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SQLAdvanced#indexes#performance

Why might an index not be used by a query?

Common causes: wrapping the column in a function, leading wildcard LIKE, implicit type conversion, low selectivity, or a stale optimizer statistic.

Example
-- index unused:
WHERE YEAR(order_date) = 2026
-- index-friendly rewrite:
WHERE order_date >= '2026-01-01' AND order_date < '2027-01-01'

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