•Need information about the relations and indexes involved. Catalogs typically
contain at least:
–# tuples (NTuples) and # pages (NPages) for
each relation.
–# distinct key values (NKeys) and NPages for each index.
–Index height, low/high key values (Low/High) for each tree index.
•Catalogs updated periodically.
–Updating whenever data changes is too expensive; lots of
approximation anyway, so slight
inconsistency ok.
•More detailed information (e.g., histograms of the
values in some field) are
sometimes stored.