由于每个索引(片段)都有自己的udi计数器，因此将根据索引(片段)的修改程度判断是否跳过或是重建索引统计。

Since each index (fragment) has its own UDI counter, the decision to skip or rebuild index statistics is done based on how much the index (fragment) has changed.

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比较原始文档的修改日期，看是否有必要重建索引。

A modification date compare against the original Document's modification date, to see if it needs to be reindexed.

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