PG-286: Reduce calls to pgstat_fetch_stat_numbackends().

After couple CPU profiling sessions with perf, it was detected that the
function pgstat_fetch_stat_numbackends() is very expensive, reading the
implementation on PostgreSQL's backend_status.c just confirmed that.

We use that function on pg_stat_monitor to retrieve the application name
and IP address of the client, we now cache the results in order to avoid
calling it for every query being processed.

pg_stat_monitor.c

@@ -104,12 +104,11 @@ PG_FUNCTION_INFO_V1(pg_stat_monitor_settings);
 PG_FUNCTION_INFO_V1(get_histogram_timings);
 PG_FUNCTION_INFO_V1(pg_stat_monitor_hook_stats);
 
-static uint pg_get_client_addr(void);
+static uint pg_get_client_addr(bool *ok);
-static int pg_get_application_name(char* application_name);
+static int pg_get_application_name(char *application_name, bool *ok);
 static PgBackendStatus *pg_get_backend_status(void);
 static Datum intarray_get_datum(int32 arr[], int len);
 
-
 #if PG_VERSION_NUM < 140000
 DECLARE_HOOK(void pgss_post_parse_analyze, ParseState *pstate, Query *query);
 #else
@@ -1132,46 +1131,44 @@ pg_get_backend_status(void)
 }
 
 static int
-pg_get_application_name(char *application_name)
+pg_get_application_name(char *application_name, bool *ok)
 {
 	PgBackendStatus *beentry = pg_get_backend_status();
 	if (!beentry)
 		return snprintf(application_name, APPLICATIONNAME_LEN, "%s", "postmaster");
 
+	if (!beentry)
+		return snprintf(application_name, APPLICATIONNAME_LEN, "%s", "unknown");
+
+	*ok = true;
+
 	return snprintf(application_name, APPLICATIONNAME_LEN, "%s", beentry->st_appname);
 }
 
 static uint
-pg_get_client_addr(void)
+pg_get_client_addr(bool *ok)
 {
 	PgBackendStatus *beentry = pg_get_backend_status();
 	char remote_host[NI_MAXHOST];
 	int ret;
-	static uint localhost = 0;
 
 	remote_host[0] = '\0';
 
 	if (!beentry)
 		return ntohl(inet_addr("127.0.0.1"));
 
+	*ok = true;
+
 	ret = pg_getnameinfo_all(&beentry->st_clientaddr.addr,
 							 beentry->st_clientaddr.salen,
 							 remote_host, sizeof(remote_host),
 							 NULL, 0,
 							 NI_NUMERICHOST | NI_NUMERICSERV);
 	if (ret != 0)
-	{
+		return ntohl(inet_addr("127.0.0.1"));
-		if (localhost == 0)
-			localhost = ntohl(inet_addr("127.0.0.1"));
-		return localhost;
-	}
 
 	if (strcmp(remote_host, "[local]") == 0)
-	{
+		return ntohl(inet_addr("127.0.0.1"));
-		if (localhost == 0)
-			localhost = ntohl(inet_addr("127.0.0.1"));
-		return localhost;
-	}
 
 	return ntohl(inet_addr(remote_host));
 }
@@ -1191,11 +1188,11 @@ pgss_update_entry(pgssEntry *entry,
 						BufferUsage *bufusage,
 						WalUsage *walusage,
 						bool reset,
-						pgssStoreKind kind)
+						pgssStoreKind kind,
+						const char *app_name,
+						size_t app_name_len)
 {
 	int                 index;
-	char			    application_name[APPLICATIONNAME_LEN];
-	int				    application_name_len = pg_get_application_name(application_name);
 	double              old_mean;
 	int             	message_len = error_info ? strlen (error_info->message) : 0;
 	int             	comments_len = comments ? strlen (comments) : 0;
@@ -1264,11 +1261,11 @@ pgss_update_entry(pgssEntry *entry,
 			e->counters.resp_calls[index]++;
 		}
 
-		if (plan_text_len > 0)
+		if (plan_text_len > 0 && !e->counters.planinfo.plan_text[0])
 			_snprintf(e->counters.planinfo.plan_text, plan_info->plan_text, plan_text_len + 1, PLAN_TEXT_LEN);
 
-		if (application_name_len > 0)
+		if (app_name_len > 0 && !e->counters.info.application_name[0])
-			_snprintf(e->counters.info.application_name, application_name, application_name_len + 1, APPLICATIONNAME_LEN);
+			_snprintf(e->counters.info.application_name, app_name, app_name_len + 1, APPLICATIONNAME_LEN);
 
 		e->counters.info.num_relations = num_relations;
 		_snprintf2(e->counters.info.relations, relations, num_relations,  REL_LEN);
@@ -1465,8 +1462,8 @@ pgss_store(uint64 queryid,
 	pgssHashKey     key;
 	pgssEntry       *entry;
 	pgssSharedState *pgss = pgsm_get_ss();
-	char            application_name[APPLICATIONNAME_LEN];
+	static char     application_name[APPLICATIONNAME_LEN] = "";
-	int             application_name_len;
+	static int      application_name_len = 0;
 	bool            reset = false;
 	uint64          bucketid;
 	uint64          prev_bucket_id;
@@ -1474,6 +1471,9 @@ pgss_store(uint64 queryid,
 	uint64          planid;
 	uint64          appid;
 	char            comments[512] = "";
+	static bool found_app_name = false;
+	static bool found_client_addr = false;
+	static uint client_addr = 0;
 
 	/*  Monitoring is disabled */
 	if (!PGSM_ENABLED)
@@ -1492,7 +1492,12 @@ pgss_store(uint64 queryid,
 	else
 		userid =  GetUserId();
 
-	application_name_len = pg_get_application_name(application_name);
+	if (!found_app_name)
+		application_name_len = pg_get_application_name(application_name, &found_app_name);
+
+	if (!found_client_addr)
+		client_addr = pg_get_client_addr(&found_client_addr);
+
 	planid = plan_info ? plan_info->planid : 0;
 	appid = djb2_hash((unsigned char *)application_name, application_name_len);
 
@@ -1508,7 +1513,7 @@ pgss_store(uint64 queryid,
 	key.userid = userid;
 	key.dbid = MyDatabaseId;
 	key.queryid = queryid;
-	key.ip = pg_get_client_addr();
+	key.ip = client_addr;
 	key.planid = planid;
 	key.appid = appid;
 #if PG_VERSION_NUM < 140000
@@ -1597,7 +1602,9 @@ pgss_store(uint64 queryid,
 					bufusage,			/* bufusage */
 					walusage,			/* walusage */
 					reset,				/* reset */
-					kind);				/* kind */
+					kind,				/* kind */
+					application_name,
+					application_name_len);
 
 	LWLockRelease(pgss->lock);
 }