Answer
A covering index includes all columns required by a query, allowing MySQL to fetch results directly from the index without accessing the table.
Quick Answer
Covering index: the index contains all columns needed to satisfy a query without accessing the actual data rows. MySQL can fulfill the query entirely from the index B-Tree. Seen in EXPLAIN as "Using index" (no "Using where" or table access). Example: index on (user_id, email) covers SELECT email FROM users WHERE user_id = 1. Dramatically faster than regular index access for frequently-run queries.
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SugharaIQ Editorial Team
Verified Answer
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Source: SugharaIQ