alamb commented on code in PR #13046:
URL: https://github.com/apache/datafusion/pull/13046#discussion_r1811262014


##########
datafusion/core/src/physical_optimizer/optimizer.rs:
##########
@@ -103,6 +103,10 @@ impl PhysicalOptimizer {
             // replacing operators with fetching variants, or adding limits
             // past operators that support limit pushdown.
             Arc::new(LimitPushdown::new()),
+            // The EliminateCommonPhysicalSubExprs rule extracts common 
physical
+            // subexpression trees into a `ProjectionExec` node under the 
actual node to
+            // calculate the common values only once.
+            Arc::new(EliminateCommonPhysicalSubexprs::new()),

Review Comment:
   I think the usecase is for systems (like Comet) that don't use the 
LogicalPlanner.
   
   That being said, I think the question of "should it be run by default" is a 
good one -- and I agree with your assertion if we can't find an example where 
this helps a plan we should probably not enable it by default.
   
   FYI @andygrove 



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