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doc: mention ORDER BY for some aggregates, add ORDER BY examples

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Discussion: https://postgr.es/m/CAKFQuwb+4SWnfrfQKB-UM1P1x97Xk+ybSar4xM32XGLd=fq9bA@mail.gmail.com

Co-authored-by: David G. Johnston

Backpatch-through: master
This commit is contained in:
Bruce Momjian
2023-11-03 13:05:27 -04:00
parent 95a610b097
commit 7751352942
2 changed files with 43 additions and 21 deletions
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38
doc/src/sgml/syntax.sgml
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@@ -1647,7 +1647,20 @@ sqrt(2)
are always just expressions and cannot be output-column names or numbers.
For example:
<programlisting>
SELECT array_agg(a ORDER BY b DESC) FROM table;
WITH vals (v) AS ( VALUES (1),(3),(4),(3),(2) )
SELECT array_agg(v ORDER BY v DESC) FROM vals;
array_agg
-------------
{4,3,3,2,1}
</programlisting>
Since <type>jsonb</type> only keeps the last matching key, ordering
of its keys can be significant:
<programlisting>
WITH vals (k, v) AS ( VALUES ('key0','1'), ('key1','3'), ('key1','2') )
SELECT jsonb_object_agg(k, v ORDER BY v) FROM vals;
jsonb_object_agg
----------------------------
{"key0": "1", "key1": "3"}
</programlisting>
</para>
@@ -1668,20 +1681,19 @@ SELECT string_agg(a ORDER BY a, ',') FROM table; -- incorrect
</para>
<para>
If <literal>DISTINCT</literal> is specified in addition to an
<replaceable>order_by_clause</replaceable>, then all the <literal>ORDER BY</literal>
expressions must match regular arguments of the aggregate; that is,
you cannot sort on an expression that is not included in the
<literal>DISTINCT</literal> list.
If <literal>DISTINCT</literal> is specified with an
<replaceable>order_by_clause</replaceable>, <literal>ORDER
BY</literal> expressions can only reference columns in the
<literal>DISTINCT</literal> list. For example:
<programlisting>
WITH vals (v) AS ( VALUES (1),(3),(4),(3),(2) )
SELECT array_agg(DISTINCT v ORDER BY v DESC) FROM vals;
array_agg
-----------
{4,3,2,1}
</programlisting>
</para>
<note>
<para>
The ability to specify both <literal>DISTINCT</literal> and <literal>ORDER BY</literal>
in an aggregate function is a <productname>PostgreSQL</productname> extension.
</para>
</note>
<para>
Placing <literal>ORDER BY</literal> within the aggregate's regular argument
list, as described so far, is used when ordering the input rows for