I have a PostgreSQL table with date field in the following format
2017-09-07T17:24:33+08:00
and I want to convert it to UTC time.
I've looked around but found no way to do that with this specific time format. What am I missing?
Thanks
timezone definition (https://www.postgresql.org/docs/9.1/functions-datetime.html) : The function timezone(zone, timestamp) is equivalent to the SQL-conforming construct timestamp AT TIME ZONE zone
SELECT timezone('UTC','2017-09-07T17:24:33+08:00');
if selecting from column,
with t as (
SELECT '2017-09-07T17:24:33+08:00' as tm
)
SELECT timezone('UTC',tm::timestamptz) as ts
from t;
I have a PostgreSQL table with date field in the following format: 2017-09-07T17:24:33+08:00
This is incorrect. Per Postgres documentation,
All timezone-aware dates and times are stored internally in UTC. They are converted to local time in the zone specified by the TimeZone configuration parameter before being displayed to the client.
To display a TIMESTAMP WITH TIME ZONE attribute (which, again, is stored internally in UTC) in a specific time zone, you have a few options:
By default, “They are converted to local time in the zone specified by the TimeZone configuration parameter before being displayed to the client.” This can be set in postgresql.conf, using the SQL command SET TIME ZONE, or using a number of other options.
Use the AT TIME ZONE construct.
So, in regards to your original question: You don't have to do anything to your timestamptz for Postgres to store it in UTC. To display your attribute in UTC, you can either change the TimeZone configuration paramter, or use the construct below:
SELECT dt_with_timezone AT TIME ZONE 'UTC' FROM my_table
Related
The behavior of the at time zone operator on the date type is not as expected in PostgreSQL.
set time zone 'UTC';
select
'2021-01-03'::date at time zone 'America/Chicago' as "value",
pg_typeof('2021-01-03'::date at time zone 'America/Chicago') as "type";
x
value
type
Expected
2021-01-03 06:00:00+00
timestamp with time zone
Actual
2021-01-02 18:00:00
timestamp without time zone
DB Fiddle
Tested on PostgreSQL server versions 10, 11, 12, and 13.
This behavior is very strange to me, and I have not been able to find anywhere it is described or discussed. It seems PSQL is implicitly casting date to timestamptz (using the system time zone) and then applying the at time zone operator. I know that the at time zone operator is only defined (in the docs) for the timestamp(tz) and time(tz) types. However, I would have assumed PSQL would implicitly cast date to timestamp and not timestamptz because:
It is a "safe" conversion (no need to assume a time zone). Simply append 00:00:00.
It results in a timestamptz which one would expect when using the at time zone operator on a type that does not have a time zone.
The current behavior simply doesn't make sense to me and I can't think of a use case that would benefit from it. You are asking for the timestamp with time zone for a date at a particular time zone and instead you get a timestamp of the date at the system time zone localized to the given time zone.
Of course, a simple workaround is to cast to timestamp first:
set time zone 'UTC';
select '2021-01-03'::date::timestamp at time zone 'America/Chicago';
I was hoping someone could shed some light on this. What am I missing? Perhaps some overarching rule about how date is implicitly cast to timestamp(tz)? Is this behavior documented somewhere?
Thanks!
Mike
You ask "why". The reason is that AT TIME ZONE does not operate on date, so PostgreSQL invokes an implicit type cast to convert it to a different data type. Lacking an explicit directive, PostgreSQL opts for the preferred data type for datetime, which happens to be timestamp with time zone.
The data type resolution rules for operators are documented here, and the preferred type for a type category can be found in the typispreferred and typcategory columns of the pg_type system catalog.
As per the Postgresql documentation:
The AT TIME ZONE converts time stamp without time zone to/from time stamp with time zone, and time values to different time zones
Here in first query, you are trying to apply at time zone to date type, not to time stamp
set time zone 'UTC';
select
'2021-01-03'::date at time zone 'America/Chicago' as "value",
pg_typeof('2021-01-03'::date at time zone 'America/Chicago') as "type";
which is eventually gets ignored, and you will get a timestamp without timezone
But in second query,
set time zone 'UTC';
select '2021-01-03'::date::timestamp at time zone 'America/Chicago';
You are casting the date to time stamp first and then converting it to desired time zone,
which gives you the expected result of time stamp with time zone.
Hope this clears your doubt.
I am getting my data: date and time in UTC, in a csv file format in separate columns. Since I will need to convert this zone to date and time of the place where I live, currently in summer to UTC+2, and maybe some other zones I was wondering what is the best practice to insert data in postgres when we are talking about type of data. Should I place both of my data in a single column or keep them separate as types: date and time, and if not should I use timestamp or timestampz (or something else).
use timestamptz it will store your time stamp in UTC. and will display it to the client according to it's locale.
https://www.postgresql.org/docs/current/static/datatype-datetime.html
For timestamp with time zone, the internally stored value is always in
UTC (Universal Coordinated Time, traditionally known as Greenwich Mean
Time, GMT). An input value that has an explicit time zone specified is
converted to UTC using the appropriate offset for that time zone. If
no time zone is stated in the input string, then it is assumed to be
in the time zone indicated by the system's TimeZone parameter, and is
converted to UTC using the offset for the timezone zone.
When a timestamp with time zone value is output, it is always
converted from UTC to the current timezone zone, and displayed as
local time in that zone. To see the time in another time zone, either
change timezone or use the AT TIME ZONE construct (see Section 9.9.3).
updated with another good point from Lukasz, I had to mention:
Also in favor of single column is the fact that if you would store
both date and time in separate columns you would still need to combine
them and convert to timestamp if you wanted to change time zone of
date.
Not doing that would lead to date '2017-12-31' with time '23:01:01' would in other time zone in fact be not only different time, but different date with all YEAR and MONTH and DAY different
another update As per Laurenz notice, don't forget the above docs quote
An input value that has an explicit time zone specified is converted to UTC using the appropriate offset for that time zone. Which means you have to manage the input dates carefully. Eg:
t=# create table t(t timestamptz);
CREATE TABLE
t=# set timezone to 'GMT+5';
SET
t=# insert into t select '2017-01-01 00:00:00';
INSERT 0 1
t=# insert into t select '2017-01-01 00:00:00' at time zone 'UTC';
INSERT 0 1
t=# insert into t select '2017-01-01 00:00:00+02';
INSERT 0 1
t=# select * from t;
t
------------------------
2017-01-01 00:00:00-05
2017-01-01 05:00:00-05
2016-12-31 17:00:00-05
(3 rows)
I have a log table in a PostgreSQL database with an event column of type timestamp without time zone.
Now I have a bash script, which creates a CSV file from the log database:
...
psql .. -c "COPY (SELECT event, ... FROM logtable order by event desc) TO STDOUT WITH CSV" logdb > log.csv
...
This is executed on the cloud server on which the DB is hosted and therefore, the timestamp strings in log.csv are in local time of the timezone of the server.
However, I like to have the timestamp strings to represent the time of my own time zone. So I shall be able to let psql transform the timestamp -> string to a given timezone. How can I achieve this?
First of all, you should use timestamptz instead of timestamp whenever working with multiple times zones. Would avoid the problem completely.
Details:
Ignoring timezones altogether in Rails and PostgreSQL
You can use the AT TIME ZONE construct like #NuLo suggests, it may even work, but not exactly as described.
AT TIME ZONE converts the type timestamp (timestamp without time zone) to timestamptz (timestamp with time zone) and vice versa. The text representation of a timestamptz value depends on the current setting of the time zone in the session in which you run the command. These two timestamptz values are 100 % identical (denote the same point in time):
'2015-09-02 15:55:00+02'::timestamptz
'2015-09-02 14:55:00+01'::timestamptz
But the text representation is not. The display is for different time zones. If you take this string literal and feed it to a timestamp type, the time zone part is just ignored and you end up with different values. Hence, if you run your COPY statement in a session with the same time zone setting as your original timestamp values are for, the suggested operation happens to work.
The clean way, however, is to produce correct timestamp values to begin with by applying AT TIME ZONE twice:
SELECT event AT TIME ZONE 'my_target_tz' AT TIME ZONE 'my_source_tz', ...
FROM logtable
ORDER BY event desc;
'my_target_tz' is "your own time zone" and 'my_source_tz' the time zone of the of the cloud server in the example. To make sure that DST is respected use time zone names, not time zone abbreviations. The documentation:
A time zone abbreviation, for example PST. Such a specification merely
defines a particular offset from UTC, in contrast to full time zone names
which can imply a set of daylight savings transition-date rules as well.
Related:
Accounting for DST in Postgres, when selecting scheduled items
Time zone names with identical properties yield different result when applied to timestamp
Or, much better yet, use timestamptz everywhere and it works correctly automatically.
I run a Postgres 8.3 database where times seem to be stored in UTC without time zone.
I am trying to display in local time but not with '+01' suffix :
With select scheduled_start_ts I get :
2014-01-20 05:01:35.663
With select scheduled_start_ts at time zone 'MET' :
2014-01-20 05:01:35.663+01
I would like to get "2014-01-20 06:01:35.663" which is in local time.
The database I am using cannot be modified and I am not allowed to modify how data are stored.
If you want to format times, use the to_char function. See formatting functions in the docs.
regress=> SELECT to_char(
(TIMESTAMP '2014-01-20 05:01:35.663' AT TIME ZONE 'UTC')
AT TIME ZONE 'MET',
'YYYY-MM-DD HH:MI:SS'
);
to_char
---------------------
2014-01-20 06:01:35
(1 row)
The (TIMESTAMP 'xxx' AT TIME ZONE 'UTC') gets me a timestamptz with the correct time, by re-interpreting the TIMESTAMP as being in UTC. The second AT TIME STAMP instead converts the timestamptz into a timestamp in timezone MET. This then gets formatted.
Whatever the SQL standards committe were smoking when they designed this, I never, ever, ever want to be anywhere near it.
I have a column added_at of type timestamp without time zone. I want it's default value to be the current date-time but without time zone. The function now() returns a timezone as well.
How do I solve that problem?
SELECT now()::timestamp;
The cast converts the timestamptz returned by now() to the corresponding timestamp in your time zone - defined by the timezone setting of the session. That's also how the standard SQL function LOCALTIMESTAMP is implemented in Postgres.
If you don't operate in multiple time zones, that works just fine. Else switch to timestamptz for added_at. The difference?
Ignoring time zones altogether in Rails and PostgreSQL
BTW, this does exactly the same, just more noisy and expensive:
SELECT now() AT TIME ZONE current_setting('timezone');
Well you can do something like:
SELECT now() AT TIME ZONE current_setting('TimeZone');
SELECT now() AT TIME ZONE 'Europe/Paris';
SELECT now() AT TIME ZONE 'UTC';
Not sure how that makes any sense for a column "added_at". You almost always want an absolute timestamp (timestamp with time zone) not a floating one.
Edit responding to points below:
Yes, should use timestamp with time zone (absolute time) unless you have a good reason not to.
The client timezone is given by SHOW TimeZone or current_setting(...) as shown above.
Do take some time to skim the manuals - they cover all this quite well.
"Current Date/Time":
CURRENT_TIME and CURRENT_TIMESTAMP deliver values with time zone; LOCALTIME and LOCALTIMESTAMP deliver values without time zone.
New, and Native Answer in 2020
In PostgreSQL, If you only want the current date-time by calling CURRENT_TIMESTAMP() without time zone, and fractional digits in the seconds field which come after the decimal point of the seconds field?
(Tested on PostgreSQL v12.4)
Then use this:
SELECT CURRENT_TIMESTAMP(0)::TIMESTAMP WITHOUT TIME ZONE;
If you define your column's data type as timestamp (not as timestamptz), then you can store the timestamp without time zone, in that case you don't neet to add TIMESTAMP WITHOUT TIME ZONE
Like this:
CREATE TABLE foo (created timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP(0))
In the above function, 0 is passed to get rid of the fractional digits in the seconds field.
If your application doesn't care about timezone, you can use SELECT LOCALTIMESTAMP for it.
Ex:
SELECT LOCALTIMESTAMP
-- Result: 2023-01-30 17:43:33.628952