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docs: improve AT TIME ZONE description

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The previous description was unclear.  Also add a third example, change
use of time zone acronyms to more verbose descriptions, and add a
mention that using 'time' with AT TIME ZONE uses the current time zone
rules.

Backpatch-through: 9.3
This commit is contained in:
Bruce Momjian 2018-09-04 22:34:07 -04:00
parent fb466d7b5d
commit 838fd62cab
1 changed files with 21 additions and 11 deletions
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doc/src/sgml/func.sgml
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@ -8082,10 +8082,11 @@ SELECT date_trunc('year', TIMESTAMP '2001-02-16 20:38:40');
</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.
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">
@ -8130,24 +8131,33 @@ SELECT date_trunc('year', TIMESTAMP '2001-02-16 20:38:40');
<para>
In these expressions, the desired time zone <replaceable>zone</replaceable> can be
specified either as a text string (e.g., <literal>'PST'</literal>)
specified either as a text string (e.g., <literal>'America/Los_Angeles'</literal>)
or as an interval (e.g., <literal>INTERVAL '-08:00'</literal>).
In the text case, a time zone name can be specified in any of the ways
described in <xref linkend="datatype-timezones"/>.
</para>
<para>
Examples (assuming the local time zone is <literal>PST8PDT</literal>):
Examples (assuming the local time zone is <literal>America/Los_Angeles</literal>):
<screen>
SELECT TIMESTAMP '2001-02-16 20:38:40' AT TIME ZONE 'MST';
SELECT TIMESTAMP '2001-02-16 20:38:40' AT TIME ZONE 'America/Denver';
<lineannotation>Result: </lineannotation><computeroutput>2001-02-16 19:38:40-08</computeroutput>
SELECT TIMESTAMP WITH TIME ZONE '2001-02-16 20:38:40-05' AT TIME ZONE 'MST';
SELECT TIMESTAMP WITH TIME ZONE '2001-02-16 20:38:40-05' AT TIME ZONE 'America/Denver';
<lineannotation>Result: </lineannotation><computeroutput>2001-02-16 18:38:40</computeroutput>
SELECT TIMESTAMP '2001-02-16 20:38:40-05' AT TIME ZONE 'Asia/Tokyo' AT TIME ZONE 'America/Chicago';
<lineannotation>Result: </lineannotation><computeroutput>2001-02-16 05:38:40</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).
The first example adds a time zone to a value that lacks it, and
displays the value using the current <varname>TimeZone</varname>
setting. 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> setting. The third example converts
Tokyo time to Chicago time. Converting <emphasis>time</emphasis>
values to other time zones uses the currently active time zone rules
since no date is supplied.
</para>
<para>