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docs: improve AT TIME ZONE description
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
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@ -7889,10 +7889,11 @@ SELECT date_trunc('year', TIMESTAMP '2001-02-16 20:38:40');
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</indexterm>
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<para>
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The <literal>AT TIME ZONE</literal> construct allows conversions
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of time stamps to different time zones. <xref
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linkend="functions-datetime-zoneconvert-table"> shows its
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variants.
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The <literal>AT TIME ZONE</literal> converts time
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stamp <emphasis>without time zone</emphasis> to/from
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time stamp <emphasis>with time zone</emphasis>, and
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<emphasis>time</emphasis> values to different time zones. <xref
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linkend="functions-datetime-zoneconvert-table"> shows its variants.
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</para>
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<table id="functions-datetime-zoneconvert-table">
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@ -7937,24 +7938,33 @@ SELECT date_trunc('year', TIMESTAMP '2001-02-16 20:38:40');
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<para>
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In these expressions, the desired time zone <replaceable>zone</> can be
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specified either as a text string (e.g., <literal>'PST'</literal>)
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specified either as a text string (e.g., <literal>'America/Los_Angeles'</literal>)
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or as an interval (e.g., <literal>INTERVAL '-08:00'</literal>).
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In the text case, a time zone name can be specified in any of the ways
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described in <xref linkend="datatype-timezones">.
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</para>
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<para>
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Examples (assuming the local time zone is <literal>PST8PDT</>):
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Examples (assuming the local time zone is <literal>America/Los_Angeles</literal>):
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<screen>
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SELECT TIMESTAMP '2001-02-16 20:38:40' AT TIME ZONE 'MST';
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SELECT TIMESTAMP '2001-02-16 20:38:40' AT TIME ZONE 'America/Denver';
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<lineannotation>Result: </lineannotation><computeroutput>2001-02-16 19:38:40-08</computeroutput>
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SELECT TIMESTAMP WITH TIME ZONE '2001-02-16 20:38:40-05' AT TIME ZONE 'MST';
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SELECT TIMESTAMP WITH TIME ZONE '2001-02-16 20:38:40-05' AT TIME ZONE 'America/Denver';
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<lineannotation>Result: </lineannotation><computeroutput>2001-02-16 18:38:40</computeroutput>
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SELECT TIMESTAMP '2001-02-16 20:38:40-05' AT TIME ZONE 'Asia/Tokyo' AT TIME ZONE 'America/Chicago';
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<lineannotation>Result: </lineannotation><computeroutput>2001-02-16 05:38:40</computeroutput>
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</screen>
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The first example takes a time stamp without time zone and interprets it as MST time
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(UTC-7), which is then converted to PST (UTC-8) for display. The second example takes
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a time stamp specified in EST (UTC-5) and converts it to local time in MST (UTC-7).
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The first example adds a time zone to a value that lacks it, and
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displays the value using the current <varname>TimeZone</varname>
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setting. The second example shifts the time stamp with time zone value
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to the specified time zone, and returns the value without a time zone.
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This allows storage and display of values different from the current
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<varname>TimeZone</varname> setting. The third example converts
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Tokyo time to Chicago time. Converting <emphasis>time</emphasis>
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values to other time zones uses the currently active time zone rules
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since no date is supplied.
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</para>
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<para>
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