MySQL for Database Administrators
面向数据库管理员的 MySQL

Explanation of Simple Join Output
简单连接输出说明

• The join in the preceding slide has a WHERE clause that references an unindexed field.
  上一张幻灯片中的联接具有引用未索引字段的 WHERE 子句。
• The first operation is an index scan of all rows in the emp_no index of the titles table.
  第一个作是对 titles 表的 emp_no 索引中的所有行进行索引扫描。
• It reads all the index entries, and filters them based on the WHERE clause against the primary key value. All secondary indexes in InnoDB tables also contain the primary key values.
  它读取所有索引条目，并根据主键值根据 WHERE 子句过滤它们。InnoDB 表中的所有二级索引也包含主键值。
• The second operation references the first one:
  第二个作引用第一个作：
• The type of the second operation is eq_ref, indicating that it matches each row for equality with its compared value.
  第二个作的类型为 eq_ref，表示它匹配每一行以使其比较值相等。
• The value in question is the qualified column name employees.titles.emp_no, indicating that the value to compare is not a constant provided in the statement, but comes from that other column.
  有问题的值是限定列名 employees.titles.emp_no，指示要比较的值不是语句中提供的常量，而是来自另一列。