MySQL 5.7

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select version();


version()


5.7.35


--
--
-- Test of NOW(p) at various values of p - checking if rounding or truncation 
-- occurs
--
SELECT 'NOW()' , NOW() UNION ALL
SELECT 'NOW(0)', NOW(0) UNION ALL
SELECT 'NOW(1)', NOW(1) UNION ALL
SELECT 'NOW(2)', NOW(2) UNION ALL
SELECT 'NOW(3)', NOW(3) UNION ALL
SELECT 'NOW(4)', NOW(4) UNION ALL
SELECT 'NOW(5)', NOW(5) UNION ALL
SELECT 'NOW(6)', NOW(6);

--
-- A few runs of the fiddle clearly shows that the value is truncated and not
-- rounded, to using NOW(3) for millisecond precision will not risk
-- chronological anomalies!
--


NOW()
NOW()


NOW()
2021-12-28 10:27:55.000000

NOW(0)
2021-12-28 10:27:55.000000

NOW(1)
2021-12-28 10:27:55.800000

NOW(2)
2021-12-28 10:27:55.880000

NOW(3)
2021-12-28 10:27:55.884000

NOW(4)
2021-12-28 10:27:55.884200

NOW(5)
2021-12-28 10:27:55.884210

NOW(6)
2021-12-28 10:27:55.884212


CREATE TABLE now_test 
(
  s TIMESTAMP,
  t TIMESTAMP(0) NULL DEFAULT NULL,
  u TIMESTAMP(1) NULL DEFAULT NULL,
  v TIMESTAMP(2) NULL DEFAULT NULL,
  w TIMESTAMP(3) NULL DEFAULT NULL,
  x TIMESTAMP(4) NULL DEFAULT NULL,
  y TIMESTAMP(5) NULL DEFAULT NULL,
  z TIMESTAMP(6) NULL DEFAULT NULL
);
INSERT INTO now_test VALUES
(NOW(), NOW(0), NOW(1), NOW(2), NOW(3), NOW(4), NOW(5), NOW(6));
SELECT 'NOW()' , 's' AS dt, s  AS ts FROM now_test  UNION
SELECT 'NOW(0)', 't'      , t        FROM now_test  UNION
SELECT 'NOW(1)', 'u'      , u        FROM now_test  UNION
SELECT 'NOW(2)', 'v'      , v        FROM now_test  UNION
SELECT 'NOW(3)', 'w'      , w        FROM now_test  UNION
SELECT 'NOW(4)', 'x'      , x        FROM now_test  UNION
SELECT 'NOW(5)', 'y'      , y        FROM now_test  UNION
SELECT 'NOW(6)', 'z'      , z        FROM now_test;

-- 
-- As with the SELECTs above, a few runs of the fiddle clearly show that the
-- value is truncated at 3 digits for the 'w' (TIMESTAMP(3)) field
--


NOW()
dt
ts


NOW()
s
2021-12-28 10:27:55.000000

NOW(0)
t
2021-12-28 10:27:55.000000

NOW(1)
u
2021-12-28 10:27:55.900000

NOW(2)
v
2021-12-28 10:27:55.900000

NOW(3)
w
2021-12-28 10:27:55.902000

NOW(4)
x
2021-12-28 10:27:55.902000

NOW(5)
y
2021-12-28 10:27:55.902070

NOW(6)
z
2021-12-28 10:27:55.902073


SELECT
  table_name  AS "The table",
  data_length AS "Data length"
FROM
  information_schema.tables
WHERE
    TABLE_NAME = 'now_test'
ORDER BY
  1 + 2
DESC;


The table
Data length


now_test
16384


SHOW VARIABLES LIKE 'innodb_page_size';


Variable_name
Value


innodb_page_size
16384

NOW()	NOW()
NOW()	2021-12-28 10:27:55.000000
NOW(0)	2021-12-28 10:27:55.000000
NOW(1)	2021-12-28 10:27:55.800000
NOW(2)	2021-12-28 10:27:55.880000
NOW(3)	2021-12-28 10:27:55.884000
NOW(4)	2021-12-28 10:27:55.884200
NOW(5)	2021-12-28 10:27:55.884210
NOW(6)	2021-12-28 10:27:55.884212