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diff --git a/doc/src/sgml/html/xfunc-overload.html b/doc/src/sgml/html/xfunc-overload.html new file mode 100644 index 0000000..0556ad0 --- /dev/null +++ b/doc/src/sgml/html/xfunc-overload.html @@ -0,0 +1,67 @@ +<?xml version="1.0" encoding="UTF-8" standalone="no"?> +<!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>38.6. 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 Vsnapshot" /><link rel="prev" href="xfunc-sql.html" title="38.5. Query Language (SQL) Functions" /><link rel="next" href="xfunc-volatility.html" title="38.7. Function Volatility Categories" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">38.6. Function Overloading</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="xfunc-sql.html" title="38.5. Query Language (SQL) Functions">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="extend.html" title="Chapter 38. Extending SQL">Up</a></td><th width="60%" align="center">Chapter 38. Extending <acronym class="acronym">SQL</acronym></th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 15.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="xfunc-volatility.html" title="38.7. Function Volatility Categories">Next</a></td></tr></table><hr /></div><div class="sect1" id="XFUNC-OVERLOAD"><div class="titlepage"><div><div><h2 class="title" style="clear: both">38.6. Function Overloading</h2></div></div></div><a id="id-1.8.3.9.2" class="indexterm"></a><p> + More than one function can be defined with the same SQL name, so long + as the arguments they take are different. In other words, + function names can be <em class="firstterm">overloaded</em>. Whether or not + you use it, this capability entails security precautions when calling + functions in databases where some users mistrust other users; see + <a class="xref" href="typeconv-func.html" title="10.3. Functions">Section 10.3</a>. When a query is executed, the server + will determine which function to call from the data types and the number + of the provided arguments. Overloading can also be used to simulate + functions with a variable number of arguments, up to a finite maximum + number. + </p><p> + When creating a family of overloaded functions, one should be + careful not to create ambiguities. For instance, given the + functions: +</p><pre class="programlisting"> +CREATE FUNCTION test(int, real) RETURNS ... +CREATE FUNCTION test(smallint, double precision) RETURNS ... +</pre><p> + it is not immediately clear which function would be called with + some trivial input like <code class="literal">test(1, 1.5)</code>. The + currently implemented resolution rules are described in + <a class="xref" href="typeconv.html" title="Chapter 10. Type Conversion">Chapter 10</a>, but it is unwise to design a system that subtly + relies on this behavior. + </p><p> + A function that takes a single argument of a composite type should + generally not have the same name as any attribute (field) of that type. + Recall that <code class="literal"><em class="replaceable"><code>attribute</code></em>(<em class="replaceable"><code>table</code></em>)</code> + is considered equivalent + to <code class="literal"><em class="replaceable"><code>table</code></em>.<em class="replaceable"><code>attribute</code></em></code>. + In the case that there is an + ambiguity between a function on a composite type and an attribute of + the composite type, the attribute will always be used. It is possible + to override that choice by schema-qualifying the function name + (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>) + </code>) but it's better to + avoid the problem by not choosing conflicting names. + </p><p> + Another possible conflict is between variadic and non-variadic functions. + For instance, it is possible to create both <code class="literal">foo(numeric)</code> and + <code class="literal">foo(VARIADIC numeric[])</code>. In this case it is unclear which one + should be matched to a call providing a single numeric argument, such as + <code class="literal">foo(10.1)</code>. The rule is that the function appearing + earlier in the search path is used, or if the two functions are in the + same schema, the non-variadic one is preferred. + </p><p> + When overloading C-language functions, there is an additional + constraint: The C name of each function in the family of + overloaded functions must be different from the C names of all + other functions, either internal or dynamically loaded. If this + rule is violated, the behavior is not portable. You might get a + run-time linker error, or one of the functions will get called + (usually the internal one). The alternative form of the + <code class="literal">AS</code> clause for the SQL <code class="command">CREATE + FUNCTION</code> command decouples the SQL function name from + the function name in the C source code. For instance: +</p><pre class="programlisting"> +CREATE FUNCTION test(int) RETURNS int + AS '<em class="replaceable"><code>filename</code></em>', 'test_1arg' + LANGUAGE C; +CREATE FUNCTION test(int, int) RETURNS int + AS '<em class="replaceable"><code>filename</code></em>', 'test_2arg' + LANGUAGE C; +</pre><p> + The names of the C functions here reflect one of many possible conventions. + </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" title="38.5. Query Language (SQL) Functions">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="extend.html" title="Chapter 38. Extending SQL">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="xfunc-volatility.html" title="38.7. Function Volatility Categories">Next</a></td></tr><tr><td width="40%" align="left" valign="top">38.5. 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