On Sat, Sep 24, 2022 at 01:41:38PM -0400, Tom Lane wrote:
> One thing I don't like about it documentation-wise is that it leaves
> the concept of backend ID pretty much completely undefined.
How specific do you think this definition ought to be? All I've come up
with so far is "internal identifier for the backend that is independent
from its PID," which is what I used in the attached patch. Do we want to
mention its uses in more detail (e.g., temp schema name), or should we keep
it vague?
--
Nathan Bossart
Amazon Web Services: https://aws.amazon.com
>From 4fd862d40ba3c703e00973d9743a0d4897fe4197 Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nathandboss...@gmail.com>
Date: Fri, 12 Aug 2022 23:07:37 -0700
Subject: [PATCH v3 1/1] Adjust pg_stat_get_backend_*() to use backends' PGPROC
backend IDs.
Presently, these functions use the index of the backend's entry in
localBackendStatusTable as the backend ID. While this might bear
some resemblance to the backend ID of the backend's PGPROC struct,
it can quickly diverge as sessions connect and disconnect. This
change modifies the pg_stat_get_backend_*() suite of functions to
use the PGPROC backend IDs instead. While we could instead
introduce a new function for retrieving PGPROC backend IDs,
presenting two sets of backend IDs to users seems likely to cause
even more confusion than what already exists.
This is primarily useful for discovering the session that owns a
resource named with its PGPROC backend ID. For example, temporary
schema names include the PGPROC backend ID, and it might be
necessary to identify the session that owns a temporary table that
is putting the cluster in danger of transaction ID wraparound.
Author: Nathan Bossart
---
doc/src/sgml/monitoring.sgml | 10 +++---
src/backend/utils/activity/backend_status.c | 40 +++++++++++++++++++--
src/backend/utils/adt/pgstatfuncs.c | 11 +++---
src/include/utils/backend_status.h | 8 +++++
4 files changed, 57 insertions(+), 12 deletions(-)
diff --git a/doc/src/sgml/monitoring.sgml b/doc/src/sgml/monitoring.sgml
index 1d9509a2f6..47f2883576 100644
--- a/doc/src/sgml/monitoring.sgml
+++ b/doc/src/sgml/monitoring.sgml
@@ -5488,10 +5488,11 @@ SELECT pid, wait_event_type, wait_event FROM pg_stat_activity WHERE wait_event i
information. In such cases, an older 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 process's backend ID number, which is an
+ internal identifier for the backend that is independent from its
+ <acronym>PID</acronym>.
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:
@@ -5526,8 +5527,7 @@ 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>
diff --git a/src/backend/utils/activity/backend_status.c b/src/backend/utils/activity/backend_status.c
index c7ed1e6d7a..159d022070 100644
--- a/src/backend/utils/activity/backend_status.c
+++ b/src/backend/utils/activity/backend_status.c
@@ -849,6 +849,7 @@ pgstat_read_current_status(void)
BackendIdGetTransactionIds(i,
&localentry->backend_xid,
&localentry->backend_xmin);
+ localentry->backend_id = i;
localentry++;
localappname += NAMEDATALEN;
@@ -1045,6 +1046,22 @@ pgstat_get_my_query_id(void)
return MyBEEntry->st_query_id;
}
+/* ----------
+ * cmp_lbestatus
+ *
+ * Comparison function for bsearch() on an array of LocalPgBackendStatus. The
+ * backend_id field is used to compare the arguments.
+ *
+ * ----------
+ */
+static int
+cmp_lbestatus(const void *a, const void *b)
+{
+ LocalPgBackendStatus *lbestatus1 = (LocalPgBackendStatus *) a;
+ LocalPgBackendStatus *lbestatus2 = (LocalPgBackendStatus *) b;
+
+ return lbestatus1->backend_id - lbestatus2->backend_id;
+}
/* ----------
* pgstat_fetch_stat_beentry() -
@@ -1052,6 +1069,10 @@ pgstat_get_my_query_id(void)
* Support function for the SQL-callable pgstat* functions. Returns
* our local copy of the current-activity entry for one backend.
*
+ * Unlike pgstat_fetch_stat_local_beentry(), the beid argument refers to the
+ * backend ID stored in the backend's PGPROC struct instead of its index in
+ * the localBackendStatusTable.
+ *
* NB: caller is responsible for a check if the user is permitted to see
* this info (especially the querystring).
* ----------
@@ -1059,12 +1080,23 @@ pgstat_get_my_query_id(void)
PgBackendStatus *
pgstat_fetch_stat_beentry(int beid)
{
+ LocalPgBackendStatus key;
+ LocalPgBackendStatus *ret;
+
pgstat_read_current_status();
- if (beid < 1 || beid > localNumBackends)
+ if (beid < 1 || beid > NumBackendStatSlots)
return NULL;
- return &localBackendStatusTable[beid - 1].backendStatus;
+ key.backend_id = beid;
+ ret = (LocalPgBackendStatus *) bsearch(&key, localBackendStatusTable,
+ localNumBackends,
+ sizeof(LocalPgBackendStatus),
+ cmp_lbestatus);
+ if (ret)
+ return &ret->backendStatus;
+
+ return NULL;
}
@@ -1074,6 +1106,10 @@ pgstat_fetch_stat_beentry(int beid)
* Like pgstat_fetch_stat_beentry() but with locally computed additions (like
* xid and xmin values of the backend)
*
+ * Unlike pgstat_fetch_stat_beentry(), the beid argument refers to the index
+ * in the localBackendStatusTable instead of the backend ID stored in the
+ * backend's PGPROC struct.
+ *
* NB: caller is responsible for a check if the user is permitted to see
* this info (especially the querystring).
* ----------
diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index be15b4b2e5..56ca18b6d0 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -415,7 +415,6 @@ pg_stat_get_backend_idset(PG_FUNCTION_ARGS)
{
FuncCallContext *funcctx;
int *fctx;
- int32 result;
/* stuff done only on the first call of the function */
if (SRF_IS_FIRSTCALL())
@@ -436,12 +435,13 @@ pg_stat_get_backend_idset(PG_FUNCTION_ARGS)
fctx = funcctx->user_fctx;
fctx[0] += 1;
- result = fctx[0];
- if (result <= fctx[1])
+ if (fctx[0] <= fctx[1])
{
+ LocalPgBackendStatus *localStatus = pgstat_fetch_stat_local_beentry(fctx[0]);
+
/* do when there is more left to send */
- SRF_RETURN_NEXT(funcctx, Int32GetDatum(result));
+ SRF_RETURN_NEXT(funcctx, Int32GetDatum(localStatus->backend_id));
}
else
{
@@ -1180,7 +1180,8 @@ pg_stat_get_db_numbackends(PG_FUNCTION_ARGS)
result = 0;
for (beid = 1; beid <= tot_backends; beid++)
{
- PgBackendStatus *beentry = pgstat_fetch_stat_beentry(beid);
+ LocalPgBackendStatus *localStatus = pgstat_fetch_stat_local_beentry(beid);
+ PgBackendStatus *beentry = &localStatus->backendStatus;
if (beentry && beentry->st_databaseid == dbid)
result++;
diff --git a/src/include/utils/backend_status.h b/src/include/utils/backend_status.h
index 7403bca25e..f7c7c6e671 100644
--- a/src/include/utils/backend_status.h
+++ b/src/include/utils/backend_status.h
@@ -13,6 +13,7 @@
#include "datatype/timestamp.h"
#include "libpq/pqcomm.h"
#include "miscadmin.h" /* for BackendType */
+#include "storage/backendid.h"
#include "utils/backend_progress.h"
@@ -258,6 +259,13 @@ typedef struct LocalPgBackendStatus
* not.
*/
TransactionId backend_xmin;
+
+ /*
+ * The backend ID. For auxiliary processes, this will be set to a value
+ * greater than MaxBackends (since auxiliary processes do not have proper
+ * backend IDs).
+ */
+ BackendId backend_id;
} LocalPgBackendStatus;
--
2.25.1
