从 timestamptz 中提取日期部分

Question

I am trying to extract the hour from a timestamp with a timezone. However, my times are coming up incorrectly.

Here's an example, I am using Dbeaver with my timezone set to EST:

SELECT '2020-01-24 14:27:12' AT TIME ZONE 'US/Pacific' as foo,
EXTRACT(HOUR FROM foo) as ex,
DATE_PART('HOUR', foo::timestamp) as dp

RETURNS:

foo                 |ex |dp
2020-01-24 17:27:12 |22 | 22

• Why is my time coming up 3 hours ahead, it should be 3 hours behind?
• Extract and DATE_PART don't seem to get me the hour I would like. It looks like it's taking 17 as EST and then converting it to UTC. Here's what I am expecting to get:

foo                 |ex |dp
2020-01-24 11:27:12 |11 | 11

Answer 1

Check if your timezone is set to EST:

SELECT current_setting('TIMEZONE');

or with:

show timezone;

If it is not you can set it like this:

set timezone to est;

AS shown in this DEMO

If that is not working try with convert_timezone

select convert_timezone('US/Pacific', '2020-01-24 14:27:12')

And exploring the mater on hand I have found this fact:

Note Amazon Redshift doesn't validate POSIX-style time zone specifications, so it is possible to set the time zone to an invalid value. For example, the following command doesn't return an error, even though it sets the time zone to an invalid value.

set timezone to 'xxx36';

from this source: https://docs.aws.amazon.com/redshift/latest/dg/CONVERT_TIMEZONE.html

Answer 2

'AT TIME ZONE' does not work like you are expecting. Use convert_timezone() instead.

SELECT
  '2020-01-24 14:27:12' AT TIME ZONE 'US/Pacific' as foo,  
    -- expected '2020-01-24 6:27:12' got '2020-01-24 22:27:12+00'
  convert_timezone('UTC', 'US/Pacific', CAST('2020-01-24 14:27:12' AS TIMESTAMP WITHOUT TIME ZONE)) as bar 
    -- expected '2020-01-24 6:27:12' got '2020-01-24 06:27:12'
  ;

'AT TIME ZONE' interprets the timestamp as being relative to the specified time zone and converts it to a TIMESTAMPTZ offset to UTC. That is in the above example it converts from US/Pacific to UTC, not the other way around.

Answer 3

It works perfectly fine for me (using dbVisualizer).

The issue is with your SQL Client.

SQL clients often impose formatting that impacts the values you see. You can test this by converting values to Text before sending them to your SQL client:

SELECT
  '2020-01-24 14:27:12' AT TIME ZONE 'US/Pacific' as foo,
  foo::text as t,
  EXTRACT(HOUR FROM foo) as ex,
  DATE_PART('HOUR', foo::timestamp) as dp

For me, this results in:

2020-01-24 22:27:12+00  2020-01-24 22:27:12+00  22.0    22.0

Try it in your SQL client and see what happens.