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Commit74207b8

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Doc: improve introductory information about procedures.
Clarify the discussion in "User-Defined Procedures", by laying outthe key differences between functions and procedures in a bulletedlist. Notably, this avoids burying the lede about procedures beingable to do transaction control. Make the back-link in the CREATEFUNCTION reference page more prominent, and add one in CREATEPROCEDURE.Per gripe from Guyren Howe. Thanks to David Johnston for discussion.Discussion:https://postgr.es/m/BYAPR03MB4903C53A8BB7EFF5EA289674A6949@BYAPR03MB4903.namprd03.prod.outlook.com
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‎doc/src/sgml/ref/create_function.sgml‎

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@@ -100,6 +100,11 @@ CREATE [ OR REPLACE ] FUNCTION
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To be able to create a function, you must have <literal>USAGE</literal>
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privilege on the argument types and the return type.
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</para>
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<para>
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Refer to <xref linkend="xfunc"/> for further information on writing
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functions.
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</para>
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</refsect1>
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<refsect1>
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</varlistentry>
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</variablelist>
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<para>
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Refer to <xref linkend="xfunc"/> for further information on writing
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functions.
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</para>
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</refsect1>
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<refsect1 id="sql-createfunction-overloading">
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<title>Examples</title>
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<para>
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Here are some trivial examples to help you get started. For more
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information and examples, see <xref linkend="xfunc"/>.
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Add two integers using a SQL function:
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<programlisting>
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CREATE FUNCTION add(integer, integer) RETURNS integer
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AS 'select $1 + $2;'

‎doc/src/sgml/ref/create_procedure.sgml‎

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@@ -76,6 +76,11 @@ CREATE [ OR REPLACE ] PROCEDURE
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To be able to create a procedure, you must have <literal>USAGE</literal>
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privilege on the argument types.
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</para>
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<para>
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Refer to <xref linkend="xproc"/> for further information on writing
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procedures.
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</para>
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</refsect1>
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<refsect1>

‎doc/src/sgml/xfunc.sgml‎

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</indexterm>
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<para>
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A procedure is a database object similar to a function. The difference is
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that a procedure does not return a value, so there is no return type
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declaration. While a function is called as part of a query or DML
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command, a procedure is called in isolation using
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the <xref linkend="sql-call"/> command. If the CALL command is not
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part of an explicit transaction, a procedure in many server-side
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languages can commit, rollback, and begin new transactions during
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its execution, which is not possible in functions.
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A procedure is a database object similar to a function.
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The key differences are:
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<itemizedlist>
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<listitem>
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<para>
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Procedures are defined with the <xref linkend="sql-createprocedure"/>
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command, not <command>CREATE FUNCTION</command>.
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</para>
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</listitem>
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<listitem>
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<para>
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Procedures do not return a function value; hence <command>CREATE
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PROCEDURE</command> lacks a <literal>RETURNS</literal> clause.
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However, procedures can instead return data to their callers via
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output parameters.
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</para>
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</listitem>
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<listitem>
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<para>
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While a function is called as part of a query or DML command, a
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procedure is called in isolation using
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the <xref linkend="sql-call"/> command.
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</para>
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</listitem>
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<listitem>
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<para>
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A procedure can commit or roll back transactions during its
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execution (then automatically beginning a new transaction), so long
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as the invoking <command>CALL</command> command is not part of an
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explicit transaction block. A function cannot do that.
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</para>
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</listitem>
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<listitem>
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<para>
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Certain function attributes, such as strictness, don't apply to
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procedures. Those attributes control how the function is
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used in a query, which isn't relevant to procedures.
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</para>
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</listitem>
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</itemizedlist>
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</para>
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<para>
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The explanations on how to define user-defined functions in the rest of
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this chapter apply to procedures as well, except that
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the <xref linkend="sql-createprocedure"/> command is used instead, there is
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no return type, and some other features such as strictness don't apply.
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The explanations in the following sections about how to define
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user-defined functions apply to procedures as well, except for the
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points made above.
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</para>
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<para>

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