MySQL Indexing

When to use

• The user needs a multi-column (composite) or covering index for a query that filters, joins, or sorts on several columns at once.
• The user wants to run EXPLAIN (or EXPLAIN ANALYZE) and interpret the plan to understand which index MySQL chose, or why it fell back to a full scan.
• The user has a slow SELECT, JOIN, or ORDER BY that scans a large table and wants to add or fix an index so the database reads far fewer rows.

When NOT to use