Add support for AT LOCAL

When converting a timestamp to/from with/without time zone, the SQL
Standard specifies an AT LOCAL variant of AT TIME ZONE which uses the
session's time zone.  This includes three system functions able to do
the work in the same way as the existing flavors for AT TIME ZONE,
except that these need to be marked as stable as they depend on the
session's TimeZone GUC.

Bump catalog version.

Author: Vik Fearing
Reviewed-by: Laurenz Albe, Cary Huang, Michael Paquier
Discussion: https://postgr.es/m/8e25dec4-5667-c1a5-6581-167d710c2182@postgresfriends.org

1 files changed, 94 insertions, 3 deletions

diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
index 0769824e46b..affd1254bb7 100644
--- a/doc/src/sgml/func.sgml
+++ b/doc/src/sgml/func.sgml
@@ -10611,7 +10611,7 @@ SELECT date_bin('15 minutes', TIMESTAMP '2020-02-11 15:44:17', TIMESTAMP '2001-0
   </sect2>
 
   <sect2 id="functions-datetime-zoneconvert">
-   <title><literal>AT TIME ZONE</literal></title>
+   <title><literal>AT TIME ZONE and AT LOCAL</literal></title>
 
    <indexterm>
     <primary>time zone</primary>
@@ -10622,6 +10622,10 @@ SELECT date_bin('15 minutes', TIMESTAMP '2020-02-11 15:44:17', TIMESTAMP '2001-0
     <primary>AT TIME ZONE</primary>
    </indexterm>
 
+   <indexterm>
+    <primary>AT LOCAL</primary>
+   </indexterm>
+
    <para>
     The <literal>AT TIME ZONE</literal> operator converts time
     stamp <emphasis>without</emphasis> time zone to/from
@@ -10632,7 +10636,7 @@ SELECT date_bin('15 minutes', TIMESTAMP '2020-02-11 15:44:17', TIMESTAMP '2001-0
    </para>
 
     <table id="functions-datetime-zoneconvert-table">
-     <title><literal>AT TIME ZONE</literal> Variants</title>
+     <title><literal>AT TIME ZONE</literal> and <literal>AT LOCAL</literal> Variants</title>
      <tgroup cols="1">
       <thead>
        <row>
@@ -10667,6 +10671,22 @@ SELECT date_bin('15 minutes', TIMESTAMP '2020-02-11 15:44:17', TIMESTAMP '2001-0
 
        <row>
         <entry role="func_table_entry"><para role="func_signature">
+         <type>timestamp without time zone</type> <literal>AT LOCAL</literal>
+         <returnvalue>timestamp with time zone</returnvalue>
+        </para>
+        <para>
+         Converts given time stamp <emphasis>without</emphasis> time zone to
+         time stamp <emphasis>with</emphasis> the session's
+         <varname>TimeZone</varname> value as time zone.
+        </para>
+        <para>
+         <literal>timestamp '2001-02-16 20:38:40' at local</literal>
+         <returnvalue>2001-02-17 03:38:40+00</returnvalue>
+        </para></entry>
+       </row>
+
+       <row>
+        <entry role="func_table_entry"><para role="func_signature">
          <type>timestamp with time zone</type> <literal>AT TIME ZONE</literal> <replaceable>zone</replaceable>
          <returnvalue>timestamp without time zone</returnvalue>
         </para>
@@ -10683,6 +10703,22 @@ SELECT date_bin('15 minutes', TIMESTAMP '2020-02-11 15:44:17', TIMESTAMP '2001-0
 
        <row>
         <entry role="func_table_entry"><para role="func_signature">
+         <type>timestamp with time zone</type> <literal>AT LOCAL</literal>
+         <returnvalue>timestamp without time zone</returnvalue>
+        </para>
+        <para>
+         Converts given time stamp <emphasis>with</emphasis> time zone to
+         time stamp <emphasis>without</emphasis> time zone, as the time would
+         appear with the session's <varname>TimeZone</varname> value as time zone.
+        </para>
+        <para>
+         <literal>timestamp with time zone '2001-02-16 20:38:40-05' at local</literal>
+         <returnvalue>2001-02-16 18:38:40</returnvalue>
+        </para></entry>
+       </row>
+
+       <row>
+        <entry role="func_table_entry"><para role="func_signature">
          <type>time with time zone</type> <literal>AT TIME ZONE</literal> <replaceable>zone</replaceable>
          <returnvalue>time with time zone</returnvalue>
         </para>
@@ -10696,6 +10732,25 @@ SELECT date_bin('15 minutes', TIMESTAMP '2020-02-11 15:44:17', TIMESTAMP '2001-0
          <returnvalue>10:34:17+00</returnvalue>
         </para></entry>
        </row>
+
+       <row>
+        <entry role="func_table_entry"><para role="func_signature">
+         <type>time with time zone</type> <literal>AT LOCAL</literal>
+         <returnvalue>time with time zone</returnvalue>
+        </para>
+        <para>
+         Converts given time <emphasis>with</emphasis> time zone to a new time
+         zone.  Since no date is supplied, this uses the currently active UTC
+         offset for the session's <varname>TimeZone</varname> value.
+        </para>
+        <para>
+         Assuming the session's <varname>TimeZone</varname> is set to <literal>UTC</literal>:
+        </para>
+        <para>
+         <literal>time with time zone '05:34:17-05' at local</literal>
+         <returnvalue>10:34:17+00</returnvalue>
+        </para></entry>
+       </row>
       </tbody>
      </tgroup>
     </table>
@@ -10711,6 +10766,13 @@ SELECT date_bin('15 minutes', TIMESTAMP '2020-02-11 15:44:17', TIMESTAMP '2001-0
    </para>
 
    <para>
+    The syntax <literal>AT LOCAL</literal> may be used as shorthand for
+    <literal>AT TIME ZONE <replaceable>local</replaceable></literal>, where
+    <replaceable>local</replaceable> is the session's
+    <varname>TimeZone</varname> value.
+   </para>
+
+   <para>
     Examples (assuming the current <xref linkend="guc-timezone"/> setting
     is <literal>America/Los_Angeles</literal>):
 <screen>
@@ -10722,6 +10784,12 @@ SELECT TIMESTAMP WITH TIME ZONE '2001-02-16 20:38:40-05' AT TIME ZONE 'America/D
 
 SELECT TIMESTAMP '2001-02-16 20:38:40' AT TIME ZONE 'Asia/Tokyo' AT TIME ZONE 'America/Chicago';
 <lineannotation>Result: </lineannotation><computeroutput>2001-02-16 05:38:40</computeroutput>
+
+SELECT TIMESTAMP WITH TIME ZONE '2001-02-16 20:38:40-05' AT LOCAL;
+<lineannotation>Result: </lineannotation><computeroutput>2001-02-16 17:38:40</computeroutput>
+
+SELECT TIME WITH TIME ZONE '20:38:40-05' AT LOCAL;
+<lineannotation>Result: </lineannotation><computeroutput>17:38:40</computeroutput>
 </screen>
     The first example adds a time zone to a value that lacks it, and
     displays the value using the current <varname>TimeZone</varname>
@@ -10729,7 +10797,18 @@ SELECT TIMESTAMP '2001-02-16 20:38:40' AT TIME ZONE 'Asia/Tokyo' AT TIME ZONE 'A
     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.
+    Tokyo time to Chicago time.  The fourth example shifts the time stamp
+    with time zone value to the time zone currently specified by the
+    <varname>TimeZone</varname> setting and returns the value without a
+    time zone.
+   </para>
+
+   <para>
+    The fifth example is a cautionary tale. Due to the fact that there is no
+    date associated with the input value, the conversion is made using the
+    current date of the session. Therefore, this static example may show a wrong
+    result depending on the time of the year it is viewed because
+    <literal>'America/Los_Angeles'</literal> observes Daylight Savings Time.
    </para>
 
    <para>
@@ -10745,6 +10824,18 @@ SELECT TIMESTAMP '2001-02-16 20:38:40' AT TIME ZONE 'Asia/Tokyo' AT TIME ZONE 'A
     <literal><replaceable>time</replaceable> AT TIME ZONE
     <replaceable>zone</replaceable></literal>.
    </para>
+
+   <para>
+    The function <literal><function>timezone</function>(<replaceable>timestamp</replaceable>)</literal>
+    is equivalent to the SQL-conforming construct <literal><replaceable>timestamp</replaceable>
+    AT LOCAL</literal>.
+   </para>
+
+   <para>
+    The function <literal><function>timezone</function>(<replaceable>time</replaceable>)</literal>
+    is equivalent to the SQL-conforming construct <literal><replaceable>time</replaceable>
+    AT LOCAL</literal>.
+   </para>
   </sect2>
 
   <sect2 id="functions-datetime-current">