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  1. <?xml version="1.0" encoding="UTF-8" standalone="no"?>

  2. <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>37.6. Function Overloading</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets V1.79.1" /><link rel="prev" href="xfunc-sql.html" title="37.5. Query Language (SQL) Functions" /><link rel="next" href="xfunc-volatility.html" title="37.7. Function Volatility Categories" /></head><body><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">37.6. Function Overloading</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="xfunc-sql.html" title="37.5. Query Language (SQL) Functions">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="extend.html" title="Chapter 37. Extending SQL">Up</a></td><th width="60%" align="center">Chapter 37. Extending <acronym xmlns="http://www.w3.org/1999/xhtml" class="acronym">SQL</acronym></th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 12.4 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="xfunc-volatility.html" title="37.7. Function Volatility Categories">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="XFUNC-OVERLOAD"><div class="titlepage"><div><div><h2 class="title" style="clear: both">37.6. Function Overloading</h2></div></div></div><a id="id-1.8.3.9.2" class="indexterm"></a><p>

  3.     More than one function can be defined with the same SQL name, so long

  4.     as the arguments they take are different.  In other words,

  5.     function names can be <em class="firstterm">overloaded</em>.  Whether or not

  6.     you use it, this capability entails security precautions when calling

  7.     functions in databases where some users mistrust other users; see

  8.     <a class="xref" href="typeconv-func.html" title="10.3. Functions">Section 10.3</a>.  When a query is executed, the server

  9.     will determine which function to call from the data types and the number

  10.     of the provided arguments.  Overloading can also be used to simulate

  11.     functions with a variable number of arguments, up to a finite maximum

  12.     number.

  13.    </p><p>

  14.     When creating a family of overloaded functions, one should be

  15.     careful not to create ambiguities.  For instance, given the

  16.     functions:

  17. </p><pre class="programlisting">

  18. CREATE FUNCTION test(int, real) RETURNS ...

  19. CREATE FUNCTION test(smallint, double precision) RETURNS ...

  20. </pre><p>

  21.     it is not immediately clear which function would be called with

  22.     some trivial input like <code class="literal">test(1, 1.5)</code>.  The

  23.     currently implemented resolution rules are described in

  24.     <a class="xref" href="typeconv.html" title="Chapter 10. Type Conversion">Chapter 10</a>, but it is unwise to design a system that subtly

  25.     relies on this behavior.

  26.    </p><p>

  27.     A function that takes a single argument of a composite type should

  28.     generally not have the same name as any attribute (field) of that type.

  29.     Recall that <code class="literal"><em class="replaceable"><code>attribute</code></em>(<em class="replaceable"><code>table</code></em>)</code>

  30.     is considered equivalent

  31.     to <code class="literal"><em class="replaceable"><code>table</code></em>.<em class="replaceable"><code>attribute</code></em></code>.

  32.     In the case that there is an

  33.     ambiguity between a function on a composite type and an attribute of

  34.     the composite type, the attribute will always be used.  It is possible

  35.     to override that choice by schema-qualifying the function name

  36.     (that is, <code class="literal"><em class="replaceable"><code>schema</code></em>.<em class="replaceable"><code>func</code></em>(<em class="replaceable"><code>table</code></em>)

  37.     </code>) but it's better to

  38.     avoid the problem by not choosing conflicting names.

  39.    </p><p>

  40.     Another possible conflict is between variadic and non-variadic functions.

  41.     For instance, it is possible to create both <code class="literal">foo(numeric)</code> and

  42.     <code class="literal">foo(VARIADIC numeric[])</code>.  In this case it is unclear which one

  43.     should be matched to a call providing a single numeric argument, such as

  44.     <code class="literal">foo(10.1)</code>.  The rule is that the function appearing

  45.     earlier in the search path is used, or if the two functions are in the

  46.     same schema, the non-variadic one is preferred.

  47.    </p><p>

  48.     When overloading C-language functions, there is an additional

  49.     constraint: The C name of each function in the family of

  50.     overloaded functions must be different from the C names of all

  51.     other functions, either internal or dynamically loaded.  If this

  52.     rule is violated, the behavior is not portable.  You might get a

  53.     run-time linker error, or one of the functions will get called

  54.     (usually the internal one).  The alternative form of the

  55.     <code class="literal">AS</code> clause for the SQL <code class="command">CREATE

  56.     FUNCTION</code> command decouples the SQL function name from

  57.     the function name in the C source code.  For instance:

  58. </p><pre class="programlisting">

  59. CREATE FUNCTION test(int) RETURNS int

  60.     AS '<em class="replaceable"><code>filename</code></em>', 'test_1arg'

  61.     LANGUAGE C;

  62. CREATE FUNCTION test(int, int) RETURNS int

  63.     AS '<em class="replaceable"><code>filename</code></em>', 'test_2arg'

  64.     LANGUAGE C;

  65. </pre><p>

  66.     The names of the C functions here reflect one of many possible conventions.

  67.    </p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="xfunc-sql.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="extend.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="xfunc-volatility.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">37.5. Query Language (<acronym class="acronym">SQL</acronym>) Functions </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> 37.7. Function Volatility Categories</td></tr></table></div></body></html>
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