filters = [f for f in filters if f]
sql = "select EqualWidth((%s), (%s), (%s), (%s));"
param = (data.table_name, attribute.name, filters if filters else None, self.n)
cur = data._execute_sql_query(sql, param)
points = [a for a, in cur.fetchall()]
After Change
filters = [f.to_sql() for f in data.row_filters]
filters = [f for f in filters if f]
att = attribute.to_sql()
cur = data._sql_query(["min(%s)" % att, "max(%s)" % att], filters)
min, max = cur.fetchone()
dif = (max - min) / self.n
points = [min + (i + 1) * dif for i in range(self.n - 1)]
else: