Fix handling of pgstat counters for TRUNCATE in a prepared transaction.

pgstat_twophase_postcommit is supposed to duplicate the math in
AtEOXact_PgStat, but it had missed out the bit about clearing
t_delta_live_tuples/t_delta_dead_tuples for a TRUNCATE.

It's harder than you might think to replicate the issue here, because
those counters would only be nonzero when a previous transaction in
the same backend had added/deleted tuples in the truncated table,
and those counts hadn't been sent to the stats collector yet.

Evident oversight in commit d42358ef.  I've not added a regression
test for this; we tried to add one in d42358ef, and had to revert it
because it was too timing-sensitive for the buildfarm.

Back-patch to 9.5 where d42358ef came in.

Stas Kelvich

Discussion: <EB57BF68-C06D-4737-BDDC-4BA778F4E62B@postgrespro.ru>

src/backend/postmaster/pgstat.c

@@ -2226,7 +2226,12 @@ pgstat_twophase_postcommit(TransactionId xid, uint16 info,
 	pgstat_info->t_counts.t_tuples_updated += rec->tuples_updated;
 	pgstat_info->t_counts.t_tuples_deleted += rec->tuples_deleted;
 	pgstat_info->t_counts.t_truncated = rec->t_truncated;
-
+	if (rec->t_truncated)
+	{
+		/* forget live/dead stats seen by backend thus far */
+		pgstat_info->t_counts.t_delta_live_tuples = 0;
+		pgstat_info->t_counts.t_delta_dead_tuples = 0;
+	}
 	pgstat_info->t_counts.t_delta_live_tuples +=
 		rec->tuples_inserted - rec->tuples_deleted;
 	pgstat_info->t_counts.t_delta_dead_tuples +=