谁能通过简单的例子告诉我索引涵盖了什么?
问题描述：
任何人都可以通过简单的示例告诉我什么内容覆盖索引吗?
Can anyone tell me what is covering Index by simple example?
答
如果查询的选择列表中请求的所有列均在索引中可用，那么查询引擎不必再次查找表，这可以显着提高查询的性能.由于所有请求的列在索引中都可用，因此索引涵盖了查询.因此，该查询称为覆盖查询，索引称为覆盖索引.
请参考 http://www.simple- talk.com/sql/learn-sql-server/using-covering-indexes-to-improve-query-performance/ [ http://connectsql.blogspot.in/2010/12/sql-server- performance-covering-index.html [ ^ ]
If all the columns requested in the select list of query, are available in the index, then the query engine doesn''t have to lookup the table again which can significantly increase the performance of the query. Since, all the requested columns are available with in the index, the index is covering the query. So, the query is called as the covering query and the index as covering index.
Refer http://www.simple-talk.com/sql/learn-sql-server/using-covering-indexes-to-improve-query-performance/[^]
http://connectsql.blogspot.in/2010/12/sql-server-performance-covering-index.html[^]