Postgresql 中文操作指南

38.6. Function Overloading #

使用相同 SQL 名称可以定义多个函数，只要它们采用的参数不同。换句话说，函数名称可以_overloaded_。无论你是否使用它，在一些用户不信任其他用户的数据库中调用函数时，此功能需要进行安全预防；请参见 Section 10.3。当执行查询时，服务器将根据提供参数的数据类型和数量确定要调用的函数。也可使用重载模拟具有有限的最大数量的可变数量参数的函数。

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 overloaded. Whether or not you use it, this capability entails security precautions when calling functions in databases where some users mistrust other users; see Section 10.3. 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.

在创建重载函数族时，应小心不要创建歧义。例如，给定如下函数：

When creating a family of overloaded functions, one should be careful not to create ambiguities. For instance, given the functions:

CREATE FUNCTION test(int, real) RETURNS ...
CREATE FUNCTION test(smallint, double precision) RETURNS ...

使用一些琐碎输入（比如 test(1, 1.5)）时，尚不确定会调用哪个函数。当前实施的解决方案规则在 Chapter 10中进行描述，但设计高度依赖此行为的系统不明智。

it is not immediately clear which function would be called with some trivial input like test(1, 1.5). The currently implemented resolution rules are described in Chapter 10, but it is unwise to design a system that subtly relies on this behavior.

通常，接受复合类型单个参数的函数不应该与该类型中的任何属性（字段）同名。请回忆一下 attribute(table)_ is considered equivalent to _ table_. attribute. 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, schema . func ( table )_ )，但最好通过不选择冲突的名称来避免问题。

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 attribute_(table)_ is considered equivalent to table_.attribute. 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, schema.func(table)_ ) but it’s better to avoid the problem by not choosing conflicting names.

另一个可能的冲突是可变参函数和不可变参函数之间的冲突。例如，可以同时创建 foo(numeric)_和 _foo(VARIADIC numeric[])。在这种情况下，不清楚在向调用提供单个数字参数（例如 foo(10.1)）时应匹配哪个。规则是使用搜索路径中较早出现的函数，或者如果这两个函数在同一架构中，则优先使用不可变参函数。

Another possible conflict is between variadic and non-variadic functions. For instance, it is possible to create both foo(numeric) and foo(VARIADIC numeric[]). In this case it is unclear which one should be matched to a call providing a single numeric argument, such as foo(10.1). 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.

重载 C 语言函数时，还有附加约束：重载函数族中的每个函数的 C 名称必须不同于所有其他函数（内部函数或动态加载函数）的 C 名称。如果违反此规则，行为将不可移植。你可能会遇到运行时链接器错误，或者一个函数将被调用（通常是内部函数）。SQL CREATE FUNCTION 命令的 AS 子句的备用形式将 SQL 函数名与 C 源代码中的函数名分离。例如：

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 AS clause for the SQL CREATE FUNCTION command decouples the SQL function name from the function name in the C source code. For instance:

CREATE FUNCTION test(int) RETURNS int
    AS 'filename', 'test_1arg'
    LANGUAGE C;
CREATE FUNCTION test(int, int) RETURNS int
    AS 'filename', 'test_2arg'
    LANGUAGE C;

此处 C 函数的名称反映了许多可能的约定之一。

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