Oracle Bitmap Index Techniques Oracle Tips by Burleson Consulting

 

Oracle bitmap indexes are very different from standard b-tree indexes. In bitmap structures, a two-dimensional array is created with one column for every row in the table being indexed. Each column represents a distinct value within the bitmapped index. This two-dimensional array represents each value within the index multiplied by the number of rows in the table.

At row retrieval time, Oracle decompresses the bitmap into the RAM data buffers so it can be rapidly scanned for matching values. These matching values are delivered to Oracle in the form of a Row-ID list, and these Row-ID values may directly access the required information.

The real benefit of bitmapped indexing occurs when one table includes multiple bitmapped indexes. Each individual column may have low cardinality. The creation of multiple bitmapped indexes provides a very powerful method for rapidly answering difficult SQL queries.

A bitmap merge operation build ROWID lists

Using this bitmap merge methodology, Oracle can provide sub-second response time when working against multiple low-cardinality columns.

Also see these important notes on Oracle bitmap index maximum distinct values

For example, assume there is a motor vehicle database with numerous low-cardinality columns such as car_color, car_make, car_model, and car_year. Each column contains less than 100 distinct values by themselves, and a b-tree index would be fairly useless in a database of 20 million vehicles.

However, combining these indexes together in a query can provide blistering response times a lot faster than the traditional method of reading each one of the 20 million rows in the base table. For example, assume we wanted to find old blue Toyota Corollas manufactured in 1981:

select
   license_plat_nbr
from
   vehicle
where
   color = "blue"
and
   make = "toyota"
and
   year = 2015;

Oracle uses a specialized optimizer method called a bitmapped index merge to service this query. In a bitmapped index merge, each Row-ID, or RID, list is built independently by using the bitmaps, and a special merge routine is used in order to compare the RID lists and find the intersecting values.

As the number if distinct values increases, the size of the bitmap increases exponentially, such that an index with 100 values may perform thousands of times faster than a bitmap index on 1,000 distinct column values. 

Also, remember that bitmap indexes are only suitable for static tables and materialized views which are updated at nigh and rebuilt after batch row loading.  If your tables experience multiple DML's per second, BE CAREFUL when implementing bitmap indexes!

• 1 - 7 distinct key values - Queries against bitmap indexes with a low cardinality are very fast.

• 8-100 distinct key values - As the number if distinct values increases, performance decreases proportionally.

• 100 - 10,000 distinct key values - Over 100 distinct values, the bitmap indexes become huge and SQL performance drops off rapidly.

• Over 10,000 distinct key values - At this point, performance is ten times slower than an index with only 100 distinct values.

 

---

Oracle Bitmap indexes are a very powerful Oracle feature, but they can be tricky!

You will want a bitmap index when:

1 - Table column is low cardinality - As a ROUGH guide, consider a bitmap for any index with less than 100 distinct values

    select region, count(*) from sales group by region;

2 - The table has LOW DML - You must have low insert./update/delete activity.  Updating bitmapped indexes take a lot of resources, and bitmapped indexes are best for largely read-only tables and tables that are batch updated nightly.

3 - Multiple columns - Your SQL queries reference multiple, low cardinality values in there where clause.  Oracle cost-based SQL optimizer (CBO) will scream when you have bitmap indexes on . 

Troubleshooting Oracle bitmap indexes:

Some of the most common problems when implementing bitmap indexes include:

1. Small table - The CBO may force a full-table scan if your table is small!

2. Bad stats - Make sure you always analyze the bitmap with dbms_stats right after creation:

CREATE BITMAP INDEX
emp_bitmap_idx
ON index_demo (gender);

exec dbms_stats.gather_index_stats(OWNNAME=>'SCOTT', INDNAME=>'EMP_BITMAP_IDX');
 

  3. Test with a hint - To force the use of your new bitmap index, just use a Oracle INDEX hint:

select /*+ index(emp emp_bitmap_idx) */
   count(*)
from
   emp, dept
where
   emp.deptno = dept.deptno;