is prohibited.
To minimize number of disk page accesses during the candidate check, it is necessary
to cluster the attribute values. A commonly used clustering (data layout) technique is
called the vertical partition or otherwise known as projection index. In general, the
vertical data layout is more efficient for searching, while the horizontal organization
(commonly used in DBMS) is more efficient for updating. To make the candidate
check more efficient, we recommend the vertical data organization.
Compression
Compression is the third strategy to reduce the size of bitmap indices. Since each
bitmap of the bitmap index may be used separately from others, compression is
typically applied on each individual bitmap. Compression is a well-researched topic
and efficient compression software packages are widely available. Even though
these general-purpose compression methods are effective in reducing the size of
bitmaps, query-processing operations on compressed bitmaps are often slower than
on uncompressed bitmaps (Johnson, 1999). This motivated a number of researchers
to improve the efficiency of compressed bitmap indices. Two of the most notable
compression methods are byte-aligned bitmap code (BBC) (Antoshenkov, 1994;
Antoshenkov & Ziauddin, 1996) and word-aligned hybrid (WAH) code (Wu et al.,
2004, 2006). Bitmaps compressed with BBC are slightly larger in size than those
compressed with the best available general-purpose compression methods.
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