The latter is actually a primary key lookup against the parent table and constitutes the foreign key constraint verification.

索引扫描实际上是对父表进行主键查找，并完成外键约束验证。

youdao

The latter is actually a primary key lookup against the parent table and constitutes the foreign key constraint verification.

索引扫描实际上是对父表进行主键查找，并完成外键约束验证。

youdao