Github user joseph-torres commented on a diff in the pull request:

    https://github.com/apache/spark/pull/18790#discussion_r131761258
  
    --- Diff: 
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala
 ---
    @@ -872,6 +886,25 @@ object PushDownPredicate extends Rule[LogicalPlan] 
with PredicateHelper {
           pushDownPredicate(filter, u.child) { predicate =>
             u.withNewChildren(Seq(Filter(predicate, u.child)))
           }
    +
    +    case filter @ Filter(condition, watermark: EventTimeWatermark) =>
    --- End diff --
    
    For filter, the logic has a subtle additional condition. We don't want to 
push down filters on the watermark attribute, because:
    * they'll be at least as expensive to evaluate as the watermark operator
    * partition pruning shouldn't apply since there won't be useful partitions 
on an event time
    
    For project, I don't see a rule for UnaryNode anywhere. I might have missed 
it.
    
    For limit, I actually removed the rule for EventTimeWatermark that I 
originally added, since it does drop rows in some situations.
    
    So I don't think that making EventTimeWatermark subclass UnaryNode would 
avoid any of the code in this PR. I agree it seems appropriate, but it also 
seems orthogonal.


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