By querying a feed from Bigtable, we are really just using Bigtable as another caching layer.
通过查询Bigtable中的提要，实际上使用了Bigtable作为另一个缓存层。
Directly using SQL makes caching problematic, since the caching layer must deal with full result sets rather than single database rows.
直接使用SQL会使缓存出问题，因为缓存层必须处理完整的结果集，而不是单独一个数据库行。
This caching layer optimizes access to the physical devices by keeping data around for a short time (or speculatively read ahead so that the data is available when needed).
这个缓存层通过将数据保留一段时间(或者随即预先读取数据以便在需要是就可用)优化了对物理设备的访问。
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