07索引

微放發表於2024-10-19
索引

索引

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索引失效的情況

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所查詢的資料在索引中都有就叫覆蓋索引

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所以當使用select *時非常容易出現回表查詢，效能就會降低

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字首取多少個，取決於需求，根據需求確定選擇性

選擇性不重複的個數/總數

使用字首索引可以降低索引體積，提高索引效率

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