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help / color / mirror / Atom feedFrom: Ian Lawrence Barwick <[email protected]>
To: pgsql-hackers <[email protected]>
Subject: pg_stat_get_backend_subxact() and backend IDs?
Date: Thu, 24 Aug 2023 10:22:49 +0900
Message-ID: <CAB8KJ=j-ACb3H4L9a_b3ZG3iCYDW5aEu3WsPAzkm2S7JzS1Few@mail.gmail.com> (raw)
Hi
I was playing around with "pg_stat_get_backend_subxact()" (commit 10ea0f924)
and see it emits NULL values for some backends, e.g.:
postgres=# \pset null NULL
Null display is "NULL".
postgres=# SELECT id, pg_stat_get_backend_pid(id), s.*,
pg_stat_get_backend_activity (id)
FROM pg_stat_get_backend_idset() id
JOIN LATERAL pg_stat_get_backend_subxact(id) AS s ON TRUE;
id | pg_stat_get_backend_pid | subxact_count |
subxact_overflowed | pg_stat_get_backend_activity
-----+-------------------------+---------------+--------------------+------------------------------------------------------------
1 | 3175972 | 0 | f
| <command string not enabled>
2 | 3175973 | 0 | f
| <command string not enabled>
3 | 3177889 | 0 | f
| SELECT id, pg_stat_get_backend_pid(id), s.*, +
| | |
| pg_stat_get_backend_activity (id) +
| | |
| FROM pg_stat_get_backend_idset() id +
| | |
| JOIN LATERAL pg_stat_get_backend_subxact(id) AS s ON TRUE;
4 | 3176027 | 5 | f
| savepoint s4;
256 | 3175969 | NULL | NULL
| <command string not enabled>
258 | 3175968 | NULL | NULL
| <command string not enabled>
259 | 3175971 | NULL | NULL
| <command string not enabled>
(7 rows)
Reading through the thread [1], it looks like 0/false are intended to be
returned for non-backend processes too [2], so it seems odd that NULL/NULL is
getting returned in some cases, especially as that's what's returned if a
non-existent backend ID is provided.
[1] https://www.postgresql.org/message-id/flat/CAFiTN-uvYAofNRaGF4R%2Bu6_OrABdkqNRoX7V6%2BPP3H_0HuYMwg%4...
[2] https://www.postgresql.org/message-id/flat/CAFiTN-ut0uwkRJDQJeDPXpVyTWD46m3gt3JDToE02hTfONEN%3DQ%40m...
Looking at the code, this is happening because
"pgstat_fetch_stat_local_beentry()"
expects to be passed the backend ID as an integer representing a 1-based index
referring to "localBackendStatusTable", but "pg_stat_get_backend_subxact()"
is presumably intended to take the actual BackendId , as per other
"pg_stat_get_XXX()"
functions.
Also, the comment for "pgstat_fetch_stat_local_beentry()" says:
Returns NULL if the argument is out of range (no current caller does that).
so the last part is currently incorrect.
Assuming I am not misunderstanding something here (always a
possibility, apologies
in advance if this is merely noise), what is actually needed is a function which
accepts a BackendId (as per "pgstat_fetch_stat_beentry()"), but returns a
LocalPgBackendStatus (as per "pgstat_fetch_stat_local_beentry()") like the
attached, clumsily named "pgstat_fetch_stat_backend_local_beentry()".
Regards
Ian Barwick
Attachments:
[text/x-patch] pg_stat_get_backend_subxact-fix.v1.patch (2.5K, ../CAB8KJ=j-ACb3H4L9a_b3ZG3iCYDW5aEu3WsPAzkm2S7JzS1Few@mail.gmail.com/2-pg_stat_get_backend_subxact-fix.v1.patch)
download | inline diff:
commit d4292d3347658f61855fd84827954f587838a324
Author: Ian Barwick <[email protected]>
Date: Thu Aug 24 10:16:51 2023 +0900
pg_stat_get_backend_subxact(): handle backend ID correctly
diff --git a/src/backend/utils/activity/backend_status.c b/src/backend/utils/activity/backend_status.c
index 38f91a495b..bce7836264 100644
--- a/src/backend/utils/activity/backend_status.c
+++ b/src/backend/utils/activity/backend_status.c
@@ -1136,6 +1136,39 @@ pgstat_fetch_stat_local_beentry(int beid)
}
+/* ----------
+ * pgstat_fetch_stat_backend_local_beentry() -
+ *
+ * Like pgstat_fetch_stat_local_beentry() but takes the BackendId of the
+ * desired session.
+ *
+ * Returns NULL if the given beid doesn't identify any known session.
+ *
+ * NB: caller is responsible for a check if the user is permitted to see
+ * this info (especially the querystring).
+ * ----------
+ */
+
+LocalPgBackendStatus *
+pgstat_fetch_stat_backend_local_beentry(BackendId beid)
+{
+ LocalPgBackendStatus key;
+
+ pgstat_read_current_status();
+
+ /*
+ * Since the localBackendStatusTable is in order by backend_id, we can use
+ * bsearch() to search it efficiently.
+ */
+ key.backend_id = beid;
+
+ return (LocalPgBackendStatus *) bsearch(&key, localBackendStatusTable,
+ localNumBackends,
+ sizeof(LocalPgBackendStatus),
+ cmp_lbestatus);
+}
+
+
/* ----------
* pgstat_fetch_stat_numbackends() -
*
diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index 2b9742ad21..8479ea130b 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -727,7 +727,7 @@ pg_stat_get_backend_subxact(PG_FUNCTION_ARGS)
BlessTupleDesc(tupdesc);
- if ((local_beentry = pgstat_fetch_stat_local_beentry(beid)) != NULL)
+ if ((local_beentry = pgstat_fetch_stat_backend_local_beentry(beid)) != NULL)
{
/* Fill values and NULLs */
values[0] = Int32GetDatum(local_beentry->backend_subxact_count);
diff --git a/src/include/utils/backend_status.h b/src/include/utils/backend_status.h
index 77939a0aed..760ddc48ae 100644
--- a/src/include/utils/backend_status.h
+++ b/src/include/utils/backend_status.h
@@ -334,6 +334,7 @@ extern uint64 pgstat_get_my_query_id(void);
*/
extern int pgstat_fetch_stat_numbackends(void);
extern PgBackendStatus *pgstat_fetch_stat_beentry(BackendId beid);
+extern LocalPgBackendStatus *pgstat_fetch_stat_backend_local_beentry(BackendId beid);
extern LocalPgBackendStatus *pgstat_fetch_stat_local_beentry(int beid);
extern char *pgstat_clip_activity(const char *raw_activity);
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