Github user cloud-fan commented on the pull request:
https://github.com/apache/spark/pull/11828#issuecomment-199646190
If my understanding is right, what we want is:
* insert a `Project` below `Filter`, so that we may have chance to push it
down further.
* push down `Filter` through `Project`, to reduce the number of input rows.
The problem is: at the time we push `Filter` through `Project`, we don't
know if this `Project` can be pushed down further, and we may mistakenly lift
it and make the plan sub-optimal.
I think logically we should put `ColumnPruning` and
`PushPredicatesThroughProject` in different batches, so that when we push down
`Filter` through `Project`, we can make sure this `Project` can't be pushed
further and it's safe to lift it.
Anyway, this PR does fix the problem, but I think we should simplify the
`Operator Optimizations` batch, it contains over 30 rules, and is very hard to
reason about how these rules interact with each other.
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