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Use actual backend IDs in pg_stat_get_backend_idset() and friends.

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Up to now, the ID values returned by pg_stat_get_backend_idset() and
used by pg_stat_get_backend_activity() and allied functions were just
indexes into a local array of sessions seen by the last stats refresh.
This is problematic for a few reasons.  The "ID" of a session can vary
over its existence, which is surprising.  Also, while these numbers
often match the "backend ID" used for purposes like temp schema
assignment, that isn't reliably true.  We can fairly cheaply switch
things around to make these numbers actually be the sessions' backend
IDs.  The added test case illustrates that with this definition, the
temp schema used by a given session can be obtained given its PID.

While here, delete some dead code that guarded against getting
a NULL return from pgstat_fetch_stat_local_beentry().  That can't
happen as long as the caller is careful to pass an in-range array
index, as all the callers are.  (This code may not have been dead
when written, but it surely is now.)

Nathan Bossart

Discussion: https://postgr.es/m/20220815205811.GA250990@nathanxps13
This commit is contained in:
Tom Lane
2022-09-29 12:14:39 -04:00
parent d5e3fe682a
commit d7e39d72ca
6 changed files with 107 additions and 51 deletions
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20
doc/src/sgml/monitoring.sgml
View File

@ -5485,20 +5485,23 @@ SELECT pid, wait_event_type, wait_event FROM pg_stat_activity WHERE wait_event i
the <structname>pg_stat_activity</structname> view, returns a set of records
containing all the available information about each backend process.
Sometimes it may be more convenient to obtain just a subset of this
information. In such cases, an older set of per-backend statistics
information. In such cases, another set of per-backend statistics
access functions can be used; these are shown in <xref
linkend="monitoring-stats-backend-funcs-table"/>.
These access functions use a backend ID number, which ranges from one
to the number of currently active backends.
These access functions use the session's backend ID number, which is a
small positive integer that is distinct from the backend ID of any
concurrent session, although a session's ID can be recycled as soon as
it exits. The backend ID is used, among other things, to identify the
session's temporary schema if it has one.
The function <function>pg_stat_get_backend_idset</function> provides a
convenient way to generate one row for each active backend for
convenient way to list all the active backends' ID numbers for
invoking these functions. For example, to show the <acronym>PID</acronym>s and
current queries of all backends:
<programlisting>
SELECT pg_stat_get_backend_pid(s.backendid) AS pid,
pg_stat_get_backend_activity(s.backendid) AS query
FROM (SELECT pg_stat_get_backend_idset() AS backendid) AS s;
SELECT pg_stat_get_backend_pid(backendid) AS pid,
pg_stat_get_backend_activity(backendid) AS query
FROM pg_stat_get_backend_idset() AS backendid;
</programlisting>
</para>
@ -5526,8 +5529,7 @@ SELECT pg_stat_get_backend_pid(s.backendid) AS pid,
<returnvalue>setof integer</returnvalue>
</para>
<para>
Returns the set of currently active backend ID numbers (from 1 to the
number of active backends).
Returns the set of currently active backend ID numbers.
</para></entry>
</row>