Add support for pg_postmaster_open_time()

It is not always reliable to depend on pg_postmaster_start_time()
for database uptime calculations, owing to how postmaster catches
child process crashes, startup recovery etc. and continues without
a restart. This could lead to multiple seconds (minutes or even
hours) of difference when although postmaster was up, the database
was not accepting connections.

This function returns the start time when the database was ready
to accept new database connections, allowing better calculation
of database availability.

On a standby, this time reflects the time the database was ready
to accept read-only connections, whereas in single-user mode
this time reflects the time the database was ready to accept
user commands.

Author: robins

doc/src/sgml/func.sgml

@@ -24916,6 +24916,22 @@ SELECT * FROM pg_ls_dir('.') WITH ORDINALITY AS t(ls,n);
        </para></entry>
       </row>
 
+      <row>
+       <entry role="func_table_entry"><para role="func_signature">
+        <indexterm>
+         <primary>pg_postmaster_open_time</primary>
+        </indexterm>
+        <function>pg_postmaster_open_time</function> ()
+        <returnvalue>timestamp with time zone</returnvalue>
+       </para>
+       <para>
+        Returns the time when the server was open to connections. On a standby
+        this will be the time when the server was open to read-only connections.
+        In single-user mode this returns the time the server was ready to accept
+        user commands.
+       </para></entry>
+      </row>
+
       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>

src/backend/postmaster/launch_backend.c

@@ -111,6 +111,7 @@ typedef struct
 	ProcSignalHeader *ProcSignal;
 	pid_t		PostmasterPid;
 	TimestampTz PgStartTime;
+	TimestampTz PgOpenStartTime;
 	TimestampTz PgReloadTime;
 	pg_time_t	first_syslogger_file_time;
 	bool		redirection_done;
@@ -771,6 +772,7 @@ save_backend_variables(BackendParameters *param,
 
 	param->PostmasterPid = PostmasterPid;
 	param->PgStartTime = PgStartTime;
+	param->PgOpenStartTime = PgOpenStartTime;
 	param->PgReloadTime = PgReloadTime;
 	param->first_syslogger_file_time = first_syslogger_file_time;
 
@@ -1031,6 +1033,7 @@ restore_backend_variables(BackendParameters *param)
 
 	PostmasterPid = param->PostmasterPid;
 	PgStartTime = param->PgStartTime;
+	PgOpenStartTime = param->PgOpenStartTime;
 	PgReloadTime = param->PgReloadTime;
 	first_syslogger_file_time = param->first_syslogger_file_time;
 

src/backend/postmaster/postmaster.c

@@ -2342,6 +2342,11 @@ process_pm_child_exit(void)
 			 */
 			StartWorkerNeeded = true;
 
+			/*
+			 * Remember time when database was open to connections
+			 */
+			PgOpenStartTime = GetCurrentTimestamp();
+
 			/* at this point we are really open for business */
 			ereport(LOG,
 					(errmsg("database system is ready to accept connections")));
@@ -3737,6 +3742,11 @@ process_pm_pmsignal(void)
 		ereport(LOG,
 				(errmsg("database system is ready to accept read-only connections")));
 
+		/*
+		 * Remember time when database was open to connections
+		 */
+		PgOpenStartTime = GetCurrentTimestamp();
+
 		/* Report status */
 		AddToDataDirLockFile(LOCK_FILE_LINE_PM_STATUS, PM_STATUS_READY);
 #ifdef USE_SYSTEMD

src/backend/tcop/postgres.c

@@ -4157,6 +4157,11 @@ PostgresSingleUserMain(int argc, char *argv[],
 	 */
 	PgStartTime = GetCurrentTimestamp();
 
+	/*
+	 * Remember when stand-alone was open to user commands.
+	 */
+	PgOpenStartTime = GetCurrentTimestamp();
+
 	/*
 	 * Create a per-backend PGPROC struct in shared memory. We must do this
 	 * before we can use LWLocks.

src/backend/utils/adt/timestamp.c

@@ -53,6 +53,9 @@
 /* Set at postmaster start */
 TimestampTz PgStartTime;
 
+/* Set when database is open to connections */
+TimestampTz PgOpenStartTime;
+
 /* Set at configuration reload */
 TimestampTz PgReloadTime;
 
@@ -1629,6 +1632,12 @@ pg_postmaster_start_time(PG_FUNCTION_ARGS)
 	PG_RETURN_TIMESTAMPTZ(PgStartTime);
 }
 
+Datum
+pg_postmaster_open_time(PG_FUNCTION_ARGS)
+{
+	PG_RETURN_TIMESTAMPTZ(PgOpenStartTime);
+}
+
 Datum
 pg_conf_load_time(PG_FUNCTION_ARGS)
 {

src/include/catalog/pg_proc.dat

@@ -8741,6 +8741,12 @@
   prorettype => 'timestamptz', proargtypes => '',
   prosrc => 'pg_postmaster_start_time' },
 
+# open to connections start time function
+{ oid => '8600', descr => 'open connections start time',
+  proname => 'pg_postmaster_open_time', provolatile => 's',
+  prorettype => 'timestamptz', proargtypes => '',
+  prosrc => 'pg_postmaster_open_time' },
+
 # config reload time function
 { oid => '2034', descr => 'configuration load time',
   proname => 'pg_conf_load_time', provolatile => 's', proparallel => 'r',

src/include/utils/timestamp.h

@@ -98,6 +98,9 @@ TimestampDifferenceMicroseconds(TimestampTz start_time,
 /* Set at postmaster start */
 extern PGDLLIMPORT TimestampTz PgStartTime;
 
+/* Set when database is open to connections */
+extern PGDLLIMPORT TimestampTz PgOpenStartTime;
+
 /* Set at configuration reload */
 extern PGDLLIMPORT TimestampTz PgReloadTime;