BUG #14027: n_tup_ins increments regardless of insertion success

The following bug has been logged on the website:

Bug reference: 14027
Logged by: Ilya Matveychikov
Email address: matvejchikov(at)gmail(dot)com
PostgreSQL version: 9.5.1
Operating system: Linux
Description:

postgres=# create table t (name text unique);
postgres=# select n_tup_ins from pg_stat_user_tables where relname='t';
n_tup_ins
-----------
0
postgres=# insert into t (name) values ('a');
INSERT 0 1
postgres=# select n_tup_ins from pg_stat_user_tables where relname='t';
n_tup_ins
-----------
1
postgres=# insert into t (name) values ('b');
INSERT 0 1
postgres=# select n_tup_ins from pg_stat_user_tables where relname='t';
n_tup_ins
-----------
2
postgres=# insert into t (name) values ('a');
ОШИБКА: повторяющееся значение ключа нарушает ограничение уникальности
"t_name_key"
ПОДРОБНОСТИ: Ключ "(name)=(a)" уже существует.
postgres=# select n_tup_ins from pg_stat_user_tables where relname='t';
n_tup_ins
-----------
3