When Should You Use Composite Indexes in Databases?

When to Use Composite Indexes

In database management, composite indexes are beneficial for optimizing queries that involve searching for data across multiple columns. They are particularly effective when queries utilize left-most prefixes of the index columns. This means that the index will be used to retrieve data when the search criteria includes all or a subset of the columns in the composite index, in the same order as they appear in the index.

For example, consider a table named homes with the following schema:

Benefits of Using Composite Indexes

Suppose you often execute queries that search for homes within a specific geographic area. You could create a composite index on geolat and geolng columns:

By using this composite index, the database can quickly retrieve homes that fall within the specified geographic range because the index covers both columns used in the search criteria. However, it's important to note that a composite index is most effective when queries utilize both columns specified in the index.

Choosing the Best Order for Composite Indexes

The order of columns in a composite index should consider the frequency and type of queries being performed. If most queries search for data based on the first column, it should be placed as the first column in the index. If queries frequently use multiple columns, consider creating a composite index with those columns in the order they appear in the query.

In the homes table, if queries typically search for homes based on location and then year built, the composite index should be structured as:

Performance Considerations

While composite indexes can improve query performance, they also consume more storage space and require additional maintenance overhead. Therefore, it's important to carefully assess the potential benefits and drawbacks before creating composite indexes.

In the example of the homes table, if queries rarely use both geolat and geolng together, creating a composite index may not yield significant performance improvements. In such cases, it's better to create separate indexes on the individual columns.

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