FUNCTION_ARGS)
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wait_event=pgstat_get_wait_event(raw_wait_event);
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leader=proc->lockGroupLeader;
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if (leader)
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/*
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* Show the leader only for active parallel workers. This
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* leaves the field as NULL for the leader of a parallel
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* group.
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*/
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if (leader&&leader->pid!=beentry->st_procpid)
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{
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values[29]=Int32GetDatum(leader->pid);
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nulls[29]= false;

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