how to use variable for select query in pl/pgsql

hello everybody,

i want use a variable to denote a table name in select query.

for example:

DECLARE
table_message_data varchar;
record_message_data record;
BEGIN
table_message_data := prefix || 'message_data';

select into record_message_data * from table_message_data;
END;

the "prefix" is the function input parameter,but i don't know how to use
the "table_message_data" in select query.

please tell me,thanks.