diff --git a/doc/src/sgml/bgworker.sgml b/doc/src/sgml/bgworker.sgml
index ef28f725114..33ff5269516 100644
--- a/doc/src/sgml/bgworker.sgml
+++ b/doc/src/sgml/bgworker.sgml
@@ -58,6 +58,7 @@ typedef struct BackgroundWorker
     char        bgw_library_name[BGW_MAXLEN];   /* only if bgw_main is NULL */
     char        bgw_function_name[BGW_MAXLEN];  /* only if bgw_main is NULL */
     Datum       bgw_main_arg;
+    char        bgw_extra[BGW_EXTRALEN];
     int         bgw_notify_pid;
 } BackgroundWorker;
 </programlisting>
@@ -136,6 +137,13 @@ typedef struct BackgroundWorker
    <structfield>bgw_main</structfield> is NULL.
   </para>
 
+  <para>
+   <structfield>bgw_extra</structfield> can contain extra data to be passed
+   to the background worker.  Unlike <structfield>bgw_main_arg</>, this data
+   is not passed as an argument to the worker's main function, but it can be
+   accessed via <literal>MyBgworkerEntry</literal>, as discussed above.
+  </para>
+
   <para>
    <structfield>bgw_notify_pid</structfield> is the PID of a PostgreSQL
    backend process to which the postmaster should send <literal>SIGUSR1</>
diff --git a/src/backend/access/transam/parallel.c b/src/backend/access/transam/parallel.c
index 9c7428f5d6c..38a4d65b1f5 100644
--- a/src/backend/access/transam/parallel.c
+++ b/src/backend/access/transam/parallel.c
@@ -77,10 +77,6 @@ typedef struct FixedParallelState
 	/* Mutex protects remaining fields. */
 	slock_t		mutex;
 
-	/* Track whether workers have attached. */
-	int			workers_expected;
-	int			workers_attached;
-
 	/* Maximum XactLastRecEnd of any worker. */
 	XLogRecPtr	last_xlog_end;
 } FixedParallelState;
@@ -286,8 +282,6 @@ InitializeParallelDSM(ParallelContext *pcxt)
 	fps->parallel_master_backend_id = MyBackendId;
 	fps->entrypoint = pcxt->entrypoint;
 	SpinLockInit(&fps->mutex);
-	fps->workers_expected = pcxt->nworkers;
-	fps->workers_attached = 0;
 	fps->last_xlog_end = 0;
 	shm_toc_insert(pcxt->toc, PARALLEL_KEY_FIXED, fps);
 
@@ -406,6 +400,7 @@ LaunchParallelWorkers(ParallelContext *pcxt)
 	worker.bgw_main = ParallelWorkerMain;
 	worker.bgw_main_arg = UInt32GetDatum(dsm_segment_handle(pcxt->seg));
 	worker.bgw_notify_pid = MyProcPid;
+	memset(&worker.bgw_extra, 0, BGW_EXTRALEN);
 
 	/*
 	 * Start workers.
@@ -417,6 +412,7 @@ LaunchParallelWorkers(ParallelContext *pcxt)
 	 */
 	for (i = 0; i < pcxt->nworkers; ++i)
 	{
+		memcpy(worker.bgw_extra, &i, sizeof(int));
 		if (!any_registrations_failed &&
 			RegisterDynamicBackgroundWorker(&worker,
 											&pcxt->worker[i].bgwhandle))
@@ -825,6 +821,10 @@ ParallelWorkerMain(Datum main_arg)
 	pqsignal(SIGTERM, die);
 	BackgroundWorkerUnblockSignals();
 
+	/* Determine and set our parallel worker number. */
+	Assert(ParallelWorkerNumber == -1);
+	memcpy(&ParallelWorkerNumber, MyBgworkerEntry->bgw_extra, sizeof(int));
+
 	/* Set up a memory context and resource owner. */
 	Assert(CurrentResourceOwner == NULL);
 	CurrentResourceOwner = ResourceOwnerCreate(NULL, "parallel toplevel");
@@ -849,18 +849,9 @@ ParallelWorkerMain(Datum main_arg)
 				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
 			   errmsg("bad magic number in dynamic shared memory segment")));
 
-	/* Determine and set our worker number. */
+	/* Look up fixed parallel state. */
 	fps = shm_toc_lookup(toc, PARALLEL_KEY_FIXED);
 	Assert(fps != NULL);
-	Assert(ParallelWorkerNumber == -1);
-	SpinLockAcquire(&fps->mutex);
-	if (fps->workers_attached < fps->workers_expected)
-		ParallelWorkerNumber = fps->workers_attached++;
-	SpinLockRelease(&fps->mutex);
-	if (ParallelWorkerNumber < 0)
-		ereport(ERROR,
-				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
-				 errmsg("too many parallel workers already attached")));
 	MyFixedParallelState = fps;
 
 	/*
diff --git a/src/backend/postmaster/bgworker.c b/src/backend/postmaster/bgworker.c
index 220e599cca2..76619e9517c 100644
--- a/src/backend/postmaster/bgworker.c
+++ b/src/backend/postmaster/bgworker.c
@@ -314,6 +314,7 @@ BackgroundWorkerStateChange(void)
 		rw->rw_worker.bgw_restart_time = slot->worker.bgw_restart_time;
 		rw->rw_worker.bgw_main = slot->worker.bgw_main;
 		rw->rw_worker.bgw_main_arg = slot->worker.bgw_main_arg;
+		memcpy(rw->rw_worker.bgw_extra, slot->worker.bgw_extra, BGW_EXTRALEN);
 
 		/*
 		 * Copy the PID to be notified about state changes, but only if the
diff --git a/src/include/postmaster/bgworker.h b/src/include/postmaster/bgworker.h
index f0a95306545..6e0b5cd9fc8 100644
--- a/src/include/postmaster/bgworker.h
+++ b/src/include/postmaster/bgworker.h
@@ -74,6 +74,7 @@ typedef enum
 #define BGW_DEFAULT_RESTART_INTERVAL	60
 #define BGW_NEVER_RESTART				-1
 #define BGW_MAXLEN						64
+#define BGW_EXTRALEN					128
 
 typedef struct BackgroundWorker
 {
@@ -85,6 +86,7 @@ typedef struct BackgroundWorker
 	char		bgw_library_name[BGW_MAXLEN];	/* only if bgw_main is NULL */
 	char		bgw_function_name[BGW_MAXLEN];	/* only if bgw_main is NULL */
 	Datum		bgw_main_arg;
+	char		bgw_extra[BGW_EXTRALEN];
 	pid_t		bgw_notify_pid; /* SIGUSR1 this backend on start/stop */
 } BackgroundWorker;