Create table and update it within one function ?

Hi list !

I read in the manual that "Functions and trigger procedures are
always executed within a transaction established by an outer query".
Should I understand, that there is no way to make complex
procedures that would create tables AND fill them with data ?

I have a function that is defined as :
CREATE OR REPLACE FUNCTION my_function() RETURNS VOID AS $$
SELECT create_graph_tables('graph', 'int8');
UPDATE graph_edges SET cost=1;
$$ LANGUAGE SQL;

The create_graph_tables() function creates a table "graph_edges",
but when I run this 'CREATE FUNCTION' statement, I have an error
'relation "graph_edges" does not exist".

Is there no way to do this ?

Thanks a lot !
--
Arnaud