•Characterizing positive
capabilities:
–Schema independent (e.g., can
always perform joins, selections).
–Schema dependent: can join R and
S, but not T.
–Given a query, tells you whether
it can be handled.
•Key issue: how do you search for
plans?
•Garlic approach (IBM): Given a
query, STAR rules determine which subqueries are executable by
the sources. Then proceed bottom-up as in System-R.