I'm gonna to save date and time in my PostgreSQL database and fetch and show it to the user in an appropriate format.
Suppose that application users are located in Iran and use Jalali date/time system. In the first half of year, Iran time is UTC+04:30, but in the rest of year, it is UTC+03:30. As a matter of fact daylight saving time is used in Iran.
IMPORTANT: Sometimes we make a decision and change database server location from Iran to Europe or elsewhere.
Now I have some questions:
What data type is more convenient to save date with time? TIMESTAMP (TIMESTAMP WITHOUT TIMEZONE) or TIMESTAMPZ (TIMESTAMP WITH TIME ZONE)?
What data type is more convenient to save time only? TIME or TIME WITH TIME ZONE?
What data type is more convenient to save date only? DATE (Gregorian) String (Jalali) or a custom data type?
How can I set TIMEZONE for our database once? I wanna set TIMEZONE for database one time and after that all queries over TIMESTAMPZ columns will be saved and fetched within that TIMEZONE and also it considered daylight saving time ?
What SQL query is the best when I wan to save current date and time ?
a- INSERT INTO test(d) VALUES(now());
b- INSERT INTO test(d) VALUES(now() at time zone 'utc');
c- INSERT INTO test(d) VALUES(now() at time zone 'Asia/Tehran');
d- INSERT INTO test(d) VALUES(current_timestamp);
e- INSERT INTO test(d) VALUES(now() at time zone 'utc');
f- INSERT INTO test(d) VALUES(now() at time zone 'Asia/Tehran');
Is number 5-c and 5-f considered daylight saving times when save it or not?
Is number 5-a and 5-d saved in 'Asia/Tehran' when database has 'Asia/Tehran' time zone ?
When I wan to query from database, what options is the best in my situation ?
a- SELECT d FROM test;
b- SELECT d at time zone 'utc' FROM test;
c- SELECT d at time zone 'Asia/Tehran' FROM test;
Is number 7-c considered daylight saving times?
Is number 7-a considered 'Asia/Tehran' time zone and daylight saving time when database has 'Asia/Tehran' time zone?
If I'd better use the timestamp to save date/time, then I have to add daylight saving time to it and convert it to Jalali date time and show it to the user, and vice versa.
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
timestamptz
timestamptz and select date only
timestamptz and select time only
database saves tz aware time stamps in UTC, no matter your locale or settings. stamps are always converted to UTC adjusting it by TimeZone parameter. Same deconvert happens on displaying data every time. TimeZone for server is only default value, used if client does not specify any. database TimeZone overrides the one in postgresql.conf for specified database, but still, client settings will override the database ones.
Related
What is the common (good) practice for the type of date+time columns in PostgreSQL? Timestamp WITH or WITHOUT time zone? Is it possible to output timestamps in all queries (selects) as a UTC string timestamp even if the column has a type TIMESTAMP WITH TIME ZONE?
Last question first: you can set the timestamp parameter to UTC or use AT TIME ZONE 'UTC'.
What data type to choose depends on how you want to handle time zones:
if you don't care about time zones, use timestamp without time zone
if you want to handle time zones in the application, use timestamp without time zone and store everything in UTC
if you want to handle time zones in the database, use timestamp with time zone and set tge timezone parameter correctly
I have a pg database with a column type timestamp with time zone. I inserted the following date:
2016-08-01 00:00:00 GMT
However, in the database, it shows up as:
2016-07-31 20:00:00-04
Does anyone know what might be going on?
Thanks in advance!
Despite the name, TIMESTAMP WITH TIME ZONE doesn't actually store the time zone. It uses the session's time zone to normalize to UTC, and stores UTC. On retrieval it converts back from UTC to the session time zone.
You can change the session time zone by using the SET TIME ZONE command. Preferably, you should use the standard IANA time zone identifiers. For example:
SET TIME ZONE 'Europe/Paris'
or
SET TIME ZONE 'UTC'
Alternatively use the TIMESTAMP [WITHOUT TIME ZONE] type instead, which does no conversions.
I have column 'event_date_time' in my 'events' table with type 'timestamp with timezone'. My python flask application is saving date like '2014-08-30 02:17:02+00:00' but postgres automatically converts it to '2014-08-30 07:17:02+05'. It converts the timestamp to my local timezone i-e Pakistan. I want to save it without converting.
I have tried
set timezone='UTC'
and it does change timezone to 'UTC' but pgadmin3 is still saving the converted time.
I am using MAC OS and Postgresql 9.3.
The reason pgadmin is displaying hours +5 is because your system timezone is set to this.
When you save a "timestamp with time zone" value at GMT + or - any value, the system offsets whatever timezone your input was to GMT (or UTC), so that when you go to retrieve it, you can specify the timezone you want it displayed in.
For example let's establish a current time for say... New York.
select now()::timestamp with time zone at time zone 'America/New_York';
At the time of asking it returned '2014-08-23 08:50:57.136817'. 8:50 Saturday morning, or 8:51 if you're being pedantic.
Now if we take that same time and display it in GMT we will see a different result:
select '2014-08-23 08:50:57.136817 America/New_York'::timestamp with time zone at time zone 'GMT';
Now have a new time of '2014-08-23 12:50:57.136817'... 5 hours into the "future"!
Finally let's get the original timestamp and display it in what I believe is the Pakistan time zone (PKT) and see what it shows
select '2014-08-23 08:50:57.136817 America/New_York'::timestamp with time zone at time zone 'PKT';
The result? '2014-08-23 17:50:57.136817' further into the future still!
Again I must stress the reason it can do this is because it is always converting the input time offset to UTC or GMT. Postgres processes all of its "timestamp with time zone" data types in this way. It is designed to avoid time zone problems such as daylight savings and so on.
Your issue appears to be that python is inserting the time at an offset of +00, and if this was supposed to be a local time then you will be 5 hours off as far as postgres is concerned. Without knowing exactly what queries python is making, I would assume you may want to look at that to make sure it is giving you the correct time, presumably set timezone='PKT' should be a fix. Either way, when you are viewing timestamp with time zone using a browser such as pgadmin, the timestamp is being converted to your local timezone and this is why you see +5.
Alternatively if you do wish to see those times at +00 then you must specify that you want this in your SELECT queries.
I need to convert local time to UTC using a function. The inputs I have is the local time, timezone of the local time (e.g. 'Pacific/Auckland'). What I need to get from the procedure is the UTC time for the local time based on given timezone.
Can someone please help?
I am using version 8.3
This is covered in the manual, but it's not always obvious how to actually work with dates/times. The SQL spec is a bit bizarre.
In the case of your question it isn't clear whether you want to store the time in UTC but display it in the server's local time (TimeZone), or whether you wan to ignore TimeZone and always display it as UTC. I'll assume the latter.
For timestamps you'd just use AT TIME ZONE twice, like:
SELECT TIMESTAMP '2013-08-13 00:00:00' AT TIME ZONE 'Australia/Sydney' AT TIME ZONE 'UTC';
You need to use AT TIME ZONE twice. Once to convert the input timestamp to timestamptz according to the argument timezone, then another to convert that to a timestamp at UTC.
Unfortunately because of the (IMO insane) way the SQL spec defines AT TIME ZONE for TIME, you can't do the same thing for TIME. You'll have to manipulate the TimeZone variable instead:
SET TimeZone = 'UTC';
SELECT TIME '07:00' AT TIME ZONE 'Australia/Sydney';
This still leaves you with a timetz not a time. So its display value changes with the timezone setting.
You can use select now() at time zone 'UTC' for postgresql.
Based on the first two answers, the question was unclear as originally posted, thus I am completely rewriting it:
The following question is concerned only with how and what data is stored, and is no way shape or form concerned with converting data upon retrieval. As such, converting at SELECT to the desired time zone is not an appropriate answer.
When inserting a value into a timestamp with time zone field, it is retrieved (and thus presumably stored) with the timestamp converted to the local time zone of the database at the time it was inserted.
That is so say, a timestamp inserted as 2012-01-01 00:00:00+00:00 is retrieved as 2011-12-31 19:00:00-05, where the local time zone of the database at the time of the insert was -05. Timestamps that were inserted during daylight savings time, when the database was at -04, are returned using the -04 time zone.
What I want is for all timestamps to use an arbitrary time zone when stored (and thus all be retrieved without any additional work as having that time zone). That is to say, were the server orbiting the planet, all times would be at +00:00 (arbitrary time zone), instead of -12:00 to +12:00.
Can I insert into a timestamp with time zone column such that all timestamps are stored relative to an arbitrary time zone? If so, how?
Original follows.
When inserting a value into a timestamp with time zone field, it is being converted to the server's current time zone.
Example: If I insert a value specifying a time zone of -1, retrieving it will give back the time at -5 (the time zone of the server at the time it was inserted).
Is it possible to specify that it should be stored using an arbitrary time zone?
Note: This question is not how to convert the returned time to another time zone, this is specific to how the time is stored.
You have to save the time zone offset in addition to the timestamp.
As #Milen already explained (and linked to the manual): a timestamp only saves a point in time (as abstract value). The time zone modifier is not saved, it only serves to adjust the timestamp relative to UTC.
Consider the following demo:
-- DROP TABLE tbl;
CREATE TEMP TABLE tbl (id int, myts timestamptz, mytz interval);
INSERT INTO tbl VALUES
(1, now() , EXTRACT (timezone from now()) * interval '1s')
,(2, '2012-01-01 00:00-05', interval '-5h')
,(3, '2012-01-01 00:00+04', interval '4h')
,(4, '2012-11-11 20:30+03', interval '3h');
SELECT *
,(myts AT TIME ZONE mytz)::text
|| CASE WHEN mytz > '0:0' THEN '+' ELSE '' END
|| to_char(mytz, 'FMHH24:mi') AS timestamp_at_origin
FROM tbl;
Run it locally to see. Pay special attention to the details of the AT TIME ZONE construct, and how I extract the time zone from the (local!) timestamp with time zone.
now() returns timestamp with time zone or timestamptz for short.
EXTRACT (timezone from now()) * interval '1s'
timestamp_at_origin displays the timestamp with time zone as seen at its origin. If I understood your question, then that is what you are looking for.
You could further improve formatting.
You may be interested in this related question which sheds some light on the ambiguities and pitfalls of time zones.
When inserting a value into a timestamp with time zone field what actually happens is the timestamp is converted to UTC. Another matter altogether is to what time zone that value is converted on output. There are a few ways to control that:
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).
You can do your select with an at time zone operator:
select insertTs at time zone 'CST' from table
See more here.
I always store times in GMT so that the client can convert based on it's current GMT offset (the GMT offest is available in most language).
I write C# - so I can easily convert all DateTime objects to GMT using DateTime.ToUniversalTime() as I store data in the database.
I am not sure what language you are using or how to convert all times to GMT in postgressql, but from a logic standpoint - storing all times in GMT will allow a uniform time zone that all other time zones can easily relate to.
Hope this helps!
Greenwich Mean Time