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Hi, I have simplified function like this:

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CREATE OR REPLACE FUNCTION f(varchar) RETURNS varchar AS $FUNC$
DECLARE
     addr ALIAS FOR $1;
BEGIN
     -- return NULL;
     -- return '';
END
$FUNC$ LANGUAGE 'plpgsql';
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This function is returning varchar and it always returns at least one row. How can I make it to return 0 rows? Is it
possibleor not and how? 
Returning NULL does not help (return NULL string in 1 row). Not to return a value leads to error output.

Many thanks for help

Tomas