CREATE OR REPLACE FUNCTION prepare_my_sql(in_name name, in_sql character varying)
  RETURNS boolean AS
$BODY$

BEGIN
	BEGIN
		EXECUTE 'DEALLOCATE '||in_name;
	EXCEPTION
	WHEN invalid_sql_statement_name THEN
	END;
	
	EXECUTE 'PREPARE '||in_name||' AS '||in_sql;

	RETURN true;
END;
$BODY$
  LANGUAGE plpgsql ;

SELECT prepare_my_sql('my_sql'
                      , CASE WHEN EXISTS (SELECT * 
                                    FROM information_schema.TABLES t 
                                    JOIN information_schema.COLUMNS c ON t.table_name = c.table_name
                                    WHERE t.table_name='table007' AND c.column_name='field007') 

                        THEN 'SELECT field007 FROM table007' 
                        ELSE 'SELECT 0' 
                   END);
EXECUTE my_sql;