SELECT a value from various tables depending on a column value

Hi,
I'm trying to write a recordset-returning function that returns a
values from a base table, and one column from a joined table, where the
joined table varies according to a field of the base table. I'm looking
for an efficieint way to do this, and I don't think I know enough about
Postgres' capabilities to know how to do this.

I imagine fetching my base table rows in order of the table reference
column, looping over my base table, and setting a refcursor to a new
joined table when the table reference column changes. I would then
fetch from the appropriate joined table cursor to get the joined value
for each row.

So my question is a performance one: is this a sensible way to do this,
or am I missing something altogether about hierarchies of tables.

Or can I fetch a bunch of rows into memory and loop over them there,
thus avoid queries to look up individual rows over and over.

Any ideas would be much appreciated.

Many thanks,

Eric