Use indexes to improve the performance of the ObjectServer
databases. The use of well-designed indexes can reduce or eliminate
the need for full table scans during the execution of SQL queries,
and result in faster data retrieval. To determine the effectiveness
of an index, check the index selectivity.
Before you begin
Make sure that you are familiar with how indexes work by reading
the indexing guidelines. Remember that, although there is no limit
to the number of indexes that you can create on a table, you must
use indexes sparingly
About this task
The index selectivity is
a rating that determines the effectiveness of an index. The selectivity
rating of an index is determined by the ratio of the number of distinct
values in the table to the number of rows in the table. Possible ratings are as follows:
- High: Values in the table are at least 90% unique. This represents
the ideal selectivity rating.
- Medium: Values are between 50% and 89% unique.
- Low: Values are between 10% and 49% unique.
- Unknown: Values are less than 9% unique.
The selectivity rating of an
index is high if few rows contain the same value, and low if multiple
rows contain the same value. When deciding which table columns to
index, also consider the type of SQL queries that are being run on
the table.
In the case of large tables, adding and deleting indexes,
and viewing the selectivity of indexes can be memory-intensive.