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。
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。
应用推荐