From: Tom Lane <tgl@sss.pgh.pa.us>
To: jiangshan.liu@tju.edu.cn
cc: pgsql-bugs@lists.postgresql.org
Subject: Re: BUG #17869: Inconsistency between PL/pgSQL Function Parameter
 Handling and SQL Query Results
In-reply-to: <17869-f049fe97d6a4491b@postgresql.org>
References: <17869-f049fe97d6a4491b@postgresql.org>
Comments: In-reply-to PG Bug reporting form <noreply@postgresql.org>
	message dated "Sun, 26 Mar 2023 13:21:37 -0000"
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Date: Sun, 26 Mar 2023 11:41:05 -0400
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PG Bug reporting form <noreply@postgresql.org> writes:
> However, when passing fixed-length character types as parameters in PL/pgSQL
> functions, the behavior seems to be different. The documentation states that
> parenthesized type modifiers are discarded by CREATE FUNCTION, meaning that
> CREATE FUNCTION foo (varchar(10)) is the same as CREATE FUNCTION foo
> (varchar) [2].

I think you've misunderstood that.  Type modifiers are not applied
to function parameters.  Thus, this function declaration:

> CREATE OR REPLACE FUNCTION test(param CHAR) RETURNS TEXT AS $$

avers that the function takes any CHAR-type value regardless of length.
Had you written, say,

regression=# SELECT * FROM test('abc'::char);
NOTICE:  a
 test 
------
 a
(1 row)

the cast operation would enforce the "defaults to length 1" business;
but the function itself does not.

Generally speaking this is desirable because you wouldn't want to have
to write a different copy of test() for each string length you might
want to use it with.  If you are really intent on getting the other
behavior you could use a domain:

regression=# create domain c1 as char(1);
CREATE DOMAIN
regression=# CREATE OR REPLACE FUNCTION test(param c1) RETURNS TEXT AS $$
  BEGIN
    RAISE NOTICE '%', param;
    RETURN param;
  END; 
$$ LANGUAGE plpgsql;
CREATE FUNCTION
regression=# SELECT * FROM test('abc');
ERROR:  value too long for type character(1)
regression=# SELECT * FROM test('abc'::char);
NOTICE:  a
 test 
------
 a
(1 row)

This happens because there is no concept of a non-typmod-enforcing
cast to a domain.

			regards, tom lane