Glossary - Relational Database Index Design and the Optimizers - page 296

Culprit A transaction that monopolizes resources, perhaps because of
inadequate indexing.
DB2 for LUW A DBMS that runs under LINUX, UNIX, and Windows.
Difficult predicate A predicate that cannot participate in defining the index
slice—that is, it cannot be a matching predicate; sometimes such predicates are called
nonindexable .
Fat index An index that contains all the columns referred to in a SELECT statement;
there is no table access necessary.
Filter factor pitfall A situation where the worst case may not be the input with the
highest filter factor; it may arise when there is no sort in the access path, the
transaction responds as soon as the first screen is built, and all the predicate columns
do not participate in defining the index slice to be scanned.
First star The index rows required by the SELECT statement are next to each other,
or at least as close to each other as possible. This star minimizes the thickness of the
index slice that must be scanned.
Ideal index An index having all three stars.
Index slice The portion of an index that is scanned; the thickness of the slice
influences the number of synchronous reads that will be required to the table.
LRT-level exception monitoring Producing exception reports that show operational
transactions whose local response time or SQL elapsed time is exceptionally long.
NLR, number of local rows The number of rows remaining after the local predicates
have been applied using the maximum filter factors. Used to predict the best table
access order for a join—the outermost table is the one with the lowest NLR.
QUBE, Quick Upper-Bound Estimate A quick and easy estimate of the local
response time using only two variables, TR and TS; it is used to reveal potentially
slow access paths at a very early stage. It can reveal performance problems relating to
index or table design, assuming that the worst-case filter factors used are reasonably
close to reality. By definition the QUBE is pessimistic; it sometimes makes
false alarms.
Really difficult predicate A predicate that cannot even participate in index screening;
therefore, each table row in a slice has to be accessed to evaluate the predicate, even
though the predicate column or columns have been copied to the index.
Second star The index rows are in the right sequence to satisfy the SELECT
statement ORDER BY requirement. This star avoids the need for a sort.
Semifat index An index that contains all the columns referenced by the WHERE
clause, thereby accessing the table only when necessary.
Spike A single operational transaction whose local response time or SQL elapsed
time is exceptionally long.
Third star The index rows contain all the columns referred to by the SELECT
statement. This star eliminates table access—the access path is index only; the third
star is often the most important one.