问题描述
用PL/pgSQL或SQL编写的功能可以定义为RETURNS void.我最近偶然发现结果有一个奇怪的差异.
Functions written in PL/pgSQL or SQL can be defined as RETURNS void. I recently stumbled upon an odd difference in the result.
请考虑以下演示:
CREATE OR REPLACE FUNCTION f_sql()
RETURNS void AS
'SELECT NULL::void' -- "do nothing", no special meaning
LANGUAGE sql;
CREATE OR REPLACE FUNCTION f_plpgsql()
RETURNS void AS
$$
BEGIN
NULL; -- "do nothing", no special meaning
END;
$$ LANGUAGE plpgsql;
现在，void是虚拟类型.虽然plpgsql函数似乎返回与void类型有效的空字符串等效的内容，实际上是''::void. sql函数似乎返回NULL::void.
Now, void is a fictive type. While the plpgsql function seems to return the equivalent of an empty string as type void, effectively ''::void. The sql function seems to return NULL::void.
db=# SELECT f_sql() IS NULL;
?column?
----------
t
db=# SELECT f_sql()::text IS NULL;
?column?
----------
t
db=# SELECT f_plpgsql() IS NULL;
?column?
----------
f
db=# SELECT f_plpgsql()::text = '';
?column?
----------
t
这可能会有细微而令人困惑的副作用.
造成这种差异的原因是什么?
This can have subtle and confusing side effects.
What is the reason behind the difference?
推荐答案
(我不是此源代码的专家.已经警告您.)
(I'm no expert in this source code. You've been warned.)
该资源在线提供此处.我省略了文件名；您可以搜索该功能名称以查找其定义.我通常保留行号是因为它更容易剪切和粘贴，而不同的行号将表示源已更改.
The source is online here. I've omitted the filenames; you can search for the functionnames to find their definitions. I left the line numbers (usually) because it's easier to cut and paste, and different line numbers will mean the source has changed.
简短的故事是，某些无效"返回可能是空的cstring(空的以null终止的字符串)，而另一些则是null指针.
The short story is that some "void" returns are probably empty cstrings (empty null-terminated strings), and others are null pointers.
以下是与资源相关的部分.
Here are the parts of the source that look relevant.
00228 /*
00229 * void_out - output routine for pseudo-type VOID.
00230 *
00231 * We allow this so that "SELECT function_returning_void(...)" works.
00232 */
00233 Datum
00234 void_out(PG_FUNCTION_ARGS)
00235 {
00236 PG_RETURN_CSTRING(pstrdup(""));
00237 }
00251 /*
00252 * void_send - binary output routine for pseudo-type VOID.
00253 *
00254 * We allow this so that "SELECT function_returning_void(...)" works
00255 * even when binary output is requested.
00256 */
00257 Datum
00258 void_send(PG_FUNCTION_ARGS)
00259 {
00260 StringInfoData buf;
00261
00262 /* send an empty string */
00263 pq_begintypsend(&buf);
00264 PG_RETURN_BYTEA_P(pq_endtypsend(&buf));
00265 }
我们也有
00285 /* To return a NULL do this: */
00286 #define PG_RETURN_NULL() \
00287 do { fcinfo->isnull = true; return (Datum) 0; } while (0)
00288
00289 /* A few internal functions return void (which is not the same as NULL!) */
00290 #define PG_RETURN_VOID() return (Datum) 0
所以对我来说有意义的是，通过PG_RETURN_VOID()返回的用户定义函数不会测试等效于通过void_out()或void_send()返回的函数.我还不知道为什么，但是我必须停下来睡一下.
So it makes sense to me that a user-defined function that returns through PG_RETURN_VOID() would not test equivalent to one that returns through void_out() or void_send(). I don't yet know why that is, but I have to stop and get some sleep.
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