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中查询了索引建以外的列，那么ORDER BY就不会使用索引了。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 *时，你没有强制指定索引，那么mysql为了得到你的查询的字段而查询文件，然后再进行排序操作，这就没有用到覆盖索引。而你使用了force index就会强制使用覆盖索引，这样就不会出现filesort.
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Your city_id is the primary key, and country_id is the composite primary key.
changed to
That’s it, you’re forcing the use of an index in
SELECT中查询了索引建以外的列，那么ORDER BY就不会使用索引了。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
selectfield based on the index without querying the file again based on the index to get the result.What happens when you use
select *时，你没有强制指定索引，那么mysql为了得到你的查询的字段而查询文件，然后再进行排序操作，这就没有用到覆盖索引。而你使用了force index就会强制使用覆盖索引，这样就不会出现filesort.