Github user cloud-fan commented on the pull request:
https://github.com/apache/spark/pull/12342#issuecomment-209209559
My general thoughts about filter push down: If the filter's condition is
non-deterministic, we shouldn't push it down through some operators that will
change the number or order of the input rows, e.g. set operations, others
should be ok, e.g. the following optimization is allowed:
```
// From:
df.select('a, 'b).filter('c > rand(42))
// To:
df.filter('c > rand(42)).select('a, 'b)
```
If the underlying operator contains non-deterministic expression, filter
push down is not allowed. Currently only 4 operator may contain
non-deterministic expression: Project, Filter, Aggregate, Window
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