This seems to answer my question. I completely forgot about the behavior of NULL value in the text concatenation.
http://www.postgresql.org/docs/9.1/static/plpgsql-statements.html#PLPGSQL-QUOTE-LITERAL-EXAMPLE Because quote_literal is labelled STRICT, it will always return null when called with a null argument. In the above example, if newvalue or keyvalue were null, the entire dynamic query string would become null, leading to an error from EXECUTE. You can avoid this problem by using the quote_nullable function, which works the same as quote_literal except that when called with a null argument it returns the string NULL. For example,
