Sparse indexes only contain entries for documents that have the indexed field.
稀疏索引只包含文档的索引字段的条目。
The content in a field can be indexed or just stored as is for use by an application.
可以为字段中的内容建立索引，或者原样储存供应用程序使用。
A field is made up of the actual content to be indexed or stored plus metadata telling Lucene how to handle that content and a boost value indicating that field's importance.
字段由需要索引和储存的实际内容、告诉Lucene如何处理该内容的元数据和表明该字段的重要性的增强值组成。
Stored in the index - even if a field is entirely indexed, it doesn't necessarily mean that it'll be easy for Lucene to reconstruct it.
存储索引——即使一个字段被整个索引，也不意味着它能很容易的被Lucene重构出来。
Any document that is missing the field is not indexed.
任何缺失的文档字段不是索引。
An indexed header field starts with the '1' 1-bit pattern, followed by the index of the matching header field, represented as an integer with a 7-bit prefix.
这时候的二进制数据格式如上图， 第一位固定为1，后面7位为映射的索引值。
An indexed header field starts with the '1' 1-bit pattern, followed by the index of the matching header field, represented as an integer with a 7-bit prefix.
这时候的二进制数据格式如上图， 第一位固定为1，后面7位为映射的索引值。
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