Why do we need an index
Without indexes, a full table scan is required, which is extremely slow when dealing with large amounts of data. Index like a book's table of contents, quickly locate data.
B+tree structure
MySQL InnoDB uses B+tree indexing by default. Leaf nodes store data, while non leaf nodes store indexes.
optimization suggestions
The principle of leftmost prefix, overwrite index, avoid index invalidation, and do not over index.
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