Criteria for Choosing Indexes

The set of indexes that you define for any given column can have dramatic impact on the speed of query processing.

Use the recommendations for all criteria in combination, rather than individually. Remember also that all columns are automatically stored in a way that facilitates fast projections. To decide on additional indexes, look closely at the data in each column. Try to anticipate the number of unique and total values expected in query results.

• Number of Unique Values in the Index
  Indexes are optimized according to the number of unique (distinct) values they include.
• Types of Queries
  Certain index types optimize particular types of queries.
• Indexes Recommended by Query Type
  Use the index type recommended for your query.
• Indexes Recommended by Column Value
  Criteria such as the number of unique values in a column help identify appropriate index types for data.
• Disk Space Usage
  The following table provides estimates of the amount of space each index uses compared to the amount of column data from the source database or flat file.
• Data Types in the Index
  When indexing a column, choose only supported index types for the column data type.
• Index Type Combinations
  When a column is used in more than one type of query, you might want to assign more than one column index type.