diff --git a/src/backend/libpq/pqcomm.c b/src/backend/libpq/pqcomm.c
index d7f425de88..53f4594056 100644
--- a/src/backend/libpq/pqcomm.c
+++ b/src/backend/libpq/pqcomm.c
@@ -286,15 +286,28 @@ socket_close(int code, Datum arg)
 		secure_close(MyProcPort);
 
 		/*
-		 * Formerly we did an explicit close() here, but it seems better to
-		 * leave the socket open until the process dies.  This allows clients
-		 * to perform a "synchronous close" if they care --- wait till the
-		 * transport layer reports connection closure, and you can be sure the
-		 * backend has exited.
+		 * On most platforms, we leave the socket open until the process dies.
+		 * This allows clients to perform a "synchronous close" if they care
+		 * --- wait till the transport layer reports connection closure, and
+		 * you can be sure the backend has exited.  Saves a kernel call, too.
 		 *
-		 * We do set sock to PGINVALID_SOCKET to prevent any further I/O,
-		 * though.
+		 * However, that does not work on Windows: if the kernel closes the
+		 * socket it will invoke an "abortive shutdown" that discards any data
+		 * not yet sent to the client.  (This is a flat-out violation of the
+		 * TCP RFCs, but count on Microsoft not to care about that.)  To get
+		 * the spec-compliant "graceful shutdown" behavior, we must invoke
+		 * closesocket() explicitly.
+		 *
+		 * This code runs late enough during process shutdown that we should
+		 * have finished all externally-visible shutdown activities, so that
+		 * in principle it's good enough to act as a synchronous close on
+		 * Windows too.  But it's a lot more fragile than the other way.
 		 */
+#ifdef WIN32
+		closesocket(MyProcPort->sock);
+#endif
+
+		/* In any case, set sock to PGINVALID_SOCKET to prevent further I/O */
 		MyProcPort->sock = PGINVALID_SOCKET;
 	}
 }