日期比较应该使用哪个函数，to_char还是to_date？
date_column >= to_date('2016-01-19', 'YYYY-MM-DD')
to_char(date_column, 'YYYY-MM-DD') <= '2016-01-19'
Do you want to convert EVERY ROW from a date to string to compare?
Also, using to_char() on the database date column would typically **obviate the use of any indexes **
to_char不仅要将每一行的值转成字符串，还会丢弃索引。
总是使用to_date。