第二条 SQL 匹配的行数太多了,MySQL 执行器认为不如全盘扫描;可以看看这个。 https://dev.mysql.com/doc/refman/8.0/en/table-scan-avoidance.html You are using a key with low cardinality (many rows match the key value) through another column. In this case, MySQL assumes that by using the key probably requires many key lookups and that a table scan would be faster.