fOn Sun, Sep 2, 2018 at 02:21:58PM -0400, Tom Lane wrote:
> Bruce Momjian <br...@momjian.us> writes:
> > ! The <literal>AT TIME ZONE</literal> construct allows the addition,
> > ! conversion, and removal of time zones for time stamp values. <xref
> > linkend="functions-datetime-zoneconvert-table"/> shows its
> > variants.
>
> Maybe it'd be more to the point to say that it allows conversion between
> "timestamp with time zone" and "timestamp without time zone".
>
> > ! The first example takes a time stamp without time zone and interprets
> > ! it in the MST time zone (UTC-7), returning a time stamp with time
> > ! zone value which is displayed in local time (PST, UTC-8). The second
> > ! example takes a time stamp with time zone value (EST, UTC-5) and
> > ! converts it to the date and time in MST (UTC-7) without time zone.
> > ! Basically, the first example takes the date and time and puts it in
> > ! the specified time zone. The second example takes a time stamp with
> > ! time zone and shifts it to the specified time zone. (No time zone
> > ! designation is returned.)
>
> I still find this to be more confusing than helpful. In particular,
> I do not think that it's possible to explain this behavior clearly
> without mentioning that timestamp with time zone values are always
> stored in UTC and what AT TIME ZONE really does is convert between UTC
> and the specified zone (in a direction dependent on which type is
> supplied as argument).
Agreed. The more I dig into this the more I learn. I have developed
the attached patch which I hope this time is an improvement.
Is there any value to showing these two queries which show how calling
AT TIME ZONE twice cancels itself out:
SELECT '2018-09-02 07:09:19'::timestamp AT TIME ZONE 'America/Chicago'
AT TIME ZONE 'America/Chicago';
timezone
---------------------
2018-09-02 07:09:19
SELECT '2018-09-02 07:09:19-04'::timestamptz AT TIME ZONE
'America/Chicago' AT TIME ZONE 'America/Chicago';
timezone
------------------------
2018-09-02 07:09:19-04
or this one which shows how to convert a date/time from one time zone to
another:
SELECT '2018-09-02 07:09:19'::timestamp AT TIME ZONE 'Asia/Tokyo' AT
TIME ZONE 'America/Chicago';
timezone
---------------------
2018-09-01 17:09:19
--
Bruce Momjian <br...@momjian.us> http://momjian.us
EnterpriseDB http://enterprisedb.com
+ As you are, so once was I. As I am, so you will be. +
+ Ancient Roman grave inscription +
diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
new file mode 100644
index bb794e0..5e3d54f
*** a/doc/src/sgml/func.sgml
--- b/doc/src/sgml/func.sgml
*************** SELECT date_trunc('year', TIMESTAMP '200
*** 8082,8091 ****
</indexterm>
<para>
! The <literal>AT TIME ZONE</literal> construct allows conversions
! of time stamps to different time zones. <xref
! linkend="functions-datetime-zoneconvert-table"/> shows its
! variants.
</para>
<table id="functions-datetime-zoneconvert-table">
--- 8082,8092 ----
</indexterm>
<para>
! The <literal>AT TIME ZONE</literal> converts time
! stamp <emphasis>without time zone</emphasis> to/from
! time stamp <emphasis>with time zone</emphasis>, and
! <emphasis>time</emphasis> values to different time zones. <xref
! linkend="functions-datetime-zoneconvert-table"/> shows its variants.
</para>
<table id="functions-datetime-zoneconvert-table">
*************** SELECT TIMESTAMP '2001-02-16 20:38:40' A
*** 8145,8153 ****
SELECT TIMESTAMP WITH TIME ZONE '2001-02-16 20:38:40-05' AT TIME ZONE 'MST';
<lineannotation>Result: </lineannotation><computeroutput>2001-02-16 18:38:40</computeroutput>
</screen>
! The first example takes a time stamp without time zone and interprets it as MST time
! (UTC-7), which is then converted to PST (UTC-8) for display. The second example takes
! a time stamp specified in EST (UTC-5) and converts it to local time in MST (UTC-7).
</para>
<para>
--- 8146,8158 ----
SELECT TIMESTAMP WITH TIME ZONE '2001-02-16 20:38:40-05' AT TIME ZONE 'MST';
<lineannotation>Result: </lineannotation><computeroutput>2001-02-16 18:38:40</computeroutput>
</screen>
! The first example adds a time zone to a value that lacks it.
! The second example shifts the time stamp with time zone value
! to the specified time zone, and returns the value without a
! time zone. This allows storage and display of values different
! from the current <varname>TimeZone</varname> value. Converting
! <emphasis>time</emphasis> values to other time zones uses the currently
! active time zone rules since no date is present.
</para>
<para>
