Beginning Oracle Database 11g Administration From Novice to Professional

CHAPTER 17 SQL TUNING 387
Database designers have several kinds of indexes at their disposal; the most common
are listed here:
b*tree (balanced tree) type and are best suited for online
transaction processing environments. Index entries are stored in a structure that
has a root node, branches, and leaves, hence the name.
Reverse key indexes are a specialized type of b*tree index in which the key values
are reversed (for example, IGGY becomes YGGI) to prevent concentrations of similar
entries in index blocks and consequent contention for blocks.
Function-based indexes and indexes on virtual columns (columns defined in terms
of other columns) are indexes not on the data values contained in columns—but
on combinations of these values. For example, an index on would be
an example of a function- based index.
Bitmap indexes are a specialized kind of index used in data warehouses for columns
that contain only a few values—for example, model, year, and color. Each
value is represented by a bitmap (an array of 0s and 1s), and each element of the
bitmap represents one record in the table.
In addition to indexes, database designers have other data structures at their disposal.
Examples include clusters, index- organized tables, and partitioned tables.
Tip Oracle does not use an index unless the query optimizer perceives a benefit in doing so. You can
verify that an index is being used by issuing the clause of the command
and reviewing the contents of the view after a suitable time has passed. If Oracle is not
using an index, you can use the techniques in the following sections to increase the chances that it will do
so. If an index is never used, you should consider whether it can be removed safely because indexes slow
down insert, update, and delete operations and occupy valuable space within the database.
Hints
Hints for the optimizer can be embedded inside an SQL statement if the optimizer does
not find an acceptable query plan for the statement. Each hint partially constrains the
optimizer, and a full set of hints completely constrains the optimizer. In fact, a desirable
query plan can be preserved by capturing the complete set of hints that describes it in
a stored outline. For details, refer to Oracle Database 11g Performance Tuning Guide.