Covered indexes enhance performance when all fields requested by a query are contained within the covered index because there is no need to pull up the full document record.

Covered索引在查询所请求的全部字段都包含在Covered索引中时能提升性能，因为不再需要取出完整的文档记录。

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But house prices still plunged by record rates in February in ten of the 20 cities covered by the index.

但在该用指数衡量的20个城市中，二月份有10个城市的房价仍在以最高记录的速度下跌。

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The University of Michigan's 2007 American Customer Satisfaction Index said that cable and satellite TV suffered "the lowest level of customer satisfaction among all industries covered."

密歇根大学2007年美国客户满意度指数显示，有线及卫星电视遭受了“所有参与调查的行业中最低的客户满意度”。

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It should be noted that not all countries are covered in the index.

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Every piece of data covered by the index seemed to come in negative in January, with 13 of the 20 MSAs showing further deceleration in annual growth rates and the 10 and 20-City Composites marking their sixth consecutive month of declines.

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