Github user kiszk commented on the issue:
https://github.com/apache/spark/pull/11956
I saw PR #13899. I understood there are two design points for now.
Hopefully, no more points :)
1. `CachedBatch` must be serializable.
2. According to my experiments, we should make a class for a `CachedBatch`
simple.
For 1, current ColumnarBatch is not serializable.
For 2, at [this
line](https://github.com/apache/spark/blob/master/core/src/main/scala/org/apache/spark/storage/memory/MemoryStore.scala#L216),
`.next()` returns `CachedBatch`, and then `+=` involves call
[`SizeEstimator.estimate()`](https://github.com/apache/spark/blob/master/core/src/main/scala/org/apache/spark/util/SizeEstimator.scala#L69).
If this method takes longer time for a complicated `CacheBatch` class, the
performance is degraded. One of example is `auto_join??` in [this
test](https://github.com/apache/spark/blob/master/sql/hive/compatibility/src/test/scala/org/apache/spark/sql/hive/execution/HiveCompatibilitySuite.scala).
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