相关子查询将极大地降低数据检索的速度，对于拥有几百万个行但没有索引的大型表的非选择性查询更是如此。
Correlated subqueries will greatly degrade the speed of data retrieval, especially for non-selective queries on huge tables with millions of rows and with no index.
以上示例展示了如何使用连续扫描和索引来进行数据检索，以及如何使用嵌套循环连接方法来连接所有表。
The above example shows that sequential scan and indexes are used to do the data retrieval, and the nested-loop join method is used to join all tables.
它包括所使用的索引、从数据库表检索字段的时间和执行查询的步骤次序。
It includes the indexes that are used, The Times at which fields are retrieved from the database tables, and the order of the steps that execute the query.
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