Composite index (database)

A composite index in a database is an index on two or more columns in a table. It is used to speed up data retrieval operations where queries frequently use combinations of these columns in their WHERE clause. Unlike creating individual indexes on each column, a composite index allows the database to efficiently locate rows that match the criteria specified in the query across multiple columns simultaneously.

The order of the columns in a composite index is significant. The index is most effective when the leftmost columns in the index are used in the WHERE clause. The database can use the index to narrow down the search space based on the leftmost columns, then refine the results using the subsequent columns in the index.

Composite indexes are particularly useful when queries often filter data based on a specific combination of attributes. By creating a composite index on those attributes, the database can avoid full table scans and significantly improve query performance. However, it's important to note that adding too many indexes, including composite indexes, can negatively impact write performance, as the database needs to update the indexes whenever data is inserted, updated, or deleted. Therefore, the decision to create a composite index should be based on a careful analysis of query patterns and performance requirements.

The efficiency of a composite index depends on several factors, including the selectivity of the columns included in the index, the order of the columns in the index, and the query patterns of the application. Properly designed composite indexes can dramatically improve query performance, especially for complex queries that involve multiple filtering criteria. Poorly designed or unnecessary indexes can degrade performance and consume valuable storage space.