feat: Implement partition_statistics API for SortMergeJoinExec (#19567)

## Which issue does this PR close?

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- Part of #15873.

## Rationale for this change

- The goal is to replace the old `statistics` API with the new
`partition_statistics`


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## What changes are included in this PR?


- implemented the partition_statistics API for SortMergeJoinExec
- Removed the special case that returned unknown statistics for specific
partitions.
- Added `test_partition_statistics` to verify both aggregate and
specific partition statistics work correctly

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## Are these changes tested?

- Addes new tests

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## Are there any user-facing changes?

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Author: kumarUjjawal

datafusion/physical-plan/src/joins/sort_merge_join/exec.rs

@@ -524,15 +524,20 @@ impl ExecutionPlan for SortMergeJoinExec {
     }
 
     fn partition_statistics(&self, partition: Option<usize>) -> Result<Statistics> {
-        if partition.is_some() {
-            return Ok(Statistics::new_unknown(&self.schema()));
-        }
+        // SortMergeJoinExec uses symmetric hash partitioning where both left and right
+        // inputs are hash-partitioned on the join keys. This means partition `i` of the
+        // left input is joined with partition `i` of the right input.
+        //
+        // Therefore, partition-specific statistics can be computed by getting the
+        // partition-specific statistics from both children and combining them via
+        // `estimate_join_statistics`.
+        //
         // TODO stats: it is not possible in general to know the output size of joins
         // There are some special cases though, for example:
         // - `A LEFT JOIN B ON A.col=B.col` with `COUNT_DISTINCT(B.col)=COUNT(B.col)`
         estimate_join_statistics(
-            self.left.partition_statistics(None)?,
-            self.right.partition_statistics(None)?,
+            self.left.partition_statistics(partition)?,
+            self.right.partition_statistics(partition)?,
             &self.on,
             &self.join_type,
             &self.schema,

datafusion/physical-plan/src/joins/sort_merge_join/tests.rs

@@ -3030,6 +3030,80 @@ async fn test_anti_join_filtered_mask() -> Result<()> {
     Ok(())
 }
 
+#[test]
+fn test_partition_statistics() -> Result<()> {
+    use crate::ExecutionPlan;
+    use datafusion_common::stats::Precision;
+
+    let left = build_table(
+        ("a1", &vec![1, 2, 3]),
+        ("b1", &vec![4, 5, 5]),
+        ("c1", &vec![7, 8, 9]),
+    );
+    let right = build_table(
+        ("a2", &vec![10, 20, 30]),
+        ("b1", &vec![4, 5, 6]),
+        ("c2", &vec![70, 80, 90]),
+    );
+
+    let on = vec![(
+        Arc::new(Column::new_with_schema("b1", &left.schema())?) as _,
+        Arc::new(Column::new_with_schema("b1", &right.schema())?) as _,
+    )];
+
+    // Test different join types to ensure partition_statistics works correctly for all
+    let join_types = vec![
+        (Inner, 6),     // left cols + right cols
+        (Left, 6),      // left cols + right cols
+        (Right, 6),     // left cols + right cols
+        (Full, 6),      // left cols + right cols
+        (LeftSemi, 3),  // only left cols
+        (LeftAnti, 3),  // only left cols
+        (RightSemi, 3), // only right cols
+        (RightAnti, 3), // only right cols
+    ];
+
+    for (join_type, expected_cols) in join_types {
+        let join_exec =
+            join(Arc::clone(&left), Arc::clone(&right), on.clone(), join_type)?;
+
+        // Test aggregate statistics (partition = None)
+        // Should return meaningful statistics computed from both inputs
+        let stats = join_exec.partition_statistics(None)?;
+        assert_eq!(
+            stats.column_statistics.len(),
+            expected_cols,
+            "Aggregate stats column count failed for {join_type:?}"
+        );
+        // Verify that aggregate statistics have a meaningful num_rows (not Absent)
+        assert!(
+            !matches!(stats.num_rows, Precision::Absent),
+            "Aggregate stats should have meaningful num_rows for {join_type:?}, got {:?}",
+            stats.num_rows
+        );
+
+        // Test partition-specific statistics (partition = Some(0))
+        // The implementation correctly passes `partition` to children.
+        // Since the child TestMemoryExec returns unknown stats for specific partitions,
+        // the join output will also have Absent num_rows. This is expected behavior
+        // as the statistics depend on what the children can provide.
+        let partition_stats = join_exec.partition_statistics(Some(0))?;
+        assert_eq!(
+            partition_stats.column_statistics.len(),
+            expected_cols,
+            "Partition stats column count failed for {join_type:?}"
+        );
+        // When children return unknown stats, the join's partition stats will be Absent
+        assert!(
+            matches!(partition_stats.num_rows, Precision::Absent),
+            "Partition stats should have Absent num_rows when children return unknown for {join_type:?}, got {:?}",
+            partition_stats.num_rows
+        );
+    }
+
+    Ok(())
+}
+
 /// Returns the column names on the schema
 fn columns(schema: &Schema) -> Vec<String> {
     schema.fields().iter().map(|f| f.name().clone()).collect()