Changing user

Dear All,

I have a simple table
CREATE TABLE table1{
user name,
si numeric
};

Also I have function:
CREATE FUNCTION check1(numeric) RETURN text AS'
DECLARE
SI ALIAS AS $1;
username name;

BEGIN
username:=(SELECT user FROM table1 WHERE si=SI)
SET SESSION AUTHORIZATION username;
RETURN username;
END;
'LANGUAGE 'plpgsql';

However, when I call the function I get the error:
ERROR: syntax error at or near "$1" at character 28
CONTEXT: PL/pgSQL function "check1" line11 at SQL statement

If I comment out the "SET SESSION ..." the function works, i.e. it outputs a
valid username. I create and execute the function as a superuser.

Any suggestions?

Many thanks

Colin

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