Answer
An index improves read performance by speeding up searches but adds slight overhead to write operations.
Quick Answer
An index is a data structure (B-Tree by default in InnoDB) that speeds up data retrieval. Without an index, MySQL scans every row (full table scan). With an index, it jumps to matching rows directly. Creates overhead on INSERT/UPDATE/DELETE (index must be maintained). Index frequently searched, joined, or sorted columns. Too many indexes slow down write operations.
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SugharaIQ Editorial Team
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Source: SugharaIQ