postgres 'at time zone' 동작

at_time_zone.sql

set timezone='Asia/Seoul';
show timezone;

SELECT '2018-12-01 00:00:00'::timestamp at time zone 'GMT';
SELECT (timestamp '2018-12-01 00:00:00') at time zone 'GMT';
-- 2018-12-01 09:00:00+09

SELECT '2018-12-01 00:00:00' at time zone 'GMT';
SELECT (date '2018-12-01') at time zone 'GMT';
SELECT '2018-12-01 00:00:00'::timestamp with time zone at time zone 'GMT';
SELECT timestamp with time zone '2018-12-01 00:00:00' at time zone 'GMT';
-- 2018-11-30 15:00:00

select now();
-- 2018-12-01 00:00:00.000000+09
select now() at time zone 'GMT';
-- 2018-11-30 15:00:00.000000
select now() at time zone 'GMT' at time zone 'GMT';
-- 2018-12-01 00:00:00.000000+09