A covering index includes all columns needed by a query, eliminating table lookups.
A covering index is an advanced optimization technique where all columns needed by a query are included directly in the index. This means the database can fulfill the query using only the index, completely skipping the main table (a technique called an index-only scan). The result? Drastically faster read performance.
Take a query like SELECT name, age FROM users WHERE email = 'user@example.com';. Without a covering index, the database must first use the index to find email, then perform a second lookup in the table to retrieve name and age. With a covering index that includes all three columns (email, name, age), that extra step is eliminated.
Covering indexes are particularly useful for high-traffic queries used in analytics dashboards or API responses, where read speed is critical. They reduce I/O, improve caching efficiency, and can improve latency by 2x–20x depending on dataset size and disk performance.
Galaxy automatically suggests covering index candidates based on historical query patterns and schema introspection. While covering indexes can increase index storage size, they’re often worth it for frequently accessed read-heavy endpoints.
SELECT name, age FROM users WHERE email = 'user@example.com';
CREATE INDEX idx_email_name_age ON users(email, name, age);
2–20x depending on dataset size
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