pgbench: When using pipelining only do PQconsumeInput() when necessary.

Up to now we did a PQconsumeInput() for each pipelined query, asking the OS
for more input - which it often won't have, as all results might already have
been sent. That turns out to have a noticeable performance impact.

Alvaro Herrera reviewed the idea to add the PQisBusy() check, but not this
concrete patch.

Author: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/20210720180039.23rivhdft3l4mayn@alap3.anarazel.de
Backpatch: 14, where libpq/pgbench pipelining was introduced.

src/bin/pgbench/pgbench.c

@@ -3460,7 +3460,14 @@ advanceConnectionState(TState *thread, CState *st, StatsData *agg)
 				 */
 			case CSTATE_WAIT_RESULT:
 				pg_log_debug("client %d receiving", st->id);
-				if (!PQconsumeInput(st->con))
+
+				/*
+				 * Only check for new network data if we processed all data
+				 * fetched prior. Otherwise we end up doing a syscall for each
+				 * individual pipelined query, which has a measurable
+				 * performance impact.
+				 */
+				if (PQisBusy(st->con) && !PQconsumeInput(st->con))
 				{
 					/* there's something wrong */
 					commandFailed(st, "SQL", "perhaps the backend died while processing");