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1·Currently a sparse index can have only one field.

目前，Sparse索引只能有一个字段。

2·Covering indexes allow data storage within the index itself while sparse indexes exclude documents that do not contain the missing field.

Covered索引允许在索引本身里存储数据，而Sparse索引则会排除掉不包含索引字段的文档。

3·The index is sparse because of the missing documents when values are missing.

失踪的指数“稀疏”，因为文件值时失踪。