Page History

(I) Experiment of the necessity of TimeseriesMetadata

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1. Although the index area structure with no TimeseriesMetadata speeds up a little in raw data query,
   it reduces the speed a lot in aggregation query. => We should reserve TimeseriesMetadata.
2. The time cost does not change in the data area of TsFile.

(II) Experiment about combine Chunk and Page

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• one level index in one TsFile.
• Suitable for small Chunk (Mass Timeseries) scenario, in which 1 chunk has only 1~2 pages
  (Note: Since 0.12, If one Chunk has only one Page, then PageStatistics will be removed, we only store statistics in ChunkMetadata)

(III) Experiment about how to store PageHeader

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(b) combine PageHeader with ChunkHeader

当前的读取方式为：将 Chunk 全都读取到内存后读取。

如果按照这样的方式，则 (a) (b) 两者所用的时间相同。

如果按照精细方式进行逐块读取，分析如下：

For raw data query in a Chunk:

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需要读 (n - m) 个 PageHeader，m 个 PageData，seek 1 次。耗时为 n * th + m * (td - th) + ts

前者比后者耗时多Δt = (n - 1) * ts + m * (th - ts)，由于 n >= 1,  th > ts（读 PageHeader 也需要 seek， 因此 th > ts），

因此 Δt >0，后者耗时一定比前者少。

举例：

假设 Chunk 中有6个 Page，其中前两个 Page 是不符合时间过滤要求的

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