MariaDB [(none)]> select count(*) from statistic;
+-----------+
| count(*)  |
+-----------+
| 717573020 |
+-----------+
1 row in set (2 min 49.312 sec)

MariaDB [(none)]> select count(*) from statistic where `post_id` is not null;
+----------+
| count(*) |
+----------+
| 67237829 |
+----------+
1 row in set (32.816 sec)

update statistic set post_id=NULL where post_id is not null and date >= FROM_UNIXTIME(1552780800) and date <= FROM_UNIXTIME(1553644800)