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PostgreSQL Tutorial/Getting Started/TIMESTAMP

PostgreSQL Timestamp Data Types

Summary: in this tutorial, you will learn about the PostgreSQL timestamp data types including timestamp and timestamptz. You will also learn how to use some handy functions to handle timestamp data effectively.

Introduction to PostgreSQL timestamp

PostgreSQL TimestampPostgreSQL provides you with two temporal data types for handling timestamps:

  • timestamp: a timestamp without a timezone one.
  • timestamptz: timestamp with a timezone.

The timestamp datatype allows you to store both date and time. However, it does not have any time zone data. It means that when you change the timezone of your database server, the timestamp value stored in the database will not change automatically.

The timestamptz datatype is the timestamp with a timezone. The timestamptz data type is a time zone-aware date and time data type.

Internally, PostgreSQL stores the timestamptz in UTC value.

  • When you insert a value into a timestamptz column, PostgreSQL converts the timestamptz value into a UTC value and stores the UTC value in the table.
  • When you retrieve data from a timestamptz column, PostgreSQL converts the UTC value back to the time value of the timezone set by the database server, the user, or the current database connection.

Notice that both timestamp and timestamptz uses 8 bytes for storing the timestamp values as shown in the following query:

SELECT
  typname,
  typlen
FROM
  pg_type
WHERE
  typname ~ '^timestamp';

Output:

typname   | typlen
-------------+--------
 timestamp   |      8
 timestamptz |      8
(2 rows)

It’s important to note that PostgreSQL stores timestamptz values in the database using UTC values. It does not store any timezone data with the timestamptz value.

PostgreSQL timestamp example

Let’s take a look at an example of using the timestamp and timestamptzto have a better understanding of how PostgreSQL handles them.

First, create a table that consists of both timestamp the timestamptz columns.

CREATE TABLE timestamp_demo (
    ts TIMESTAMP,
    tstz TIMESTAMPTZ
);

Next, set the time zone of the database server to  America/Los_Angeles.

SET timezone = 'America/Los_Angeles';

By the way, you can see the current time zone using the SHOW TIMEZONE command:

SHOW TIMEZONE;
TimeZone
---------------------
 America/Los_Angeles
(1 row)

Then, insert a new row into the timstamp_demotable:

INSERT INTO timestamp_demo (ts, tstz)
VALUES('2016-06-22 19:10:25-07','2016-06-22 19:10:25-07');

After that, query data from the timestamp and timestamptz columns.

SELECT
   ts, tstz
FROM
   timestamp_demo;
ts          |          tstz
---------------------+------------------------
 2016-06-22 19:10:25 | 2016-06-22 19:10:25-07
(1 row)

The query returns the same timestamp values as the inserted values.

Finally, change the timezone of the current session to America/New_York and query data again.

SET timezone = 'America/New_York';
SELECT
  ts,
  tstz
FROM
  timestamp_demo;
ts          |          tstz
---------------------+------------------------
 2016-06-22 19:10:25 | 2016-06-22 22:10:25-04
(1 row)

The value in the timestamp column does not change whereas the value in the timestamptz column is adjusted to the new time zone of 'America/New_York'.

Generally, it is a good practice to use the timestamptz data type to store the timestamp data.

PostgreSQL timestamp functions

To handle timestamp data effectively, PostgreSQL provides some handy functions as follows:

Getting the current time

To get the current timestamp you use the NOW() function as follows:

SELECT NOW();

Output:

now
-------------------------------
 2024-01-31 21:01:58.985943-05
(1 row)

Alternatively, you can use the CURRENT_TIMESTAMP function:

SELECT CURRENT_TIMESTAMP;

Output:

current_timestamp
-------------------------------
 2024-01-31 21:02:04.715486-05
(1 row)

To get the current time without a date, you use the CURRENT_TIME function:

SELECT CURRENT_TIME;

Output:

current_time
--------------------
 21:02:13.648512-05
(1 row)

Note that both CURRENT_TIMESTAMP and CURRENT_TIME return the current time with the time zone.

To get the time of day in the string format, you use the timeofday() function.

SELECT TIMEOFDAY();
timeofday
-------------------------------------
 Wed Jan 31 21:02:20.840159 2024 EST
(1 row)

Convert between timezones

To convert a timestamp to another time zone, you use the timezone(zone, timestamp) function.

SHOW TIMEZONE;
TimeZone
------------------
 America/New_York
(1 row)

The current timezone is America/New_York.

To convert 2016-06-01 00:00 to America/Los_Angeles timezone, you use the timezone() function as follows:

SELECT timezone('America/Los_Angeles','2016-06-01 00:00');
timezone
---------------------
 2016-05-31 21:00:00
(1 row)

Note that we pass the timestamp as a string to the timezone() function, PostgreSQL casts it to timestamptz implicitly. It is better to cast a timestamp value to the timestamptz data type explicitly as the following statement:

SELECT timezone('America/Los_Angeles','2016-06-01 00:00'::timestamptz);

Output:

timezone
---------------------
 2016-05-31 21:00:00
(1 row)

Using default values for timestamp columns

First, create a new table called department:

CREATE TABLE department (
    id SERIAL PRIMARY KEY,
    name VARCHAR NOT NULL,
    created_at TIMESTAMPTZ DEFAULT CURRENT_TIMESTAMP,
    updated_at TIMESTAMPTZ DEFAULT CURRENT_TIMESTAMP
);

The default values for the created_at and updated_at columns are the current timestamp provided by the CURRENT_TIMESTAMP function.

Second, insert a new row into the department table without specifying the values for the created_at and updated_at columns:

INSERT INTO department(name)
VALUES('IT')
RETURNING *;

Output:

id | name |          created_at           |          updated_at
----+------+-------------------------------+-------------------------------
  1 | IT   | 2024-01-31 21:25:31.162808-05 | 2024-01-31 21:25:31.162808-05
(1 row)

The output indicates that PostgreSQL uses the current time to insert into the created_at and updated_at columns.

When you update a row in the department table, the updated_at column will not be updated to the current time automatically.

To update the value in the updated_at column to the time the row is updated, you can create a BEFORE UPDATE trigger to change the value in the updated_at column.

Note that MySQL offers the ON UPDATE CURRENT_TIMESTAMP to automatically update a TIMESTAMP column to the current timestamp. PostgreSQL does not support this feature at the moment.

Third, create a BEFORE UPDATE trigger to update the updated_at column of the department table:

CREATE OR REPLACE FUNCTION update_updated_at()
RETURNS TRIGGER AS $$
BEGIN
    NEW.updated_at = current_timestamp;
    RETURN NEW;
END;
$$ LANGUAGE plpgsql;

CREATE TRIGGER department_updated_at_trigger
BEFORE UPDATE ON department
FOR EACH ROW
EXECUTE FUNCTION update_updated_at();

Fourth, update the name of the IT department to ITD without specifying a value for the updated_at column:

UPDATE department
SET name = 'ITD'
WHERE id = 1
RETURNING *;

Output:

id | name |          created_at           |          updated_at
----+------+-------------------------------+-------------------------------
  1 | ITD  | 2024-01-31 21:25:31.162808-05 | 2024-01-31 21:25:51.318803-05
(1 row)

The output indicates that the value in the updated_at column has been updated automatically by the trigger.

Summary

  • Use timestamp and timestamptz to store timestamp data.
  • PostgreSQL stores the timestamptz values in the database as UTC values.
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NextPostgreSQL Interval Data Type

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