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Docs: add paragraph about date/timestamp subtraction 

per suggestion from Francisco Olart
This commit is contained in:
Bruce Momjian 2013-09-03 13:24:39 -04:00
parent 9d323bda2e
commit 1c20857661
1 changed files with 31 additions and 1 deletions
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doc/src/sgml/func.sgml
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@ -6431,7 +6431,7 @@ SELECT SUBSTRING('XY1234Z', 'Y*?([0-9]{1,3})');
</entry>
<entry><type>interval</type></entry>
<entry>Subtract arguments, producing a <quote>symbolic</> result that
uses years and months</entry>
uses years and months</entry>, rather than just days
<entry><literal>age(timestamp '2001-04-10', timestamp '1957-06-13')</literal></entry>
<entry><literal>43 years 9 mons 27 days</literal></entry>
</row>
@ -6794,6 +6794,36 @@ SELECT (DATE '2001-10-30', DATE '2001-10-30') OVERLAPS
days</> because May has 31 days, while April has only 30.
</para>
<para>
Subtraction of dates and timestamps can also be complex. The most
accurate way to perform subtraction is to convert each value to a number
of seconds using <literal>EXTRACT(EPOCH FROM ...)</> and compute the
number of <emphasis>seconds</> between the two values. This will adjust
for the number of days in each month, timezone changes, and daylight
saving time adjustments. Operator subtraction of date or timestamp
values returns the number of days (24-hours) and hours/minutes/seconds
between the values, making the same adjustments. The <function>age</>
function returns years, months, days, and hours/minutes/seconds,
performing field-by-field subtraction and then adjusting for negative
field values. The following queries, produced with <literal>timezone
= 'US/Eastern'</> and including a daylight saving time change,
illustrates these issues:
</para>
<screen>
SELECT EXTRACT(EPOCH FROM timestamptz '2013-07-01 12:00:00') -
EXTRACT(EPOCH FROM timestamptz '2013-03-01 12:00:00');
<lineannotation>Result: </lineannotation><computeroutput>10537200</computeroutput>
SELECT (EXTRACT(EPOCH FROM timestamptz '2013-07-01 12:00:00') -
EXTRACT(EPOCH FROM timestamptz '2013-03-01 12:00:00'))
/ 60 / 60 / 24;
<lineannotation>Result: </lineannotation><computeroutput>121.958333333333</computeroutput>
SELECT timestamptz '2013-07-01 12:00:00' - timestamptz '2013-03-01 12:00:00';
<lineannotation>Result: </lineannotation><computeroutput>121 days 23:00:00</computeroutput>
SELECT age(timestamptz '2013-07-01 12:00:00', timestamptz '2013-03-01 12:00:00');
<lineannotation>Result: </lineannotation><computeroutput>4 mons</computeroutput>
</screen>
<sect2 id="functions-datetime-extract">
<title><function>EXTRACT</function>, <function>date_part</function></title>