我们如何在存储过程中定义输出参数大小？

Question

我们如何在存储过程中定义输出参数大小？

Answer 1

You can't. Of course, you are in control of how much data you put into the OUT parameter in the stored procedure. If you want you can create a sized local variable to hold the data and then assign the value of that variable to the OUT parameter.

The calling program determines the size of the variable that receives the OUT parameter.

Answer 2

Here is a simple package which declares and uses a subtype:

SQL> create or replace package my_pkg as
  2      subtype limited_string is varchar2(10);
  3      procedure pad_string (p_in_str varchar
  4                          , p_length number
  5                          , p_out_str out limited_string);
  6  end my_pkg;
  7  /

Package created.

SQL> create or replace package body my_pkg as
  2      procedure pad_string
  3          (p_in_str varchar
  4              , p_length number
  5              , p_out_str out limited_string)
  6      as
  7      begin
  8          p_out_str := rpad(p_in_str, p_length, 'A');
  9      end  pad_string;
 10  end my_pkg;
 11  /

Package body created.

SQL>

However, if we call PAD_STRING() in such a way that the output string exceeds the subtype's precision it still completes successfully. Bother!

SQL> var out_str varchar2(128)
SQL>
SQL> exec my_pkg.pad_string('PAD THIS!', 12, :out_str)

PL/SQL procedure successfully completed.

SQL>
SQL> select length(:out_str) from dual
  2  /

LENGTH(:OUT_STR)
----------------
              12

SQL>

This is annoying but it's the way PL/SQL works so we have to live with it.

The way to resolve the situaton is basically to apply DBC principles and validate our parameters. So, we can assert business rules against the inputs like this:

SQL> create or replace package body my_pkg as
  2      procedure pad_string
  3          (p_in_str varchar
  4              , p_length number
  5              , p_out_str out limited_string)
  6      as
  7      begin
  8          if length(p_in_str) + p_length > 10 then
  9              raise_application_error(
 10                      -20000
 11                      , 'Returned string cannot be longer than 10 characters!');
 12          end if;
 13          p_out_str := rpad(p_in_str, p_length, 'A');
 14      end  pad_string;
 15  end my_pkg;
 16  /

Package body created.

SQL>
SQL> exec my_pkg.pad_string('PAD THIS!', 12, :out_str)
BEGIN my_pkg.pad_string('PAD THIS!', 12, :out_str); END;

*
ERROR at line 1:
ORA-20000: Returned string cannot be longer than 10 characters!
ORA-06512: at "APC.MY_PKG", line 9
ORA-06512: at line 1


SQL>

Or we can assert business rules against the output like this:

SQL> create or replace package body my_pkg as
  2      procedure pad_string
  3          (p_in_str varchar
  4              , p_length number
  5              , p_out_str out limited_string)
  6      as
  7          l_str limited_string;
  8      begin
  9          l_str := rpad(p_in_str, p_length, 'A');
 10          p_out_str := l_str;
 11      end  pad_string;
 12  end my_pkg;
 13  /

Package body created.

SQL>
SQL> exec my_pkg.pad_string('PAD THIS!', 12, :out_str)
BEGIN my_pkg.pad_string('PAD THIS!', 12, :out_str); END;

*
ERROR at line 1:
ORA-06502: PL/SQL: numeric or value error: character string buffer too small
ORA-06512: at "APC.MY_PKG", line 9
ORA-06512: at line 1

SQL>

In most scenarios we should do both. This is the polite way to build interfaces, because it means other routines can call our procedures with the confidence that they will return the values they say they will.

Answer 3

You could use a subtype in a package header and type check that in the body...

CREATE OR REPLACE PACKAGE my_test
AS
   SUBTYPE   my_out   IS   VARCHAR2( 10 ); 

   PROCEDURE do_something( pv_variable IN OUT my_out );
END;
/

CREATE OR REPLACE PACKAGE BODY my_test
AS 
   PROCEDURE do_something( pv_variable IN OUT my_out )
   IS
      lv_variable   my_out;
   BEGIN
      -- Work on a local copy of the variable in question
      lv_variable := 'abcdefghijklmnopqrstuvwxyz';

      pv_variable := lv_variable;
   END do_something;

END;
/

Then when you run this

DECLARE
   lv_variable VARCHAR2(30);
BEGIN
   my_test.do_something( lv_variable );
   DBMS_OUTPUT.PUT_LINE( '['||lv_variable||']');
END;
/

You would get the error

ORA-06502: PL/SQL: numeric or value error: character string buffer too small

Seems to go against the spirit of using an out parameter, but after Tony's comment this was the only thing I could think of to control data within the called code.