date with month and year

On Thu, May 21, 2015 at 12:01 PM, Daniel Torres <nobeeakon@gmail.com> wrote:

I everybody, I'm new in the Postgresql world, and have an easy question:
Is it possible to have date type data that only contain month and year?,
how can I obtain that from a timestamp (without time zone) column?

I've made this, but I think the result is a text, not a date

select extract (Year from '2001-05-01 20:21:00'::TIMESTAMP WITHOUT TIME
ZONE)||'-'|| extract(Month from '2001-05-01 20:21:00'::TIMESTAMP WITHOUT
TIME ZONE);

Any help is welcome, thanks

Daniel

​I don't think so. Mainly because a date, at least in PostgreSQL, is by
definition a month, day, and year. You could just arbitrarily set the day
to "01" because every month starts with day 1, I guess.​ Perhaps if you
said what you want to do with this type of date field? Of course, if you
really wanted to, you could create your own data type and conversions. But
that still wouldn't be a "date", exactly.

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On Thursday, May 21, 2015, Daniel Torres <nobeeakon@gmail.com> wrote:

I everybody, I'm new in the Postgresql world, and have an easy question:
Is it possible to have date type data that only contain month and year?,
how can I obtain that from a timestamp (without time zone) column?

I've made this, but I think the result is a text, not a date

select extract (Year from '2001-05-01 20:21:00'::TIMESTAMP WITHOUT TIME
ZONE)||'-'|| extract(Month from '2001-05-01 20:21:00'::TIMESTAMP WITHOUT
TIME ZONE);

You have to settle for the first of the month if you want a date type.
Date_trunc(day,...) will give you that.

I do end up having a lookup tha gassing sequential integers to sequential
year-months to make calculations easier without having to carry around a
date type for that sole purpose. For presentation I want text, not a date.

User defined functions are nice here - I have a todo to publish my set to
PGXN...maybe someone else already has?

David J.

Is it possible to have date type data that only contain month and year?,
how can I obtain that from a timestamp (without time zone) column?

I think you want date_trunc, which will cut everything down to the first
of the month, e.g 2015-01-01, 2015-02-01, etc. The results will still be
dates, so you can still use date functions to manipulate them.

Paul

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On 05/21/2015 10:01 AM, Daniel Torres wrote:

I everybody, I'm new in the Postgresql world, and have an easy question:
Is it possible to have date type data that only contain month and year?,
how can I obtain that from a timestamp (without time zone) column?

I've made this, but I think the result is a text, not a date

select extract (Year from '2001-05-01 20:21:00'::TIMESTAMP WITHOUT TIME
ZONE)||'-'|| extract(Month from '2001-05-01 20:21:00'::TIMESTAMP WITHOUT
TIME ZONE);

date_part will get you what you want as will to_char. The above you
could cast if you needed. You really shouldn't use WITHOUT TIME ZONE.

JD

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On 05/21/2015 10:01 AM, Daniel Torres wrote:

I everybody, I'm new in the Postgresql world, and have an easy
question: Is it possible to have date type data that only contain
month and year?, how can I obtain that from a timestamp (without time
zone) column?...

Others have offered good tips but if you tell us more about the problem
you are attempting to solve you may get some better advice and/or
warnings about pitfalls.

-Steve

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You really shouldn't use WITHOUT TIME ZONE.

I'd like to know more about this. Can you say why? Are there any
articles you'd recommend? I'm fond of normalizing all times to UTC and
only presenting them in a time zone when I know the current
"perspective". I've written about that approach in a Rails context here:

http://illuminatedcomputing.com/posts/2014/04/timezones/

I find that this helps me to ignore time zones in most parts of my
application and cut down on my timezone-related bugs.

Thanks!
Paul

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Sorry, forgot to told you what I'm trying, I have climate data and want to
obtain mean temperature and total precipitation and that sort of things per
month and year. Think date_trunc is a good solution, but any other advice
would be very welcome.

(I need to read more about time zones, I'm new at using postgresql)

Thank you,
Daniel

2015-05-21 12:45 GMT-05:00 Paul Jungwirth <pj@illuminatedcomputing.com>:

On 05/21/2015 10:45 AM, Paul Jungwirth wrote:

You really shouldn't use WITHOUT TIME ZONE.

I'd like to know more about this. Can you say why?

Start by reading about the date and time data types with special
attention to section 8.5.3:
www.postgresql.org/docs/current/static/datatype-datetime.html

Now go back and read it again and experiment a while until it makes
sense. As Adrian Klaver so eloquently put it, "If I have learned
anything about dealing with dates and times, is that it is a set of
exceptions bound together by a few rules. Every time you think you have
the little rascals cornered, one gets away." This is also a very good
reason to avoid reinventing the wheel.

When you need a break, watch this:
https://www.youtube.com/watch?v=-5wpm-gesOY

His conclusion is a good one: be very happy that someone else has done
the dirty work for you.

The Ruby article does make one good point which is that we are talking
about what they call an "instant" or what I like to refer to as a "point
in time." The "point in time" is actually a better way of thinking of
"timestamp with time zone" since the "timestamp with time zone" does not
actually store any timezone information - it stores a point in time that
can be manipulated in the time-zone of your choosing whereas timestamp
without time zone is not a point in time and must be combined with other
information to do proper manipulation.

The article does also display a couple attitudes that I feel are
especially rampant in the web-development community. The first is that
web developers shouldn't become educated about the capabilities of a
database but rather use the database as a dumb data-store and redo
everything themselves (often this includes an utter failure to use the
data-integrity capabilities of the database).

The second is the assumption that they are the only users of the
database and that nobody will ever access the data except through their
custom-written Ruby/PHP/Perl/Python code and that no other programming
language will ever be used. Woe be to the poor slob who has to deal with
ad-hoc queries, analytics platforms or reporting systems that weren't so
brilliantly reinvented or who wants to use range-types or other nice
PostgreSQL features.

Internally PostgreSQL stores timestamp without time zone in UTC but that
is entirely irrelevant. What is relevant is that you can provide an
"instant"/"point in time" in whatever time-zone representation you want
and get it back the same way. Want to use a Unix epoch in your code. Go
ahead:
extract(epoch from yourtstzcol)
abstime(yourepochint)

Want to assume everything is UTC? No problem:
Konsole output
set timezone to 'UTC';

Then you can reinvent wheels to your heart's content without wrecking
the ability to easily use other tools.

By the way, use full timezone names to avoid ambiguity. I don't know
what Ruby cooked up but PostgreSQL uses industry-standard names:
Konsole output
select * from pg_timezone_names;

Your original question had to do with month/year. You will have to
define this for your use-case but beware that it won't necessarily get
you away from time-zone issues as the month ticks over on a zone-by-zone
basis.

Also note that time-intervals can be a source of interesting
side-effects. Operator precedence is important. For example, what is one
month? 28-days? 29? 30? 31? Every system must make a judgment call. Add
a month to January 31 and you will get February 28. But add/subtract a
month from February 28 and you get January 28/March 28. So you can
create a query that takes a date, adds a month and subtracts a month and
results in a different date. There is nothing to do here but to read the
docs and try things.

There are similar issues when crossing DST boundaries. If I want to push
something out a day in my time-zone on the day that DST changes I can do
it easily and understand that PostgreSQL will handle the extra/missing
hour. Or I can use an explicit increment of '24 hours' if that is what I
want. No extra steps of converting the timestamp without time zone to
UTC, converting that to the desired local zone, doing the calculations,
converting back to UTC and back to timezone without timestamp all the
while potentially adding an easy error such as doing things in the wrong
order and checking for DST changeover in the wrong time-zone.

Cheers,
Steve

It's probably worth noting that both the Ruby 'best practice' AND
Postgres have a failure case when dealing with future dates precisely
because they are storing the data as UTC with a time zone. This is
one case where storing the data WITHOUT TIME ZONE would actually save
your bacon.

From the postgres docs: "For times in the future, the assumption is
that the latest known rules for a given time zone will continue to be
observed indefinitely far into the future."

Imagine scheduling a meeting for a certain time a few years from now.
This will be stored as UTC + time zone. A year later, that
government decides to change the time zone rules for their country.
Your operating system will get the new timezone data in an update (as
it should). However when the meeting comes around, you're going to be
early/late because the wall time that you get converting back from
UTC+time zone is no longer the time that you were supposed to have
been at the meeting. If you had stored that future date as a
timestamp WITHOUT time zone you would have still been on-time.

This is only an issue for future dates, not past ones.

-Brian Dunavant
(time is hard, so if I'm wrong anywhere here, someone please correct me)

On Thu, May 21, 2015 at 2:56 PM, Steve Crawford
<scrawford@pinpointresearch.com> wrote:

On 05/21/2015 10:45 AM, Paul Jungwirth wrote:

You really shouldn't use WITHOUT TIME ZONE.

I'd like to know more about this. Can you say why?

Start by reading about the date and time data types with special attention
to section 8.5.3:
www.postgresql.org/docs/current/static/datatype-datetime.html

Now go back and read it again and experiment a while until it makes sense.
As Adrian Klaver so eloquently put it, "If I have learned anything about
dealing with dates and times, is that it is a set of exceptions bound
together by a few rules. Every time you think you have the little rascals
cornered, one gets away." This is also a very good reason to avoid
reinventing the wheel.

When you need a break, watch this:
https://www.youtube.com/watch?v=-5wpm-gesOY

His conclusion is a good one: be very happy that someone else has done the
dirty work for you.

The Ruby article does make one good point which is that we are talking about
what they call an "instant" or what I like to refer to as a "point in time."
The "point in time" is actually a better way of thinking of "timestamp with
time zone" since the "timestamp with time zone" does not actually store any
timezone information - it stores a point in time that can be manipulated in
the time-zone of your choosing whereas timestamp without time zone is not a
point in time and must be combined with other information to do proper
manipulation.

The article does also display a couple attitudes that I feel are especially
rampant in the web-development community. The first is that web developers
shouldn't become educated about the capabilities of a database but rather
use the database as a dumb data-store and redo everything themselves (often
this includes an utter failure to use the data-integrity capabilities of the
database).

The second is the assumption that they are the only users of the database
and that nobody will ever access the data except through their
custom-written Ruby/PHP/Perl/Python code and that no other programming
language will ever be used. Woe be to the poor slob who has to deal with
ad-hoc queries, analytics platforms or reporting systems that weren't so
brilliantly reinvented or who wants to use range-types or other nice
PostgreSQL features.

Internally PostgreSQL stores timestamp without time zone in UTC but that is
entirely irrelevant. What is relevant is that you can provide an
"instant"/"point in time" in whatever time-zone representation you want and
get it back the same way. Want to use a Unix epoch in your code. Go ahead:
extract(epoch from yourtstzcol)
abstime(yourepochint)

Want to assume everything is UTC? No problem:
set timezone to 'UTC';

Then you can reinvent wheels to your heart's content without wrecking the
ability to easily use other tools.

By the way, use full timezone names to avoid ambiguity. I don't know what
Ruby cooked up but PostgreSQL uses industry-standard names:
select * from pg_timezone_names;

Your original question had to do with month/year. You will have to define
this for your use-case but beware that it won't necessarily get you away
from time-zone issues as the month ticks over on a zone-by-zone basis.

Also note that time-intervals can be a source of interesting side-effects.
Operator precedence is important. For example, what is one month? 28-days?
29? 30? 31? Every system must make a judgment call. Add a month to January
31 and you will get February 28. But add/subtract a month from February 28
and you get January 28/March 28. So you can create a query that takes a
date, adds a month and subtracts a month and results in a different date.
There is nothing to do here but to read the docs and try things.

There are similar issues when crossing DST boundaries. If I want to push
something out a day in my time-zone on the day that DST changes I can do it
easily and understand that PostgreSQL will handle the extra/missing hour. Or
I can use an explicit increment of '24 hours' if that is what I want. No
extra steps of converting the timestamp without time zone to UTC, converting
that to the desired local zone, doing the calculations, converting back to
UTC and back to timezone without timestamp all the while potentially adding
an easy error such as doing things in the wrong order and checking for DST
changeover in the wrong time-zone.

Cheers,
Steve

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On May 21, 2015 11:56:52 AM Steve Crawford wrote:

The article does also display a couple attitudes that I feel are especially
rampant in the web-development community. The first is that web developers
shouldn't become educated about the capabilities of a database but rather
use the database as a dumb data-store and redo everything themselves (often
this includes an utter failure to use the data-integrity capabilities of
the database).

Having been at the receiving end of web developer rants many times, the reason
more often than not is that the database does the checking after the fact, i.e.
after the user spend the time providing the data. Web developers need to know
what's allowed when they throw up the page. And frameworks offer little or no
help in retrieving these validation rules. So the web developer is almost forced to
roll his own.

Hi Steve,

Thanks for such a thorough response! I agree that time is a lot trickier
and trappier than one might expect, so it's good to learn how others
grapple with it.

Your original question had to do with month/year.

Just to clarify, that was Daniel's original question, but you're
replying to my follow-up question.

The first is that
web developers shouldn't become educated about the capabilities of a
database but rather use the database as a dumb data-store and redo
everything themselves (often this includes an utter failure to use the
data-integrity capabilities of the database).

That's not a debate I can hope to settle, but for what it's worth, I
mostly agree with you. That's why I've written these tools to let Rails
users leverage more of the capabilities inside Postgres, especially
integrity constraints:

https://github.com/pjungwir/db_leftovers
https://github.com/pjungwir/aggs_for_arrays/

also these efforts at education:

https://github.com/pjungwir/rails-and-sql-talk
http://illuminatedcomputing.com/posts/2015/02/postgres_lateral_join/
http://illuminatedcomputing.com/posts/2015/03/generate_series_for_time_series/

Anyway, I agree that you have to store the time zone *somewhere*, and I
suppose that's the reason Joshua remarked that you really shouldn't use
WITHOUT TIME ZONE. And often a time has one perspective that is
"canonical" or "preferred", e.g. the time zone of the user who created
the object. And in that case WITH TIME ZONE gives you a convenient place
to store that. I think I still prefer a more "relativistic" approach
where times have no preferred perspective, and input strings are
converted to a bare "instant" as quickly as possible (using whatever
time zone is appropriate). For instance that avoids the failure scenario
Brian described. I concede that storing the time zone separately as a
string makes it tricker for other database clients, at least when the
string is a name only meaningful to Rails. In the future I'll keep an
eye out for when WITH might be handy. And maybe I'll do some research to
see how well Rails would handle those columns.

Thanks again for your generosity!

Yours,
Paul

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On Thu, May 21, 2015 at 2:10 PM, Paul Jungwirth <pj@illuminatedcomputing.com

wrote:

Anyway, I agree that you have to store the time zone *somewhere*, and I
suppose that's the reason Joshua remarked that you really shouldn't use
WITHOUT TIME ZONE. And often a time has one perspective that is "canonical"
or "preferred", e.g. the time zone of the user who created the object. And
in that case WITH TIME ZONE gives you a convenient place to store that. I
think I still prefer a more "relativistic" approach where times have no
preferred perspective, and input strings are converted to a bare "instant"
as quickly as possible (using whatever time zone is appropriate). For
instance that avoids the failure scenario Brian described. I concede that
storing the time zone separately as a string makes it tricker for other
database clients, at least when the string is a name only meaningful to
Rails. In the future I'll keep an eye out for when WITH might be handy. And
maybe I'll do some research to see how well Rails would handle those
columns.

​I'm not sure Brian is correct - but my head started to hurt when I
attempted to reason it out - but what you've said above is incorrect. All
"WITH TIME ZONE" does is tell PostgreSQL to apply timezone conversions
during various operations. The stored data is represented as an epoch
without any concept of the source data's timezone representation. i.e. if
I store '2015-05-20T15:23:00-MST'::timestamptz into a table and later
retrieve it I have no way to knowing that MST was part of the original
specification.

David J.

Brian Dunavant wrote on 21.05.2015 21:51:

It's probably worth noting that both the Ruby 'best practice' AND
Postgres have a failure case when dealing with future dates precisely
because they are storing the data as UTC with a time zone. This is
one case where storing the data WITHOUT TIME ZONE would actually save
your bacon.

Postgres does not store the time zone. When storing a timestamp with time zone, it
is normalized to UTC based on the timezone of the client. When you retrieve it,
it is adjusted to the time zone of the client.

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Anyway, I agree that you have to store the time zone *somewhere*, and I
suppose that's the reason Joshua remarked that you really shouldn't use
WITHOUT TIME ZONE. And often a time has one perspective that is
"canonical" or "preferred", e.g. the time zone of the user who created
the object. And in that case WITH TIME ZONE gives you a convenient place
to store that.

No it doesn't.

WITH TIME ZONE simply means that whatever the user sends to
the database is meant to represent that time zone.

As far as PostgreSQL is concerned it will convert that
to UTC, store UTC

AND THROW AWAY THE TIMEZONE INFORMATION

While that's correct in mathematical terms (the stored instant-in-time
is *known* to be "good" at UTC) it does throw away information, namely
the storing-client-timezone data. If you want to retain that (rather
than convert UTC to the retrieving client's timezone) you'll have to
store that yourself in an extra field.

I think I still prefer a more "relativistic" approach
where times have no preferred perspective,

That's meaningless. Time has, by its very definition, a perspective. It's
just that for "canonical" times people usually agree on storing the
perspective "UTC". Other than that it would be akin to a byte string
without encoding -- you can't tell what it means.

Karsten

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what you've said above is incorrect.
All "WITH TIME ZONE" does is tell PostgreSQL to apply timezone
conversions during various operations. The stored data is represented
as an epoch without any concept of the source data's timezone
representation.

Oh, very interesting! Thank you for pointing that out. I'll have to
think some more about when I'd want that behavior.

Paul

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On Thu, May 21, 2015 at 5:27 PM, Thomas Kellerer <spam_eater@gmx.net> wrote:

Postgres does not store the time zone. When storing a timestamp with time
zone, it
is normalized to UTC based on the timezone of the client. When you retrieve
it,
it is adjusted to the time zone of the client.

Sorry, I misspoke. Thank you for correcting it. It is storing it as
UTC time zone. The rest of my post still applies. You will get the
wrong wall-clock time for the future date because it is stored as UTC
and the conversion rules will have changed giving you a different time
when you convert it back to the local time zone.

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It's probably worth noting that both the Ruby 'best practice' AND
Postgres have a failure case when dealing with future dates precisely
because they are storing the data as UTC with a time zone. This is
one case where storing the data WITHOUT TIME ZONE would actually save
your bacon.

From the postgres docs: "For times in the future, the assumption is
that the latest known rules for a given time zone will continue to be
observed indefinitely far into the future."

Imagine scheduling a meeting for a certain time a few years from now.
This will be stored as UTC + time zone. A year later, that
government decides to change the time zone rules for their country.
Your operating system will get the new timezone data in an update (as
it should). However when the meeting comes around, you're going to be
early/late because the wall time that you get converting back from
UTC+time zone is no longer the time that you were supposed to have
been at the meeting. If you had stored that future date as a
timestamp WITHOUT time zone you would have still been on-time.

This is only an issue for future dates, not past ones.

-Brian Dunavant
(time is hard, so if I'm wrong anywhere here, someone please correct me)

You are wrong (or me, but I'll try).

Imagine scheduling a meeting for a certain time a few years from now.
This will be stored as UTC + time zone.

No it won't. It will store as UTC but will not store any timezone information
(apart from the fact that it "knows" that what is stored on disk is converted
to UTC from what the client sent in for storage).

You are right in the following aspect:

- client sends in "NOW at HERE"
- server knows HERE = UTC+2
- hence NOW_UTC = NOW - 2
- server stores "NOW_UTC"
- 2 years pass
- government at HERE says that from today on "HERE = UTC + 4
- 2 years pass
- client retrieves at HERE
- server knows HERE = UTC + 4
- server also knows that HERE used to mean UTC + 2
- but server can not derive what HERE meant when NOW was stored ...
- server converts stored NOW_UTC to HERE by doing NOW_UTC + 4
- client receives NOW_HERE but this became NOW - 2 + 4

IOW, the server would need to know what HERE meant when "now"
was stored. This can only be solved by tracking insertion/update
timestamps.

Karsten

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On 05/21/2015 11:02 AM, Daniel Torres wrote:

Sorry, forgot to told you what I'm trying, I have climate data and want
to obtain mean temperature and total precipitation and that sort of
things per month and year. Think date_trunc is a good solution, but any
other advice would be very welcome.

As it turns out I am working on something similar with regards to school
days. A quick and dirty query:

SELECT
extract (
YEAR
FROM
school_day ) AS YEAR,
extract (
MONTH
FROM
school_day ) AS MONTH,
count (
school_day )
FROM
school_calendar
GROUP BY
extract (
YEAR
FROM
school_day ),
extract (
MONTH
FROM
school_day )
ORDER BY
extract (
YEAR
FROM
school_day ),
extract (
MONTH
FROM
school_day );

Reformatting courtesy of pgFormatter(http://sqlformat.darold.net/).

Results:

year | month | count
------+-------+-------
2005 | 3 | 7
2005 | 4 | 12
2005 | 5 | 17
2005 | 6 | 14
2005 | 7 | 11
2005 | 8 | 15
2005 | 9 | 16
2005 | 10 | 15
2005 | 11 | 17
2005 | 12 | 10
2006 | 1 | 15
2006 | 2 | 12
2006 | 3 | 18
2006 | 4 | 12
2006 | 5 | 18
2006 | 6 | 13
2006 | 7 | 11
2006 | 8 | 15
2006 | 9 | 15
2006 | 10 | 18
2006 | 11 | 13
2006 | 12 | 10

(I need to read more about time zones, I'm new at using postgresql)

Thank you,
Daniel

2015-05-21 12:45 GMT-05:00 Paul Jungwirth <pj@illuminatedcomputing.com
<mailto:pj@illuminatedcomputing.com>>:

You really shouldn't use WITHOUT TIME ZONE.

I'd like to know more about this. Can you say why? Are there any
articles you'd recommend? I'm fond of normalizing all times to UTC
and only presenting them in a time zone when I know the current
"perspective". I've written about that approach in a Rails context here:

http://illuminatedcomputing.com/posts/2014/04/timezones/

I find that this helps me to ignore time zones in most parts of my
application and cut down on my timezone-related bugs.

Thanks!

Paul

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adrian.klaver@aklaver.com

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On 05/21/2015 11:56 AM, Steve Crawford wrote:

On 05/21/2015 10:45 AM, Paul Jungwirth wrote:

You really shouldn't use WITHOUT TIME ZONE.

I'd like to know more about this. Can you say why?

Start by reading about the date and time data types with special
attention to section 8.5.3:
www.postgresql.org/docs/current/static/datatype-datetime.html

Now go back and read it again and experiment a while until it makes
sense. As Adrian Klaver so eloquently put it, "If I have learned
anything about dealing with dates and times, is that it is a set of
exceptions bound together by a few rules. Every time you think you have
the little rascals cornered, one gets away." This is also a very good
reason to avoid reinventing the wheel.

The check is in the mail:)

Cheers,
Steve

--
Adrian Klaver
adrian.klaver@aklaver.com

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