Question

我在pg_catalog.pg_stat_get_activity中偶然发现了一个好奇的函数签名：

CREATE OR REPLACE FUNCTION pg_stat_get_activity(
    IN pid integer, 
    OUT datid oid,
    OUT pid integer, 
    -- more parameters...)
  RETURNS SETOF record AS 'pg_stat_get_activity'
  LANGUAGE internal STABLE
  COST 1
  ROWS 100;

此函数声明两次相同的参数名称，也会从information_schema报告。

select 
  parameter_mode,
  parameter_name
from information_schema.parameters
where specific_schema = 'pg_catalog'
and specific_name like 'pg_stat_get_activity%'
order by ordinal_position

上述收益率（另见SQLFiddle）：

+--------------+----------------+
|parameter_mode|parameter_name  |
+--------------+----------------+
|IN            |pid             |
|OUT           |datid           |
|OUT           |pid             |
|...           |...             |
+--------------+----------------+

天真地，我尝试创建一个类似的功能，但没有用：

CREATE FUNCTION f_2647(p1 IN int, p1 OUT int)
AS $$
BEGIN
    p1 := p1;
END;
$$ LANGUAGE plpgsql;

我的问题：

为什么内部pg_stat_get_activity函数重新声明两次相同的参数名称？这是为了什么目的？例如。为什么不只使用INOUT参数？
内部pg_stat_get_activity功能与我的功能有何不同？为什么我不能使用这种语法？

我知道这些是相当学术性的问题，但我需要正确理解这一点，以便在issue代码生成器中修复jOOQ。

Answer 1

我注意到它出现在9.2中。在9.1版中，out字段名为procpid：

 parameter_mode |  parameter_name  
----------------+------------------
 IN             | pid
 OUT            | datid
 OUT            | procpid
 OUT            | usesysid
 ...

寻找postgres git历史记录中的更改会导致此提交：

commit 4f42b546fd87a80be30c53a0f2c897acb826ad52
Author: Magnus Hagander 
Date:   Thu Jan 19 14:19:20 2012 +0100

    Separate state from query string in pg_stat_activity

    This separates the state (running/idle/idleintransaction etc) into
    it's own field ("state"), and leaves the query field containing just
    query text.

    The query text will now mean "current query" when a query is running
    and "last query" in other states. Accordingly,the field has been
    renamed from current_query to query.

    Since backwards compatibility was broken anyway to make that, the procpid
    field has also been renamed to pid - along with the same field in
    pg_stat_replication for consistency.

    Scott Mead and Magnus Hagander, review work from Greg Smith

在改变的行中，这是感兴趣的行：

-DATA(insert OID = 2022 (  pg_stat_get_activity         PGNSP PGUID 12 1 100 0 0 f f f f t s 1 0 2249 "23" "{23,26,23,26,25,25,16,1184,1184,1184,869,25,23}" "{i,o,o,o,o,o,o,o,o,o,o,o,o}" "{pid,datid,procpid,usesysid,application_name,current_query,waiting,xact_start,query_start,backend_start,client_addr,client_hostname,client_port}" _null_ pg_stat_get_activity _null_ _null_ _null_ ));
+DATA(insert OID = 2022 (  pg_stat_get_activity         PGNSP PGUID 12 1 100 0 0 f f f f t s 1 0 2249 "23" "{23,26,23,26,25,25,25,16,1184,1184,1184,1184,869,25,23}" "{i,o,o,o,o,o,o,o,o,o,o,o,o,o,o}" "{pid,datid,pid,usesysid,application_name,state,query,waiting,xact_start,query_start,backend_start,state_change,client_addr,client_hostname,client_port}" _null_ pg_stat_get_activity _null_ _null_ _null_ ));

在这种预先消化的形式中，作者没有注意到pid的双重使用，或者他们并不关心，因为它在实践中是无害的，这是合理的。

它是通过的，因为这些内部函数是由initdb在跳过普通用户函数的创建检查的快速路径中创建的。

具有重复参数的PostgreSQL函数

1 个答案: