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A bitmap
index is a special kind of database
index that
uses bitmaps, Oracle bitmap indexes are very different from
standard b-tree indexes , Bitmap Indexes were first introduced by O’Neil
and implemented in the Model 204 DBMS. The bitmap technique is largely used for
typical data warehouse applications, which are mainly characterized and Used by complex query
types and read-mostly environments that are more or less static.

In data warehouse
environments insert, delete or update operations are not common, therefore, the
bitmap index is coming up to optimizes the query performance rather than others.

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Bitmap indexes
are used by DBMS to accelerate decision support queries. The main advantage of
Bitmap indexes is that complex logical selection operations can be performed
very quickly and faster than B-tree by applying low-cost Boolean operations
such as OR, AND, and NOT, this will reduce search space before going to the
primary source data.


Characteristic of Bitmap indexing


·       For columns with very few unique values (low

·       Columns that have low cardinality are
good candidates

·       Tables that have no or little
insert/update are good candidates

·       Stream of bits: each bit relates to a
column value in a single row of table


Advantage of Bitmap Indexes

The xmain
xadvantage xof
xthe xBitmap xindexesx
thatx arex
high xcompressedx
structure, which xwillx make xthex read xis
xreally xfast.
in xaddition, the
bitmap structure makes the systemx
mergex multiplex
indexes xtogetherx
for faster access to thex underlying

Compressed indexes are like a bitmapx indexes, x
its represent a trade-off between CPU and Disk usage. The compressed structure
is faster to read-only fromx disk but unfortunately it takes
additional CPU xcycles tox decompress for access
anx uncompressed

One belief
concerning bitmap indexes is that they are only suitable for indexing
low-cardinality data. This is not necessarilyx true, because byx compressingx the indexes in bitmap indexes, it can be used veryx successfully
forx indexingx columnsx with manyx millions ofx different


Disadvantage of Bitmap


The main disadvantage is the
modificationx on bitmap indexing which is really
dreadful, because itx
requires a huge dealx
work on behalf of the system whichx is worst than modification on B-tree index, x on other hand the overheadx on maintaining the bits is enormous.


Bitmap index Design


Basic Bitmap Index


Bitmap indices are one of the mostx efficient indexingx methods availablex for speeding up multi-dimensional
range queries for reading only (projection), the queries are evaluated with bitwise
logical operations that are well supported byx computer hardware,” for xan attribute with C distinct
values, the basic bitmap index generates C bitmaps with N bits
each, where N is the number ofx records in the datax set. Each bit in a bitmap is set to
“1” if the attribute in the record is of a specific value; otherwise the bit
is set to “0””.

Figure 1 below
will demonstrate a simple bitmap index with 6 bitmaps. “Each bitmap represents
a distinct attribute value. For instance, the attribute value 3 is highlighted
to demonstrate the encoding. In this case, bitmap 3 is set to “1”, all other
bits on the same horizontal position are set to “0””.






The basic bitmap
index introduced above is also called equality encoded bitmap index
since each bitmap indicates whether or not an attribute value equals to the
key. This strategy is the most efficient for equality queries such as “temperature
= 100.” Chan and Ioannidis (1998; 1999) developed two other encodingx strategies that are called

range encoding and interval encoding. These
bitmap indices are optimized for one-sided and two-sided range queries,
respectively. An

example of a
one-sidedx range query is “pressure

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