Mark factorial operator, and postfix operators in general, as deprecated.

Per discussion, we're planning to remove parser support for postfix
operators in order to simplify the grammar.  So it behooves us to
put out a deprecation notice at least one release before that.

There is only one built-in postfix operator, ! for factorial.
Label it deprecated in the docs and in pg_description, and adjust
some examples that formerly relied on it.  (The sister prefix
operator !! is also deprecated.  We don't really have to remove
that one, but since we're suggesting that people use factorial()
instead, it seems better to remove both operators.)

Also state in the CREATE OPERATOR ref page that postfix operators
in general are going away.

Although this changes the initial contents of pg_description,
I did not force a catversion bump; it doesn't seem essential.

In v13, also back-patch 4c5cf5431, so that there's someplace for
the <link>s to point to.

Mark Dilger and John Naylor, with some adjustments by me

Discussion: https://postgr.es/m/BE2DF53D-251A-4E26-972F-930E523580E9@enterprisedb.com

doc/src/sgml/func.sgml

@@ -1055,6 +1055,7 @@ repeat('Pg', 4) <returnvalue>PgPgPgPg</returnvalue>
        </para>
        <para>
         Factorial
+        (deprecated, use <link linkend="function-factorial"><function>factorial()</function></link> instead)
        </para>
        <para>
         <literal>5 !</literal>
@@ -1068,7 +1069,8 @@ repeat('Pg', 4) <returnvalue>PgPgPgPg</returnvalue>
         <returnvalue>numeric</returnvalue>
        </para>
        <para>
-        Factorial (as a prefix operator)
+        Factorial as a prefix operator
+        (deprecated, use <link linkend="function-factorial"><function>factorial()</function></link> instead)
        </para>
        <para>
         <literal>!! 5</literal>
@@ -1347,6 +1349,23 @@ repeat('Pg', 4) <returnvalue>PgPgPgPg</returnvalue>
        </para></entry>
       </row>
 
+      <row>
+       <entry role="func_table_entry"><para role="func_signature">
+        <indexterm id="function-factorial">
+         <primary>factorial</primary>
+        </indexterm>
+        <function>factorial</function> ( <type>bigint</type> )
+        <returnvalue>numeric</returnvalue>
+       </para>
+       <para>
+        Factorial
+       </para>
+       <para>
+        <literal>factorial(5)</literal>
+        <returnvalue>120</returnvalue>
+       </para></entry>
+      </row>
+
       <row>
        <entry role="func_table_entry"><para role="func_signature">
         <indexterm>

doc/src/sgml/ref/create_operator.sgml

@@ -87,11 +87,18 @@ CREATE OPERATOR <replaceable>name</replaceable> (
 
   <para>
    At least one of <literal>LEFTARG</literal> and <literal>RIGHTARG</literal> must be defined.  For
-   binary operators, both must be defined. For right  unary
+   binary operators, both must be defined. For right unary
    operators, only <literal>LEFTARG</literal> should be defined, while for left
    unary operators only <literal>RIGHTARG</literal> should be defined.
   </para>
 
+  <note>
+   <para>
+    Right unary, also called postfix, operators are deprecated and will be
+    removed in <productname>PostgreSQL</productname> version 14.
+   </para>
+  </note>
+
   <para>
    The <replaceable class="parameter">function_name</replaceable>
    function must have been previously defined using <command>CREATE

doc/src/sgml/syntax.sgml

@@ -977,27 +977,8 @@ CAST ( '<replaceable>string</replaceable>' AS <replaceable>type</replaceable> )
     Most operators have the same precedence and are left-associative.
     The precedence and associativity of the operators is hard-wired
     into the parser.
-   </para>
-
-   <para>
-    You will
-    sometimes need to add parentheses when using combinations of
-    binary and unary operators.  For instance:
-<programlisting>
-SELECT 5 ! - 6;
-</programlisting>
-   will be parsed as:
-<programlisting>
-SELECT 5 ! (- 6);
-</programlisting>
-    because the parser has no idea &mdash; until it is too late
-    &mdash; that <token>!</token> is defined as a postfix operator,
-    not an infix one.  To get the desired behavior in this case, you
-    must write:
-<programlisting>
-SELECT (5 !) - 6;
-</programlisting>
-    This is the price one pays for extensibility.
+    Add parentheses if you want an expression with multiple operators
+    to be parsed in some other way than what the precedence rules imply.
    </para>
 
    <table id="sql-precedence-table">

doc/src/sgml/typeconv.sgml

@@ -354,20 +354,19 @@ Some examples follow.
 </para>
 
 <example>
-<title>Factorial Operator Type Resolution</title>
+<title>Square Root Operator Type Resolution</title>
 
 <para>
-There is only one factorial operator (postfix <literal>!</literal>)
+There is only one square root operator (prefix <literal>|/</literal>)
 defined in the standard catalog, and it takes an argument of type
-<type>bigint</type>.
+<type>double precision</type>.
 The scanner assigns an initial type of <type>integer</type> to the argument
 in this query expression:
 <screen>
-SELECT 40 ! AS "40 factorial";
-
-                   40 factorial
---------------------------------------------------
- 815915283247897734345611269596115894272000000000
+SELECT |/ 40 AS "square root of 40";
+ square root of 40
+-------------------
+ 6.324555320336759
 (1 row)
 </screen>
 
@@ -375,7 +374,7 @@ So the parser does a type conversion on the operand and the query
 is equivalent to:
 
 <screen>
-SELECT CAST(40 AS bigint) ! AS "40 factorial";
+SELECT |/ CAST(40 AS double precision) AS "square root of 40";
 </screen>
 </para>
 </example>

src/include/catalog/pg_operator.dat

@@ -218,10 +218,10 @@
   oprname => '>=', oprleft => 'xid8', oprright => 'xid8', oprresult => 'bool',
   oprcom => '<=(xid8,xid8)', oprnegate => '<(xid8,xid8)', oprcode => 'xid8ge',
   oprrest => 'scalargesel', oprjoin => 'scalargejoinsel' },
-{ oid => '388', descr => 'factorial',
+{ oid => '388', descr => 'deprecated, use factorial() instead',
   oprname => '!', oprkind => 'r', oprleft => 'int8', oprright => '0',
   oprresult => 'numeric', oprcode => 'numeric_fac' },
-{ oid => '389', descr => 'deprecated, use ! instead',
+{ oid => '389', descr => 'deprecated, use factorial() instead',
   oprname => '!!', oprkind => 'l', oprleft => '0', oprright => 'int8',
   oprresult => 'numeric', oprcode => 'numeric_fac' },
 { oid => '385', descr => 'equal',

src/include/catalog/pg_proc.dat

@@ -328,6 +328,7 @@
   proname => 'unknownout', prorettype => 'cstring', proargtypes => 'unknown',
   prosrc => 'unknownout' },
 { oid => '111',
+  descr => 'implementation of deprecated ! and !! factorial operators',
   proname => 'numeric_fac', prorettype => 'numeric', proargtypes => 'int8',
   prosrc => 'numeric_fac' },