I'm currently using CURRENTTIMESTAMP AT TIME ZONE 'UTC' and inserting into columns defined as TIMESTAMP WITHOUT TIME ZONE which appears to work. However, PostgreSQL parses this into the much more confusing 'timezone('UTC'::text, ('now'::text)::timestamp(6) with time zone)' which is what is appearing on my generated documentation.
EXTRACT, datepart EXTRACT(field FROM source) The extract function retrieves subfields such as year or hour from date/time values.source must be a value expression of type timestamp, time, or interval. (Expressions of type date are cast to timestamp and can therefore be used as well.) field is an identifier or string that selects what field to extract from the source value. If you have a timestamp without time zone column and you're storing timestamps as UTC, you need to tell PostgreSQL that, and then tell it to convert it to your local time zone. Select createdat at time zone 'utc' at time zone 'america/losangeles'. The current timestamp and now function is similar work in PostgreSQL. Transaction timestamp and the current timestamp is equivalent to each other in PostgreSQL. But transaction timestamp will reflect the same which was return by the function. Examples to Implement PostgreSQL CURRENTTIMESTAMP The below example shows that now, current. We can get the current date and time by using the PostgreSQL NOW function. The PostgreSQL NOW function’s return type is the timestamp with the time zone. Depending upon the current database server’s time zone setting, the PostgreSQL NOW function returns us the current date and time.
Since 4.0 the recommended way of working with date/time types is the NodaTime plugin.
Handling date and time values usually isn't hard, but you must pay careful attention to differences in how the .NET types and PostgreSQL represent dates.It's worth reading the PostgreSQL date/time type documentation to familiarizeyourself with PostgreSQL's types.
.NET types and PostgreSQL types
A common mistake is for users to think that the PostgreSQL
timestamp with timezone type stores the timezone in the database. This is not the case: only the timestamp is stored. There is no single PostgreSQL type that stores both a date/time and a timezone, similar to .NET DateTimeOffset.
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The .NET and PostgreSQL types differ in the resolution and range they provide; the .NET type usually have a higher resolution but a lower range than the PostgreSQL types:
|PostgreSQL type||Precision/Range||.NET Native Type||Precision/Range||Npgsql .NET Provider-Specific Type|
|timestamp without time zone||1 microsecond, 4713BC-294276AD||DateTime||100 nanoseconds, 1AD-9999AD||NpgsqlDateTime|
|timestamp with time zone||1 microsecond, 4713BC-294276AD||DateTime||100 nanoseconds, 1AD-9999AD||NpgsqlDateTime|
|date||1 day, 4713BC-5874897AD||DateTime||100 nanoseconds, 1AD-9999AD||NpgsqlDate|
|time without time zone||1 microsecond, 0-24 hours||TimeSpan||100 nanoseconds, -10,675,199 - 10,675,199 days||N/A|
|time with time zone||1 microsecond, 0-24 hours||DateTimeOffset (ignore date)||100 nanoseconds, 1AD-9999AD||N/A|
|interval||1 microsecond, -178000000-178000000 years||TimeSpan||100 nanoseconds, -10,675,199 - 10,675,199 days||NpgsqlTimeSpan|
If your needs are met by the .NET native types, it is best that you use them directly with Npgsql.If, however, you require the extended range of a PostgreSQL type you can use Npgsql's provider-specific types, which represent PostgreSQL types in an exact way.
It's critical to understand exactly how timezones and timezone conversions are handled between .NET types and PostgreSQL.In particular, .NET's DateTime has a Kind property which impacts howNpgsql reads and writes the value.
DateTime is sent to PostgreSQL as a
timestamp without time zone - no timezone conversion of any kind will occur, and your
DateTime instance will be transferred as-is to PostgreSQL. This is the recommended way to store timestamps in the database. Note that you may still send
timestamp with time zone by setting
NpgsqlDbType.TimestampTz on your
NpgsqlParameter; in this case, if the
Local, Npgsql will convert the value to UTC before sending it to PostgreSQL. Otherwise, it will be sent as-is.
You can also send
DateTimeOffset values, which are written as
timestamptz and are converted to UTC before sending.
time with time zone is the only date/time type which actually stores a timezone in the database. You can use a
DateTimeOffset to send one to PostgreSQL, in which case the date component is dropped and the time and timezone are preserved. You can also send a
DateTime, in which case the
Kind will determine the the timezone sent to the database.
Detailed Behavior: Sending values to the database
|DateTime||NpgsqlDbType.Timestamp (default)||Send as-is|
|DateTime(Kind=Local)||NpgsqlDbType.TimestampTz||Convert to UTC locally before sending|
|DateTimeOffset||NpgsqlDbType.TimestampTz (default)||Convert to UTC locally before sending|
|TimeSpan||NpgsqlDbType.Time (default)||Send as-is|
|DateTimeOffset||NpgsqlDbType.TimeTz||Send time and timezone|
|DateTime(Kind=UTC)||NpgsqlDbType.TimeTz||Send time and UTC timezone|
|DateTime(Kind=Local)||NpgsqlDbType.TimeTz||Send time and local system timezone|
|DateTime(Kind=Unspecified)||NpgsqlDbType.TimeTz||Assume local, send time and local system timezone|
Detailed Behavior: Reading values from the database
|PG type||.NET value||Action|
|timestamptz||DateTime (default)||Kind=Local (according to system timezone)|
|timestamptz||DateTimeOffset||In local timezone offset|
|timetz||DateTimeOffset (default)||Date component is empty|
|timetz||TimeSpan||Strip offset, read as-is|
|timetz||DateTime||Strip offset, date is empty|
If you're really interested in some of the mapping decisions above, check out this issue.
Continuing our series of PostgreSQL Data Types todaywe’re going to introduce date, timestamp, and interval data types.
PostgreSQL implementation of the calendar is very good, and we’re going toshow some mice example about how confusing this matter is. The time zonenotion in particular is mainly a political tool these days, and it makes nosense on an engineering principle: there’s no way to solve time zoneproblems from first hand principles!
Handling dates and time and time zones is a very complex matter, and on thistopic, you can read Erik Naggum’s piece The Long, Painful History ofTime.
The PostgreSQL documentation chapters with the titles Date/TimeTypes,Data Type FormattingFunctions,and Date/Time Functions andOperatorscover all you need to know about date, time, timestamps, and time zones withPostgreSQL.
The first question we need to answer here is about using timestamps with orwithout time zones from our applications. The answer is simple: always usetimestamps WITH time zones.
A common myth is that storing time zones will certainly add to your storageand memory footprint. It’s actually not the case:
PostgreSQL defaults to using bigint internally to store timestamps, andthe on-disk and in-memory format are the same with or without time zonesupport. Here’s their whole type definition in the PostgreSQL source code(in
From the PostgreSQL documentation for timestamps, here’s how it works:
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 convertedto UTC using the appropriate offset for that time zone. If no time zone isstated in the input string, then it is assumed to be in the time zoneindicated by the system’s TimeZone parameter, and is converted to UTCusing the offset for the timezone zone.
PostgreSQL doesn’t store the time zone they come from with your timestamp.Instead it converts to and from the input and output timezone much likewe’ve seen for text with client_encoding.
In this script, we play with the client’s setting timezone and change froma French value to another French value, as Tahiti is an island in thePacific that is part of France. Here’s the full output as seen when runningthis script, when launched with
psql -a -f tz.sql:
First, we see that the now() function always returns the same timestampwithin a single transaction. If you want to see the clock running while in atransaction, use the clock_timestamp() function instead.
Then, we see that when we change the timezone client setting, PostgreSQLoutputs timestamps as expected, in the selected timezone. If you manage anapplication with users in different time zones and you want to display timein their own local preferred time zone, then you can set timezone in yourapplication code before doing any timestamp related processing, and havePostgreSQL do all the hard work for you.
Finally, when selecting back from the tstz table, we see that the columntstz realizes that both the inserted values actually are the same point intime, but seen from different places in the world, whereas the ts columnmakes it impossible to compare the entries and realize they actuallyhappened at exactly the same time.
As said before, even when using timestamps with time zone, PostgreSQL willnot store the time zone in use at input time, so there’s no way from ourtstz table to know that the entries are at the same time but just fromdifferent places.
The opening of this section links to The Long, Painful History ofTime, and if you didn’t read it yet, maybenow is a good time. Allow me to quote a relevant part of the article here:
The basic problem with time is that we need to express both time andplace whenever we want to place some event in time and space, yet we tendto assume spatial coordinates even more than we assume temporalcoordinates, and in the case of time in ordinary communication, it issimply left out entirely. Despite the existence of time zones and strangedaylight saving time regimes around the world, most people are blithelyunaware of their own time zone and certainly of how it relates to standardreferences. Most people are equally unaware that by choosing a notationthat is close to the spoken or written expression of dates, they make itmeaningless to people who may not share the culture, but can still readthe language. It is unlikely that people will change enough to put theseissues to rest, so responsible computer people need to address the issuesand resist the otherwise overpowering urge to abbreviate and dropcontext.
Several options are available to input timestamp values in PostgreSQL. Theeasiest is to use the ISO format, so if your application’s code allows thatyou’re all set. In the following example we leave the time zone out, asusually, it’s handled by the timezone session parameter, as seen above. Ifyou need to, of course, you can input the time zone in the timestamp valuesdirectly:
At insert or update time, use the same literal strings without the typedecoration: PostgreSQL already knows the type of the target column, and ituses that to parse the values literal in the DML statement.
Some application use-cases only need the date. Then use the date data typein PostgreSQL. It is of course then possible to compare a date and atimestamp with time zone in your SQL queries, and even to append a timeoffset on top of your date to construct a timestamp.
Postgresql Get Now Utc
PostgreSQL implements an interval data type along with the time, dateand timestamptz data types. An interval describes a duration, like amonth or two weeks, or even a millisecond:
The default PostgreSQL output looks like this:
Several intervalstyle values are possible, and the settingpostgres_verbose is quite nice for interactive psql sessions:
This time we get a user-friendly output:
How long is a month? Well, it depends on which month, and PostgreSQL knowsthat:
Postgresql Utc Nowhere
When you attach an interval to a date or timestamp in PostgreSQL then thenumber of days in that interval adjusts to the specific calendar entryyou’ve picked. Otherwise, an interval of a month is considered to be 30days. Here we see that computing the last day of February is very easy:
PostgreSQL’s implementation of the calendar is very good, so use it!
Postgresql Timezone Utc
This article is an extract from my book Mastering PostgreSQL in ApplicationDevelopment, which teaches SQL todevelopers so that they may replace thousands of lines of code with verysimple queries. The book has a full chapter about data types in PostgreSQL,check it out!