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Oracle 8 Tips  

by Burleson Consulting

The Data Warehouse Development Life Cycle

Oracle Features for the Data Warehouse

Oracle Bitmapped Indexes

select 'spool off;' from dual;
spool off;

set termout on;


@idx1

!sort idx2.lst

Listing 8.13 shows the listing from bitmap.sql, and displays the indexes according to the number of distinct keys.

Listing 8.13 The results of bitmap.sql.

3 is the cardinality of Index GEO_LOC_PK on column GEO_LOC_TY_CD of table DON.GEM_LCC
4 is the cardinality of Index REGION_IDX on column REHION of table DON.CUSTOMER
7 is the cardinality of Index GM_LCK on column GEO_LC_TCD of table DON.GEM_LCC
8 is the cardinality of Index USR_IDX on column USR_CD of table DON.CUSTOMER
50 is the cardinality of Index STATE_IDX on column STATE_ABBR of table DON.CUSTOMER
3117 is the cardinality of Index ZIP_IDX on column ZIP_CD of table DON.GEM_LCC
71,513 is the cardinality of Index GEO_LOC_PK on column GEO_LOC_CD of table DON.GEM_LCC
83,459 is the cardinality of Index GEO_KEY_PK on column GEO_LOC_TY_CD of table DON.GEM_LCC


Of course, columns such as GENDER and REGION should be made into bitmaps--but what about STATE with 50 values, or area_code with a few hundred values? Intuition tells us that the benefits from bitmapped indexes are a function of the cardinality and the number of rows in the table, but no hard and fast rule exists for identifying what type of index is always best. An heuristic approach is called for, and it is relatively easy for the Remote DBA to create a b-tree index, run a timed query, re-create the index with the BITMAP option, and re-execute the timed query.

Regardless, bitmapped indexes are critical to the performance of decision support system, especially those that are queried in an ad-hoc fashion against hundreds of column values. See Chapter 10, Tuning Oracle Data Warehouse And OLAP Applications, for a discussion of the applications of bitmapped indexes.


This is an excerpt from "High Performance Data Warehousing", copyright 1997. To learn more about Oracle, try "Oracle Tuning: The Definitive Reference", by Donald K. Burleson.  You can buy it direct from the publisher at 30% off here:
http://www.rampant-books.com/book_1002_oracle_tuning_definitive_reference_2nd_ed.htm

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