Add a timezone-specific variant of date_trunc().

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

doc/src/sgml/func.sgml

@@ -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>

src/backend/utils/adt/timestamp.c

@@ -3925,14 +3925,15 @@ timestamp_trunc(PG_FUNCTION_ARGS)
 	PG_RETURN_TIMESTAMP(result);
 }
 
-/* timestamptz_trunc()
- * Truncate timestamp to specified units.
+/*
+ * Common code for timestamptz_trunc() and timestamptz_trunc_zone().
+ *
+ * tzp identifies the zone to truncate with respect to.  We assume
+ * infinite timestamps have already been rejected.
  */
-Datum
-timestamptz_trunc(PG_FUNCTION_ARGS)
+static TimestampTz
+timestamptz_trunc_internal(text *units, TimestampTz timestamp, pg_tz *tzp)
 {
-	text	   *units = PG_GETARG_TEXT_PP(0);
-	TimestampTz timestamp = PG_GETARG_TIMESTAMPTZ(1);
 	TimestampTz result;
 	int			tz;
 	int			type,
@@ -3943,9 +3944,6 @@ timestamptz_trunc(PG_FUNCTION_ARGS)
 	struct pg_tm tt,
 			   *tm = &tt;
 
-	if (TIMESTAMP_NOT_FINITE(timestamp))
-		PG_RETURN_TIMESTAMPTZ(timestamp);
-
 	lowunits = downcase_truncate_identifier(VARDATA_ANY(units),
 											VARSIZE_ANY_EXHDR(units),
 											false);
@@ -3954,7 +3952,7 @@ timestamptz_trunc(PG_FUNCTION_ARGS)
 
 	if (type == UNITS)
 	{
-		if (timestamp2tm(timestamp, &tz, tm, &fsec, NULL, NULL) != 0)
+		if (timestamp2tm(timestamp, &tz, tm, &fsec, NULL, tzp) != 0)
 			ereport(ERROR,
 					(errcode(ERRCODE_DATETIME_VALUE_OUT_OF_RANGE),
 					 errmsg("timestamp out of range")));
@@ -4055,7 +4053,7 @@ timestamptz_trunc(PG_FUNCTION_ARGS)
 		}
 
 		if (redotz)
-			tz = DetermineTimeZoneOffset(tm, session_timezone);
+			tz = DetermineTimeZoneOffset(tm, tzp);
 
 		if (tm2timestamp(tm, fsec, &tz, &result) != 0)
 			ereport(ERROR,
@@ -4071,6 +4069,83 @@ timestamptz_trunc(PG_FUNCTION_ARGS)
 		result = 0;
 	}
 
+	return result;
+}
+
+/* timestamptz_trunc()
+ * Truncate timestamptz to specified units in session timezone.
+ */
+Datum
+timestamptz_trunc(PG_FUNCTION_ARGS)
+{
+	text	   *units = PG_GETARG_TEXT_PP(0);
+	TimestampTz timestamp = PG_GETARG_TIMESTAMPTZ(1);
+	TimestampTz result;
+
+	if (TIMESTAMP_NOT_FINITE(timestamp))
+		PG_RETURN_TIMESTAMPTZ(timestamp);
+
+	result = timestamptz_trunc_internal(units, timestamp, session_timezone);
+
+	PG_RETURN_TIMESTAMPTZ(result);
+}
+
+/* timestamptz_trunc_zone()
+ * Truncate timestamptz to specified units in specified timezone.
+ */
+Datum
+timestamptz_trunc_zone(PG_FUNCTION_ARGS)
+{
+	text	   *units = PG_GETARG_TEXT_PP(0);
+	TimestampTz timestamp = PG_GETARG_TIMESTAMPTZ(1);
+	text	   *zone = PG_GETARG_TEXT_PP(2);
+	TimestampTz result;
+	char		tzname[TZ_STRLEN_MAX + 1];
+	char	   *lowzone;
+	int			type,
+				val;
+	pg_tz	   *tzp;
+
+	/*
+	 * timestamptz_zone() doesn't look up the zone for infinite inputs, so we
+	 * don't do so here either.
+	 */
+	if (TIMESTAMP_NOT_FINITE(timestamp))
+		PG_RETURN_TIMESTAMP(timestamp);
+
+	/*
+	 * Look up the requested timezone (see notes in timestamptz_zone()).
+	 */
+	text_to_cstring_buffer(zone, tzname, sizeof(tzname));
+
+	/* DecodeTimezoneAbbrev requires lowercase input */
+	lowzone = downcase_truncate_identifier(tzname,
+										   strlen(tzname),
+										   false);
+
+	type = DecodeTimezoneAbbrev(0, lowzone, &val, &tzp);
+
+	if (type == TZ || type == DTZ)
+	{
+		/* fixed-offset abbreviation, get a pg_tz descriptor for that */
+		tzp = pg_tzset_offset(-val);
+	}
+	else if (type == DYNTZ)
+	{
+		/* dynamic-offset abbreviation, use its referenced timezone */
+	}
+	else
+	{
+		/* try it as a full zone name */
+		tzp = pg_tzset(tzname);
+		if (!tzp)
+			ereport(ERROR,
+					(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+					 errmsg("time zone \"%s\" not recognized", tzname)));
+	}
+
+	result = timestamptz_trunc_internal(units, timestamp, tzp);
+
 	PG_RETURN_TIMESTAMPTZ(result);
 }
 

src/include/catalog/catversion.h

@@ -53,6 +53,6 @@
  */
 
 /*							yyyymmddN */
-#define CATALOG_VERSION_NO	201811061
+#define CATALOG_VERSION_NO	201811141
 
 #endif

src/include/catalog/pg_proc.dat

@@ -2280,6 +2280,10 @@
   descr => 'truncate timestamp with time zone to specified units',
   proname => 'date_trunc', provolatile => 's', prorettype => 'timestamptz',
   proargtypes => 'text timestamptz', prosrc => 'timestamptz_trunc' },
+{ oid => '1284',
+  descr => 'truncate timestamp with time zone to specified units in specified time zone',
+  proname => 'date_trunc', provolatile => 's', prorettype => 'timestamptz',
+  proargtypes => 'text timestamptz text', prosrc => 'timestamptz_trunc_zone' },
 { oid => '1218', descr => 'truncate interval to specified units',
   proname => 'date_trunc', prorettype => 'interval',
   proargtypes => 'text interval', prosrc => 'interval_trunc' },
@@ -5825,8 +5829,8 @@
   prorettype => 'timestamptz', proargtypes => '',
   prosrc => 'pg_backup_start_time' },
 { oid => '3436', descr => 'promote standby server',
-  proname => 'pg_promote', provolatile => 'v',
-  prorettype => 'bool', proargtypes => 'bool int4', proargnames => '{wait,wait_seconds}',
+  proname => 'pg_promote', provolatile => 'v', prorettype => 'bool',
+  proargtypes => 'bool int4', proargnames => '{wait,wait_seconds}',
   prosrc => 'pg_promote' },
 { oid => '2848', descr => 'switch to new wal file',
   proname => 'pg_switch_wal', provolatile => 'v', prorettype => 'pg_lsn',
@@ -10007,10 +10011,11 @@
   proallargtypes => '{text,int8,timestamptz}', proargmodes => '{o,o,o}',
   proargnames => '{name,size,modification}', prosrc => 'pg_ls_waldir' },
 { oid => '5031', descr => 'list of files in the archive_status directory',
-  proname => 'pg_ls_archive_statusdir', procost => '10', prorows => '20', proretset => 't',
-  provolatile => 'v', prorettype => 'record', proargtypes => '',
-  proallargtypes => '{text,int8,timestamptz}', proargmodes => '{o,o,o}',
-  proargnames => '{name,size,modification}', prosrc => 'pg_ls_archive_statusdir' },
+  proname => 'pg_ls_archive_statusdir', procost => '10', prorows => '20',
+  proretset => 't', provolatile => 'v', prorettype => 'record',
+  proargtypes => '', proallargtypes => '{text,int8,timestamptz}',
+  proargmodes => '{o,o,o}', proargnames => '{name,size,modification}',
+  prosrc => 'pg_ls_archive_statusdir' },
 { oid => '5029', descr => 'list files in the pgsql_tmp directory',
   proname => 'pg_ls_tmpdir', procost => '10', prorows => '20', proretset => 't',
   provolatile => 'v', prorettype => 'record', proargtypes => '',
@@ -10036,6 +10041,6 @@
   proallargtypes => '{regclass,regclass,regclass,bool,int4}',
   proargmodes => '{i,o,o,o,o}',
   proargnames => '{rootrelid,relid,parentrelid,isleaf,level}',
-  prosrc => 'pg_partition_tree' }
+  prosrc => 'pg_partition_tree' },
 
 ]

src/test/regress/expected/timestamptz.out

@@ -649,6 +649,24 @@ SELECT '' AS date_trunc_week, date_trunc( 'week', timestamp with time zone '2004
                  | Mon Feb 23 00:00:00 2004 PST
 (1 row)
 
+SELECT '' AS date_trunc_at_tz, date_trunc('day', timestamp with time zone '2001-02-16 20:38:40+00', 'Australia/Sydney') as sydney_trunc;  -- zone name
+ date_trunc_at_tz |         sydney_trunc         
+------------------+------------------------------
+                  | Fri Feb 16 05:00:00 2001 PST
+(1 row)
+
+SELECT '' AS date_trunc_at_tz, date_trunc('day', timestamp with time zone '2001-02-16 20:38:40+00', 'GMT') as gmt_trunc;  -- fixed-offset abbreviation
+ date_trunc_at_tz |          gmt_trunc           
+------------------+------------------------------
+                  | Thu Feb 15 16:00:00 2001 PST
+(1 row)
+
+SELECT '' AS date_trunc_at_tz, date_trunc('day', timestamp with time zone '2001-02-16 20:38:40+00', 'VET') as vet_trunc;  -- variable-offset abbreviation
+ date_trunc_at_tz |          vet_trunc           
+------------------+------------------------------
+                  | Thu Feb 15 20:00:00 2001 PST
+(1 row)
+
 -- Test casting within a BETWEEN qualifier
 SELECT '' AS "54", d1 - timestamp with time zone '1997-01-02' AS diff
   FROM TIMESTAMPTZ_TBL

src/test/regress/sql/timestamptz.sql

@@ -193,6 +193,10 @@ SELECT '' AS "54", d1 - timestamp with time zone '1997-01-02' AS diff
 
 SELECT '' AS date_trunc_week, date_trunc( 'week', timestamp with time zone '2004-02-29 15:44:17.71393' ) AS week_trunc;
 
+SELECT '' AS date_trunc_at_tz, date_trunc('day', timestamp with time zone '2001-02-16 20:38:40+00', 'Australia/Sydney') as sydney_trunc;  -- zone name
+SELECT '' AS date_trunc_at_tz, date_trunc('day', timestamp with time zone '2001-02-16 20:38:40+00', 'GMT') as gmt_trunc;  -- fixed-offset abbreviation
+SELECT '' AS date_trunc_at_tz, date_trunc('day', timestamp with time zone '2001-02-16 20:38:40+00', 'VET') as vet_trunc;  -- variable-offset abbreviation
+
 -- Test casting within a BETWEEN qualifier
 SELECT '' AS "54", d1 - timestamp with time zone '1997-01-02' AS diff
   FROM TIMESTAMPTZ_TBL