
36 Automated Physical Database Design and Tuning
belongs to group 2. In this way, a MEMO compactly represents a potentially
very large number of operator trees. Also note that the children of physical
groupExpressions also point to the most efficient groupExpression in the cor-
responding groups. For instance, groupExpression 3.8 represents a hash join
operator whose left-hand child is the second groupExpression in group 1 and
whose right-hand child is the second groupExpression in group 2.
In addition to representing operator trees, the MEMO provides basic in-
frastructure for management of groupExpression properties. There are two
kinds of properties. ...