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

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
This commit is contained in:
Tom Lane 2021-01-01 15:51:09 -05:00
parent 32d6287d2e
commit 4d3f03f422
1 changed files with 16 additions and 3 deletions
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19
doc/src/sgml/func.sgml
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@ -9354,9 +9354,11 @@ 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>
@ -9365,18 +9367,29 @@ 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>
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 to_timestamp(982384720.12);
<lineannotation>Result: </lineannotation><computeroutput>2001-02-17 04:38:40.12+00</computeroutput>
</screen>
<para>
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>