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Add a timezone-specific variant of date_trunc().

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date_trunc(field, timestamptz, zone_name) performs truncation using
the named time zone as reference, rather than working in the session
time zone as is the default behavior.  It's equivalent to

date_trunc(field, timestamptz at time zone zone_name) at time zone zone_name

but it's faster, easier to type, and arguably easier to understand.

Vik Fearing and Tom Lane

Discussion: https://postgr.es/m/6249ffc4-2b22-4c1b-4e7d-7af84fedd7c6@2ndquadrant.com
This commit is contained in:
Tom Lane
2018-11-14 15:41:07 -05:00
parent 06c723447b
commit 600b04d6b5
6 changed files with 166 additions and 27 deletions
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53
doc/src/sgml/func.sgml
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@@ -7179,16 +7179,25 @@ SELECT regexp_match('abc01234xyz', '(?:(.*?)(\d+)(.*)){1,1}');
<literal><function>date_trunc(<type>text</type>, <type>timestamp</type>)</function></literal>
</entry>
<entry><type>timestamp</type></entry>
<entry>Truncate to specified precision; see also <xref linkend="functions-datetime-trunc"/>
<entry>Truncate to specified precision; see <xref linkend="functions-datetime-trunc"/>
</entry>
<entry><literal>date_trunc('hour', timestamp '2001-02-16 20:38:40')</literal></entry>
<entry><literal>2001-02-16 20:00:00</literal></entry>
</row>
<row>
<entry><literal><function>date_trunc(<type>text</type>, <type>timestamp with time zone</type>, <type>text</type>)</function></literal></entry>
<entry><type>timestamp with time zone</type></entry>
<entry>Truncate to specified precision in the specified time zone; see <xref linkend="functions-datetime-trunc"/>
</entry>
<entry><literal>date_trunc('day', timestamptz '2001-02-16 20:38:40+00', 'Australia/Sydney')</literal></entry>
<entry><literal>2001-02-16 13:00:00+00</literal></entry>
</row>
<row>
<entry><literal><function>date_trunc(<type>text</type>, <type>interval</type>)</function></literal></entry>
<entry><type>interval</type></entry>
<entry>Truncate to specified precision; see also <xref linkend="functions-datetime-trunc"/>
<entry>Truncate to specified precision; see <xref linkend="functions-datetime-trunc"/>
</entry>
<entry><literal>date_trunc('hour', interval '2 days 3 hours 40 minutes')</literal></entry>
<entry><literal>2 days 03:00:00</literal></entry>
@@ -8078,17 +8087,19 @@ SELECT date_part('hour', INTERVAL '4 hours 3 minutes');
<para>
<synopsis>
date_trunc('<replaceable>field</replaceable>', <replaceable>source</replaceable>)
date_trunc(<replaceable>field</replaceable>, <replaceable>source</replaceable> [, <replaceable>time_zone</replaceable> ])
</synopsis>
<replaceable>source</replaceable> is a value expression of type
<type>timestamp</type> or <type>interval</type>.
<type>timestamp</type>, <type>timestamp with time zone</type>,
or <type>interval</type>.
(Values of type <type>date</type> and
<type>time</type> are cast automatically to <type>timestamp</type> or
<type>interval</type>, respectively.)
<replaceable>field</replaceable> selects to which precision to
truncate the input value. The return value is of type
<type>timestamp</type> or <type>interval</type>
with all fields that are less significant than the
truncate the input value. The return value is likewise of type
<type>timestamp</type>, <type>timestamp with time zone</type>,
or <type>interval</type>,
and it has all fields that are less significant than the
selected one set to zero (or one, for day and month).
</para>
@@ -8112,13 +8123,39 @@ date_trunc('<replaceable>field</replaceable>', <replaceable>source</replaceable>
</para>
<para>
Examples:
When the input value is of type <type>timestamp with time zone</type>,
the truncation is performed with respect to a particular time zone;
for example, truncation to <literal>day</literal> produces a value that
is midnight in that zone. By default, truncation is done with respect
to the current <xref linkend="guc-timezone"/> setting, but the
optional <replaceable>time_zone</replaceable> argument can be provided
to specify a different time zone. The time zone name can be specified
in any of the ways described in <xref linkend="datatype-timezones"/>.
</para>
<para>
A time zone cannot be specified when processing <type>timestamp without
time zone</type> or <type>interval</type> inputs. These are always
taken at face value.
</para>
<para>
Examples (assuming the local time zone is <literal>America/New_York</literal>):
<screen>
SELECT date_trunc('hour', TIMESTAMP '2001-02-16 20:38:40');
<lineannotation>Result: </lineannotation><computeroutput>2001-02-16 20:00:00</computeroutput>
SELECT date_trunc('year', TIMESTAMP '2001-02-16 20:38:40');
<lineannotation>Result: </lineannotation><computeroutput>2001-01-01 00:00:00</computeroutput>
SELECT date_trunc('day', TIMESTAMP WITH TIME ZONE '2001-02-16 20:38:40+00');
<lineannotation>Result: </lineannotation><computeroutput>2001-02-16 00:00:00-05</computeroutput>
SELECT date_trunc('day', TIMESTAMP WITH TIME ZONE '2001-02-16 20:38:40+00', 'Australia/Sydney');
<lineannotation>Result: </lineannotation><computeroutput>2001-02-16 08:00:00-05</computeroutput>
SELECT date_trunc('hour', INTERVAL '3 days 02:47:33');
<lineannotation>Result: </lineannotation><computeroutput>3 days 02:00:00</computeroutput>
</screen>
</para>
</sect2>