gabotechs commented on code in PR #19957:
URL: https://github.com/apache/datafusion/pull/19957#discussion_r2732799159


##########
datafusion/physical-expr/src/projection.rs:
##########
@@ -660,9 +660,25 @@ impl ProjectionExprs {
                     }
                 }
             } else {
-                // TODO stats: estimate more statistics from expressions
-                // (expressions should compute their statistics themselves)
-                ColumnStatistics::new_unknown()
+                // TODO: expressions should compute their own statistics

Review Comment:
   For `StatisticsProvider`, `StatisticsRegistry` and `CardinalityEstimator`. 
Do you think there is some overlap with the existing 
`ExecutionPlan::partition_statistics()` and 
`PhysicalExpr::evaluate_statistics()`? would it be appropriate to extend the 
functionality shipped in both methods? or are you considering 
`StatisticsProvider`, `StatisticsRegistry` and `CardinalityEstimator` to be a 
separate mechanism for providing statistics?



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