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Doc: improve index entry for "median".

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We had an index entry for "median" attached to the percentile_cont function
entry, which was pretty useless because a person following the link would
never realize that that function was the one they were being hinted to use.

Instead, make the index entry point at the example in syntax-aggregates,
and add a <seealso> link to "percentile".

Also, since that example explicitly claims to be calculating the median,
make it use percentile_cont not percentile_disc.  This makes no difference
in terms of the larger goals of that section, but so far as I can find,
nearly everyone thinks that "median" means the continuous not discrete
calculation.

Per gripe from Steven Winfield.  Back-patch to 9.4 where we introduced
percentile_cont.

Discussion: https://postgr.es/m/20161223102056.25614.1166@wrigleys.postgresql.org
This commit is contained in:
Tom Lane 2016-12-23 12:53:09 -05:00
parent 51e9df7a10
commit a377c8d921
2 changed files with 12 additions and 5 deletions
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3
doc/src/sgml/func.sgml
View File

@ -13700,9 +13700,6 @@ SELECT xmlagg(x) FROM (SELECT x FROM test ORDER BY y DESC) AS tab;
<primary>percentile</primary>
<secondary>continuous</secondary>
</indexterm>
<indexterm>
<primary>median</primary>
</indexterm>
<function>percentile_cont(<replaceable class="parameter">fraction</replaceable>) WITHIN GROUP (ORDER BY <replaceable class="parameter">sort_expression</replaceable>)</function>
</entry>
<entry>

14
doc/src/sgml/syntax.sgml
View File

@ -1694,11 +1694,21 @@ SELECT string_agg(a ORDER BY a, ',') FROM table; -- incorrect
case, write just <literal>()</> not <literal>(*)</>.
(<productname>PostgreSQL</> will actually accept either spelling, but
only the first way conforms to the SQL standard.)
</para>
<para>
<indexterm>
<primary>median</primary>
</indexterm>
<indexterm>
<primary>median</primary>
<seealso>percentile</seealso>
</indexterm>
An example of an ordered-set aggregate call is:
<programlisting>
SELECT percentile_disc(0.5) WITHIN GROUP (ORDER BY income) FROM households;
percentile_disc
SELECT percentile_cont(0.5) WITHIN GROUP (ORDER BY income) FROM households;
percentile_cont
-----------------
50489
</programlisting>