Major Description
In SPARK-16980, we removed the full in-memory cache of table partitions in favor of loading only needed partitions from the metastore. This greatly improves the initial latency of queries that only read a small fraction of table partitions.
However, since the metastore does not store file statistics, we need to discover those from remote storage. With the loss of the in-memory file status cache this has to happen on each query, increasing the latency of repeated queries over the same partitions.
The proposal is to add back a per-table cache of partition contents, i.e. Map[Path, Array[FileStatus]]. This cache would be retained per-table, and can be invalidated through refreshTable() and refreshByPath(). Unlike the prior cache, it can be incrementally updated as new partitions are read.
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