【mysql】对于a,b,c三个字段的复合索引的测试

2023年2月18日 130次阅读 来源: 乐乐Java路漫漫 
结论，在abc三个字段的符合索引，只要用到了a这个字段，都能够使用索引，与顺序无关。

CREATE TABLE key_test (id int(11), name VARCHAR(20), priority int(4), desc VARCHAR(255));

EXPLAIN SELECT * FROM key_test WHERE id = 1 and priority = 1 AND name = 'xly';//a,c,b（使用索引）
EXPLAIN SELECT * FROM key_test WHERE priority = 1 AND name = 'xly' AND id = 1;//c,b,a（使用索引）
EXPLAIN SELECT * FROM key_test WHERE id = 1 and name = 'xly';//a,b（使用索引）
EXPLAIN SELECT * FROM key_test WHERE id = 1 and priority = 1;//a,c （使用索引）
EXPLAIN SELECT * FROM key_test WHERE priority = 1 AND id = 1;//c,a （使用索引）
EXPLAIN SELECT * FROM key_test WHERE priority = 1;//c (未使用索引)
EXPLAIN SELECT * FROM key_test WHERE name = 'xly';//b (未使用索引)
EXPLAIN SELECT * FROM key_test WHERE name = 'xly' AND priority = 1;//b,c(未使用索引)
EXPLAIN SELECT * FROM key_test WHERE priority = 1 AND name = 'xly';//c,b(未使用索引)

-- 使用order来排序
EXPLAIN SELECT * FROM key_test WHERE priority = 1 AND name = 'xly' ORDER BY id;//where c,b order a (未使用索引)
EXPLAIN SELECT id,name,priority FROM key_test ORDER BY id;//查询索引覆盖以外的字段，然后进行order by 会出现filesort

-- 给id添加主键索引
EXPLAIN SELECT * FROM key_test WHERE priority = 1 AND name = 'xly' ORDER BY id;//where c,b order a (使用主键索引)
EXPLAIN SELECT * FROM key_test WHERE priority = 1 AND name = 'xly' ORDER BY name;//where c,b order b (全表扫描，没有使用filesort)
EXPLAIN SELECT * FROM key_test WHERE priority = 1 AND name = 'xly' ORDER BY `desc`;//where c,b order d(全表扫描，使用filesort)
    原文作者：乐乐Java路漫漫
    原文地址: https://blog.csdn.net/b1303110335/article/details/81976289
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