From the 10g Oracle Database Application Developers Guide - Fundamentals, Chapter 5:
In general, you should create an index on a column in any of the following situations:
- The column is queried frequently.
- A referential integrity constraint exists on the column.
- A UNIQUE key integrity constraint exists on the column.
Use the following guidelines for determining when to create an index:
- Create an index if you frequently want to retrieve less than about 15% of the rows in a large table. This threshold percentage varies greatly, however, according to the relative speed of a table scan and how clustered the row data is about the index key. The faster the table scan, the lower the percentage; the more clustered the row data, the higher the percentage.
- Index columns that are used for joins to improve join performance.
- Primary and unique keys automatically have indexes, but you might want to create an index on a foreign key; see Chapter 6, "Maintaining Data Integrity in Application Development" for more information.
- Small tables do not require indexes; if a query is taking too long, then the table might have grown from small to large.
Some columns are strong candidates for indexing. Columns with one or more of the following characteristics are good candidates for indexing:
- Values are unique in the column, or there are few duplicates.
- There is a wide range of values (good for regular indexes).
- There is a small range of values (good for bitmap indexes).
The column contains many nulls, but queries often select all rows having a value. In this case, a comparison that matches all the non-null values, such as:
WHERE COL_X >= -9.99 *power(10,125)
is preferable to
WHERE COL_X IS NOT NULL
This is because the first uses an index on COL_X (assuming that COL_X is a numeric column).
Columns with the following characteristics are less suitable for indexing:
- There are many nulls in the column and you do not search on the non-null values.
