leftmost prefixing
The joint index matches from the leftmost column. Index (a, b, c) can search for a, a, b, a, b, c, but cannot search for b or b, c.
Coverage index
The fields queried are all in the index, so there is no need to look up data in the table. The speed is much faster.
Index invalid
Perform function operations on index columns, implicit type conversion, and like starting with%.
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