Declare variable of composite type in PostgreSQL using %TYPE

问题

Question: How can I declare a variable of the same type a parameter in a stored function?

The simple answer is use %TYPE, this works:

CREATE OR REPLACE FUNCTION test_function_1(param1 text)
  RETURNS integer AS
$BODY$ 
DECLARE
    myVariable param1%TYPE;
BEGIN
    return 1;
END;
$BODY$
  LANGUAGE plpgsql VOLATILE
  COST 100;

But the problem is when param1 is a composite type:

CREATE TYPE comp_type as
(
    field1 text
)

CREATE OR REPLACE FUNCTION test_function_2(param1 comp_type)
  RETURNS integer AS
$BODY$ 
DECLARE
    myVariable param1%TYPE;
BEGIN
    return 1;
END;
$BODY$
  LANGUAGE plpgsql VOLATILE
  COST 100;

This doesn't work:

ERROR: type comp_type does not exist [SQL State=42704]

So how can I do when param1 is a composite type?

(Note: Just myVariable comp_type is not a good option because my function is slightly more complex.)

Edited: I had a mistake on copy&paste, the real error is:

ERROR: invalid type name "param1%TYPE"
  Position: 130 [SQL State=42601]

And using param1%ROWTYPE the error is:

ERROR: relation "param1" does not exist
  Where: compilation of PL/pgSQL function "test_function_2" near line 3 [SQL State=42P01]

回答1:

~~Use %ROWTYPE in that case.~~

Edit - simple case

Tests by A.H. and DavidEG have shown this won't work. Interesting problem!
You could try a workaround. As long as your definition is like the example you can simply resort to

CREATE FUNCTION test(param1 comp_type)
  RETURNS integer AS
$BODY$ 
DECLARE
    myvar comp_type;
BEGIN
    return 1;
END;
$BODY$
  LANGUAGE plpgsql VOLATILE;

But your real problem is probably not as simple as that?

Edit 2 - the real problem

As expected, the real problem is more complex: a polymorphic input type.
Workaround for that scenario was harder, but should work flawlessly:

CREATE FUNCTION test(param1 anyelement, OUT a integer, OUT myvar anyelement)
  RETURNS record AS
$BODY$
BEGIN
    myvar := $1;  -- myvar has now the required type.

    --- do stuff with myvar.

    myvar := NULL;  -- reset if you don't want to output ..
    a := 1;
END;
$BODY$
  LANGUAGE plpgsql VOLATILE;

Call:

SELECT a FROM test('("foo")'::comp_type); -- just retrieve a, ignore myvar

See full output:

SELECT * FROM test('("foo")'::comp_type);

Note for PostgreSQL 9.0+

There has been a crucial update in v9.0. I quote the release notes:

Allow input parameters to be assigned values within PL/pgSQL functions (Steve Prentice)

Formerly, input parameters were treated as being declared CONST, so the function's code could not change their values. This restriction has been removed to simplify porting of functions from other DBMSes that do not impose the equivalent restriction. An input parameter now acts like a local variable initialized to the passed-in value.

Ergo, in addition to my workaround, you can utilize input variables directly.