Doc: improve explanation of EXTRACT(EPOCH) for timestamp without tz.

tglsfdc · tglsfdc · commit 4a4cad91dab7 · 2021-01-01T15:51:09.000-05:00
Try to be clearer about what computation is actually happening here. Per bug #16797 from Dana Burd. Discussion: https://postgr.es/m/16797-f264b0b980b53b8b@postgresql.org
diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
@@ -7298,9 +7298,11 @@ SELECT EXTRACT(DOY FROM TIMESTAMP '2001-02-16 20:38:40');
       <listitem>
        <para>
         For <type>timestamp with time zone</type> values, the
-        number of seconds since 1970-01-01 00:00:00 UTC (can be negative);
+        number of seconds since 1970-01-01 00:00:00 UTC (negative for
+        timestamps before that);
         for <type>date</type> and <type>timestamp</type> values, the
-        number of seconds since 1970-01-01 00:00:00 local time;
+        nominal number of seconds since 1970-01-01 00:00:00,
+        without regard to timezone or daylight-savings rules;
         for <type>interval</type> values, the total number
         of seconds in the interval
        </para>
@@ -7309,20 +7311,28 @@ SELECT EXTRACT(DOY FROM TIMESTAMP '2001-02-16 20:38:40');
 SELECT EXTRACT(EPOCH FROM TIMESTAMP WITH TIME ZONE '2001-02-16 20:38:40.12-08');
 <lineannotation>Result: </lineannotation><computeroutput>982384720.12</computeroutput>
 
+SELECT EXTRACT(EPOCH FROM TIMESTAMP '2001-02-16 20:38:40.12');
+<lineannotation>Result: </lineannotation><computeroutput>982355920.12</computeroutput>
+
 SELECT EXTRACT(EPOCH FROM INTERVAL '5 days 3 hours');
 <lineannotation>Result: </lineannotation><computeroutput>442800</computeroutput>
 </screen>
 
        <para>
-        Here is how you can convert an epoch value back to a time
-        stamp:
+        You can convert an epoch value back to a <type>timestamp with time zone</type>
+        with <function>to_timestamp</function>:
        </para>
 <screen>
-SELECT TIMESTAMP WITH TIME ZONE 'epoch' + 982384720.12 * INTERVAL '1 second';
+SELECT to_timestamp(982384720.12);
+<lineannotation>Result: </lineannotation><computeroutput>2001-02-17 04:38:40.12+00</computeroutput>
 </screen>
+
        <para>
-        (The <function>to_timestamp</> function encapsulates the above
-        conversion.)
+        Beware that applying <function>to_timestamp</function> to an epoch
+        extracted from a <type>date</type> or <type>timestamp</type> value
+        could produce a misleading result: the result will effectively
+        assume that the original value had been given in UTC, which might
+        not be the case.
        </para>
       </listitem>
      </varlistentry>
