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Your city_id is the primary key, and country_id is the composite primary key.

changed to

SELECT * FROM city FORCE INDEX(idx_fk_country_id) ORDER BY country_id;

That’s it, you’re forcing the use of an index in SELECT中查詢了索引建以外的列，那么ORDER BY就不會(huì)使用索引了。你可以用FORCE INDEX.

Another point is the so-called covering index. The definition of a covering index is: MySQL can return the select field based on the index without querying the file again based on the index to get the result.

What happens when you use select *時(shí)，你沒有強(qiáng)制指定索引，那么mysql為了得到你的查詢的字段而查詢文件，然后再進(jìn)行排序操作，這就沒有用到覆蓋索引。而你使用了force index就會(huì)強(qiáng)制使用覆蓋索引，這樣就不會(huì)出現(xiàn)filesort.