
204 Automated Physical Database Design and Tuning
The following constraint enforces that no SELECT query degrades by more
than 10% compared with the currently deployed configuration:
FORQinW
WHERE type(Q) = SELECT
ASSERT cost(Q, C) ≤ 1.1 * cost(Q, COrig)
The last constraint enforces that no index can be replaced by the corre-
sponding narrow alternative without at least doubling the cost of some query:
FORIinC
FORQinW
ASSERT cost(Q, C-I+narrow(I))/cost(Q, C) ≤ 2
where narrow(I) results in a single-column index with I’s leading column.
For instance, we have that narrow(R(a, b, c))=R(a).
11.1.3 Language Semantics
Constrained physical design is a multiconstraint, ...