Oracle的CURRENT_TIMESTAMP函数真的是一个函数吗？

问题描述

我的印象是，在函数名称后面可以用空括号调用无参数函数，即其他数据库允许执行的操作：

  code>函数（没有括号，没有下划线）
 
 CUBRID，MySQL，Sybase ASE允许使用 CURRENT_TIMESTAMP（）。在兼容的dbms上使用CURRENT_TIMESTAMP（）应该会失败，并且会出现语法错误。
 
 
我不确定标准对用户定义的函数有什么要说的。
 
I was under the impression that argument-less functions can be called with empty parentheses after the function name, i.e. what some other databases allow to do:
current_timestamp()
Whereas in Oracle, I have to write
current_timestamp
With user-defined functions, this rule doesn't apply (in 11g). I can write both
my_function
my_function()
My question is: Is CURRENT_TIMESTAMP really a true function or should I consider it to be a language construct / pseudo-column of the Oracle SQL dialect (compatible with the SQL standard)? Is there any formal definition about when I can (optionally, mandatorily) add the () and when I have to omit them?
Background-info:
SQL 1992 defines:
<current timestamp value function> ::=
  CURRENT_TIMESTAMP [ <left paren> <timestamp precision> <right paren> ]
Derby, HSQLDB, Ingres, Postgres, SQLite, SQL Server behave like Oracle, where there are no parentheses allowed for CURRENT_TIMESTAMP
Sybase SQL Anywhere knows a CURRENT TIMESTAMP function (without parentheses, without underscore)
CUBRID, MySQL, Sybase ASE allow for using CURRENT_TIMESTAMP()
 解决方案 
SQL standards back to 1992 refer to CURRENT_TIMESTAMP as both a "time-varying system variable" and a "datetime value function". See, for example, Database Language SQL. 
But AFAIK the standards always use CURRENT_TIMESTAMP, never CURRENT_TIMESTAMP().  Using CURRENT_TIMESTAMP() on a compliant dbms should fail with a syntax error.
I'm not sure what the standards have to say about user-defined functions.
                        
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