sql PostgreSQL相关timestamp.sql

Posted 2021-05-13

SHOW TIMEZONE; --> Japan

--- comparison between timestamptz and timestamp
SELECT timestamptz '1970-01-01 00:00:00' = timestamp '1970-01-01 00:00:00'; --> t

-- assumption #1
-- timestamptz '1970-01-01 00:00:00'
-- ↓ evaluated
-- timestamptz '1970-01-01 00:00:00+09'
-- ↓ cast to timestamp and then be truncated timezone-part.
-- timestamp '1970-01-01 00:00:00'
-- ↓
-- is equivalent to right hand side.

-- assumption #2
-- timestamp '1970-01-01 00:00:00'
-- ↓ cast to timestamptz and then be added default timezone
-- timestamptz '1970-01-01 00:00:00+09'
-- ↓
-- is equivalent to left hand side.


--- timezone setting doesn't affect `timestamp`.
CREATE TABLE timestamp_t (id serial, data timestamp);
INSERT INTO timestamp_t (data) VALUES (timestamp '1970-01-01 00:00:00');
INSERT INTO timestamp_t (data) VALUES (to_timestamp(0));
-- `to_timestamp(0)` is same as '1970-01-01 09:00:00+09' in timezone `Japan`.
-- Timezone-part of `to_timestamp(0)` `+09` will be truncated
-- thus '1970-01-01 09:00:00' will be stored.
-- note: to_timestamp() returns timestamptz not timestamp.

SELECT * FROM timestamp_t;
-- 1 | 1970-01-01 00:00:00
-- 2 | 1970-01-01 09:00:00

SET TIMEZONE TO 'UTC';
SELECT * FROM timestamp_t;
-- 1 | 1970-01-01 00:00:00
-- 2 | 1970-01-01 09:00:00

INSERT INTO timestamp_t (data) VALUES (timestamp '1970-01-01 00:00:00');
SELECT * FROM timestamp_t;
-- 1 | 1970-01-01 00:00:00
-- 2 | 1970-01-01 09:00:00
-- 3 | 1970-01-01 00:00:00

SET TIMEZONE TO 'Japan';
SELECT * FROM timestamp_t;
-- 1 | 1970-01-01 00:00:00
-- 2 | 1970-01-01 09:00:00
-- 3 | 1970-01-01 00:00:00


--- timezone setting is a mere input/output filter for `timestamptz`.
CREATE TABLE timestamptz_t (id serial, data timestamptz);
INSERT INTO timestamptz_t (data) VALUES (to_timestamp(0)); -- same as '1970-01-01 00:00:00-00'
INSERT INTO timestamptz_t (data) VALUES ('1970-01-01 00:00:00'); -- evaluated in timezone `Japan`
INSERT INTO timestamptz_t (data) VALUES ('1970-01-01 00:00:00-01');
INSERT INTO timestamptz_t (data) VALUES ('1970-01-01 00:00:00+01');
SELECT * FROM timestamptz_t;
-- 1 | 1970-01-01 09:00:00+09
-- 2 | 1970-01-01 00:00:00+09
-- 3 | 1970-01-01 10:00:00+09
-- 4 | 1970-01-01 08:00:00+09
  
SET TIMEZONE TO 'UTC';
SELECT * FROM timestamptz_t;
-- 1 | 1970-01-01 00:00:00+00
-- 2 | 1969-12-31 15:00:00+00
-- 3 | 1970-01-01 01:00:00+00
-- 4 | 1969-12-31 23:00:00+00



DROP TABLE timestamp_t;
DROP TABLE timestamptz_t;