When Should I Use Composite Indices in My Database?

When to Utilize Composite Indices

Composite indices, denoted as index(column_A, column_B, column_C), enhance database performance for queries that leverage designated columns for purposes such as joining, filtering, and even selecting data. Importantly, composite indices also benefit queries that utilize left-most subsets of the indexed columns. Consequently, an index like the one illustrated above will prove advantageous for queries involving:

• index(column_A, column_B, column_C)
• index(column_A, column_B)
• index(column_A)

Performance Considerations for Composite Indices

Using a composite index can significantly impact performance due to the following factors:

• Faster Retrieval: When queries utilize all or a part of the composite index's columns, the database can swiftly retrieve data from the indexed columns without examining the entire table.
• Reduced IO Operations: Composite indices help alleviate disk IO operations by retrieving data directly from memory, thus minimizing access to the physical storage medium.

Benefits of Using Composite Indices

Composite indices offer several advantages:

• Improved Query Execution Speed: Organizing data according to composite indices dramatically accelerates query processing, leading to noticeable performance enhancements.
• Reduced Resource Consumption: By directing queries to specific subsets of data, composite indices optimize server resource utilization, reducing overall system load.
• Enhanced Scalability: As data volumes escalate, composite indices prove invaluable in maintaining efficient query execution, even for larger datasets.

Example Use Case

Consider the provided example with the homes table:

To determine whether a composite index on geolat and geolng is appropriate, examine the most frequently executed query:

SELECT * FROM homes
WHERE geolat BETWEEN ??? AND ???
AND geolng BETWEEN ??? AND ???

For this query, a composite index on (geolat, geolng) would be optimal as it directly aligns with the specified filtering criteria.

Interpretation of the EXPLAIN Output

Analyzing the EXPLAIN output:

EXPLAIN SELECT ...
WHERE homes.geolat BETWEEN -100 AND 100
AND homes.geolng BETWEEN -100 AND 100

• geolat,geolng,display_status is the index used.
• using where indicates that the index is being used by the query's filtering conditions.

The output suggests that the existing indices on geolat and geolng are adequate for the specified query. However, if queries predominantly involve filtering on both geolat and geolng simultaneously, creating a composite index as suggested (index(geolat_geolng)) could further enhance performance.

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