postgresql如何计算两日期间有多少天

select now()

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2018-02-01 15:49:15

select date_part('year',now())

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2018

select date_part('day',now())

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1

select date_part('day','2018-01-31 15:49:15'::timestamp-'2018-01-10 10:12:15'::timestamp)

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21