diff --git a/doc/src/sgml/libpq.sgml b/doc/src/sgml/libpq.sgml
index 0cd91c544fb..615a7f167dd 100644
--- a/doc/src/sgml/libpq.sgml
+++ b/doc/src/sgml/libpq.sgml
@@ -4272,7 +4272,14 @@ int PQflush(PGconn *conn);
   <para>
    After sending any command or data on a nonblocking connection, call
    <function>PQflush</function>.  If it returns 1, wait for the socket
-   to be write-ready and call it again; repeat until it returns 0.  Once
+   to become read- or write-ready.  If it becomes write-ready, call
+   <function>PQflush</function> again.  If it becomes read-ready, call
+   <function>PQconsumeInput</function>, then call
+   <function>PQflush</function> again.  Repeat until
+   <function>PQflush</function> returns 0.  (It is necessary to check for
+   read-ready and drain the input with <function>PQconsumeInput</function>,
+   because the server can block trying to send us data, e.g. NOTICE
+   messages, and won't read our data until we read its.)  Once
    <function>PQflush</function> returns 0, wait for the socket to be
    read-ready and then read the response as described above.
   </para>
diff --git a/src/interfaces/libpq/fe-misc.c b/src/interfaces/libpq/fe-misc.c
index 05655c70645..8b0d8ef2bc6 100644
--- a/src/interfaces/libpq/fe-misc.c
+++ b/src/interfaces/libpq/fe-misc.c
@@ -908,16 +908,6 @@ pqSendSome(PGconn *conn, int len)
 			/*
 			 * We didn't send it all, wait till we can send more.
 			 *
-			 * If the connection is in non-blocking mode we don't wait, but
-			 * return 1 to indicate that data is still pending.
-			 */
-			if (pqIsnonblocking(conn))
-			{
-				result = 1;
-				break;
-			}
-
-			/*
 			 * There are scenarios in which we can't send data because the
 			 * communications channel is full, but we cannot expect the server
 			 * to clear the channel eventually because it's blocked trying to
@@ -928,12 +918,29 @@ pqSendSome(PGconn *conn, int len)
 			 * again.  Furthermore, it is possible that such incoming data
 			 * might not arrive until after we've gone to sleep.  Therefore,
 			 * we wait for either read ready or write ready.
+			 *
+			 * In non-blocking mode, we don't wait here directly, but return
+			 * 1 to indicate that data is still pending.  The caller should
+			 * wait for both read and write ready conditions, and call
+			 * PQconsumeInput() on read ready, but just in case it doesn't, we
+			 * call pqReadData() ourselves before returning.  That's not
+			 * enough if the data has not arrived yet, but it's the best we
+			 * can do, and works pretty well in practice.  (The documentation
+			 * used to say that you only need to wait for write-ready, so
+			 * there are still plenty of applications like that out there.)
 			 */
 			if (pqReadData(conn) < 0)
 			{
 				result = -1;	/* error message already set up */
 				break;
 			}
+
+			if (pqIsnonblocking(conn))
+			{
+				result = 1;
+				break;
+			}
+
 			if (pqWait(TRUE, TRUE, conn))
 			{
 				result = -1;