0xPoe commented on code in PR #16956:
URL: https://github.com/apache/datafusion/pull/16956#discussion_r2267819416


##########
datafusion/physical-plan/src/joins/hash_join.rs:
##########
@@ -892,21 +892,56 @@ impl ExecutionPlan for HashJoinExec {
     }
 
     fn partition_statistics(&self, partition: Option<usize>) -> 
Result<Statistics> {
-        if partition.is_some() {
-            return Ok(Statistics::new_unknown(&self.schema()));
+        match (partition, self.mode) {
+            // For CollectLeft mode, the left side is collected into a single 
partition,
+            // so all left partitions are available to each output partition.
+            // For the right side, we need the specific partition statistics.
+            (Some(partition), PartitionMode::CollectLeft) => {
+                let left_stats = self.left.partition_statistics(None)?;
+                let right_stats = 
self.right.partition_statistics(Some(partition))?;
+
+                let stats = estimate_join_statistics(
+                    left_stats,
+                    right_stats,
+                    self.on.clone(),
+                    &self.join_type,
+                    &self.join_schema,
+                )?;
+                Ok(stats.project(self.projection.as_ref()))
+            }
+
+            // For Partitioned mode, both sides are partitioned, so each 
output partition
+            // only has access to the corresponding partition from both sides.
+            (Some(partition), PartitionMode::Partitioned) => {
+                let left_stats = 
self.left.partition_statistics(Some(partition))?;
+                let right_stats = 
self.right.partition_statistics(Some(partition))?;
+
+                let stats = estimate_join_statistics(
+                    left_stats,
+                    right_stats,
+                    self.on.clone(),
+                    &self.join_type,
+                    &self.join_schema,
+                )?;
+                Ok(stats.project(self.projection.as_ref()))
+            }
+
+            // For Auto mode or when no specific partition is requested, fall 
back to

Review Comment:
   Oh, I see. However, it seems challenging to get the threshold here for 
conducting the same evaluation. Do you have any other ideas on how to share the 
same logic from the `JoinSelection` to determine the PartitionMode? I'm 
uncertain how to obtain the optimizer config for accessing the threshold 
settings in the current API design.



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