feat: Add selectivity metric to NestedLoopJoinExec for EXPLAIN ANALYZE (apache#18481)

## Which issue does this PR close?

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- Closes apache#18407

## Rationale for this change

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in the issue then this section is not needed.
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This new metric will give the user better visibility to see what portion
of the possibilities is actually being matched.

## What changes are included in this PR?

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is sometimes worth providing a summary of the individual changes in this
PR.
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Add selectivity metric to NestedLoopJoinExec for EXPLAIN ANALYZE

## Are these changes tested?

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Added tests

## Are there any user-facing changes?

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Yes, new metric in explain analyze
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---------

Co-authored-by: Yongting You <2010youy01@gmail.com>

Author: petern48

datafusion/core/tests/sql/explain_analyze.rs

@@ -1110,3 +1110,33 @@ async fn csv_explain_analyze_with_statistics() {
         ", statistics=[Rows=Absent, Bytes=Absent, [(Col[0]:)]]"
     );
 }
+
+#[tokio::test]
+async fn nested_loop_join_selectivity() {
+    for (join_type, expected_selectivity) in [
+        ("INNER", "1% (1/100)"),
+        ("LEFT", "10% (10/100)"),
+        ("RIGHT", "10% (10/100)"),
+        // 1 match + 9 left + 9 right = 19
+        ("FULL", "19% (19/100)"),
+    ] {
+        let ctx = SessionContext::new();
+        let sql = format!(
+            "EXPLAIN ANALYZE SELECT * \
+                FROM generate_series(1, 10) as t1(a) \
+                {join_type} JOIN generate_series(1, 10) as t2(b) \
+                ON (t1.a + t2.b) = 20"
+        );
+
+        let actual = execute_to_batches(&ctx, sql.as_str()).await;
+        let formatted = arrow::util::pretty::pretty_format_batches(&actual)
+            .unwrap()
+            .to_string();
+
+        assert_metrics!(
+            &formatted,
+            "NestedLoopJoinExec",
+            &format!("selectivity={expected_selectivity}")
+        );
+    }
+}

datafusion/physical-plan/src/joins/nested_loop_join.rs

@@ -36,7 +36,9 @@ use crate::joins::utils::{
     OnceAsync, OnceFut,
 };
 use crate::joins::SharedBitmapBuilder;
-use crate::metrics::{Count, ExecutionPlanMetricsSet, MetricsSet};
+use crate::metrics::{
+    Count, ExecutionPlanMetricsSet, MetricBuilder, MetricType, MetricsSet, RatioMetrics,
+};
 use crate::projection::{
     try_embed_projection, try_pushdown_through_join, EmbeddedProjection, JoinData,
     ProjectionExec,
@@ -496,7 +498,7 @@ impl ExecutionPlan for NestedLoopJoinExec {
             );
         }
 
-        let join_metrics = BuildProbeJoinMetrics::new(partition, &self.metrics);
+        let metrics = NestedLoopJoinMetrics::new(&self.metrics, partition);
 
         // Initialization reservation for load of inner table
         let load_reservation =
@@ -508,7 +510,7 @@ impl ExecutionPlan for NestedLoopJoinExec {
 
             Ok(collect_left_input(
                 stream,
-                join_metrics.clone(),
+                metrics.join_metrics.clone(),
                 load_reservation,
                 need_produce_result_in_final(self.join_type),
                 self.right().output_partitioning().partition_count(),
@@ -535,7 +537,7 @@ impl ExecutionPlan for NestedLoopJoinExec {
             probe_side_data,
             build_side_data,
             column_indices_after_projection,
-            join_metrics,
+            metrics,
             batch_size,
         )))
     }
@@ -749,7 +751,7 @@ pub(crate) struct NestedLoopJoinStream {
     /// the join filter (e.g., `JOIN ON (b+c)>0`).
     pub(crate) column_indices: Vec<ColumnIndex>,
     /// Join execution metrics
-    pub(crate) join_metrics: BuildProbeJoinMetrics,
+    pub(crate) metrics: NestedLoopJoinMetrics,
 
     /// `batch_size` from configuration
     batch_size: usize,
@@ -794,6 +796,24 @@ pub(crate) struct NestedLoopJoinStream {
     current_right_batch_matched: Option<BooleanArray>,
 }
 
+pub(crate) struct NestedLoopJoinMetrics {
+    /// Join execution metrics
+    pub(crate) join_metrics: BuildProbeJoinMetrics,
+    /// Selectivity of the join: output_rows / (left_rows * right_rows)
+    pub(crate) selectivity: RatioMetrics,
+}
+
+impl NestedLoopJoinMetrics {
+    pub fn new(metrics: &ExecutionPlanMetricsSet, partition: usize) -> Self {
+        Self {
+            join_metrics: BuildProbeJoinMetrics::new(partition, metrics),
+            selectivity: MetricBuilder::new(metrics)
+                .with_type(MetricType::SUMMARY)
+                .ratio_metrics("selectivity", partition),
+        }
+    }
+}
+
 impl Stream for NestedLoopJoinStream {
     type Item = Result<RecordBatch>;
 
@@ -844,7 +864,7 @@ impl Stream for NestedLoopJoinStream {
                     // -side batches), related metrics except build time will be
                     // updated.
                     // stop on drop
-                    let build_metric = self.join_metrics.build_time.clone();
+                    let build_metric = self.metrics.join_metrics.build_time.clone();
                     let _build_timer = build_metric.timer();
 
                     match self.handle_buffering_left(cx) {
@@ -878,7 +898,7 @@ impl Stream for NestedLoopJoinStream {
                 NLJState::FetchingRight => {
                     debug!("[NLJState] Entering: {:?}", self.state);
                     // stop on drop
-                    let join_metric = self.join_metrics.join_time.clone();
+                    let join_metric = self.metrics.join_metrics.join_time.clone();
                     let _join_timer = join_metric.timer();
 
                     match self.handle_fetching_right(cx) {
@@ -905,13 +925,13 @@ impl Stream for NestedLoopJoinStream {
                     debug!("[NLJState] Entering: {:?}", self.state);
 
                     // stop on drop
-                    let join_metric = self.join_metrics.join_time.clone();
+                    let join_metric = self.metrics.join_metrics.join_time.clone();
                     let _join_timer = join_metric.timer();
 
                     match self.handle_probe_right() {
                         ControlFlow::Continue(()) => continue,
                         ControlFlow::Break(poll) => {
-                            return self.join_metrics.baseline.record_poll(poll)
+                            return self.metrics.join_metrics.baseline.record_poll(poll)
                         }
                     }
                 }
@@ -926,13 +946,13 @@ impl Stream for NestedLoopJoinStream {
                     debug!("[NLJState] Entering: {:?}", self.state);
 
                     // stop on drop
-                    let join_metric = self.join_metrics.join_time.clone();
+                    let join_metric = self.metrics.join_metrics.join_time.clone();
                     let _join_timer = join_metric.timer();
 
                     match self.handle_emit_right_unmatched() {
                         ControlFlow::Continue(()) => continue,
                         ControlFlow::Break(poll) => {
-                            return self.join_metrics.baseline.record_poll(poll)
+                            return self.metrics.join_metrics.baseline.record_poll(poll)
                         }
                     }
                 }
@@ -956,13 +976,13 @@ impl Stream for NestedLoopJoinStream {
                     debug!("[NLJState] Entering: {:?}", self.state);
 
                     // stop on drop
-                    let join_metric = self.join_metrics.join_time.clone();
+                    let join_metric = self.metrics.join_metrics.join_time.clone();
                     let _join_timer = join_metric.timer();
 
                     match self.handle_emit_left_unmatched() {
                         ControlFlow::Continue(()) => continue,
                         ControlFlow::Break(poll) => {
-                            return self.join_metrics.baseline.record_poll(poll)
+                            return self.metrics.join_metrics.baseline.record_poll(poll)
                         }
                     }
                 }
@@ -972,13 +992,13 @@ impl Stream for NestedLoopJoinStream {
                     debug!("[NLJState] Entering: {:?}", self.state);
 
                     // stop on drop
-                    let join_metric = self.join_metrics.join_time.clone();
+                    let join_metric = self.metrics.join_metrics.join_time.clone();
                     let _join_timer = join_metric.timer();
                     // counting it in join timer due to there might be some
                     // final resout batches to output in this state
 
                     let poll = self.handle_done();
-                    return self.join_metrics.baseline.record_poll(poll);
+                    return self.metrics.join_metrics.baseline.record_poll(poll);
                 }
             }
         }
@@ -1000,7 +1020,7 @@ impl NestedLoopJoinStream {
         right_data: SendableRecordBatchStream,
         left_data: OnceFut<JoinLeftData>,
         column_indices: Vec<ColumnIndex>,
-        join_metrics: BuildProbeJoinMetrics,
+        metrics: NestedLoopJoinMetrics,
         batch_size: usize,
     ) -> Self {
         Self {
@@ -1010,7 +1030,7 @@ impl NestedLoopJoinStream {
             right_data,
             column_indices,
             left_data,
-            join_metrics,
+            metrics,
             buffered_left_data: None,
             output_buffer: Box::new(BatchCoalescer::new(schema, batch_size)),
             batch_size,
@@ -1057,8 +1077,8 @@ impl NestedLoopJoinStream {
                 Some(Ok(right_batch)) => {
                     // Update metrics
                     let right_batch_size = right_batch.num_rows();
-                    self.join_metrics.input_rows.add(right_batch_size);
-                    self.join_metrics.input_batches.add(1);
+                    self.metrics.join_metrics.input_rows.add(right_batch_size);
+                    self.metrics.join_metrics.input_batches.add(1);
 
                     // Skip the empty batch
                     if right_batch_size == 0 {
@@ -1108,6 +1128,17 @@ impl NestedLoopJoinStream {
             Ok(false) => {
                 // Left exhausted, transition to FetchingRight
                 self.left_probe_idx = 0;
+
+                // Selectivity Metric: Update total possibilities for the batch (left_rows * right_rows)
+                // If memory-limited execution is implemented, this logic must be updated accordingly.
+                if let (Ok(left_data), Some(right_batch)) =
+                    (self.get_left_data(), self.current_right_batch.as_ref())
+                {
+                    let left_rows = left_data.batch().num_rows();
+                    let right_rows = right_batch.num_rows();
+                    self.metrics.selectivity.add_total(left_rows * right_rows);
+                }
+
                 if self.should_track_unmatched_right {
                     debug_assert!(
                         self.current_right_batch_matched.is_some(),
@@ -1138,7 +1169,6 @@ impl NestedLoopJoinStream {
                 && self.current_right_batch.is_some(),
             "This state is yielding output for unmatched rows in the current right batch, so both the right batch and the bitmap must be present"
         );
-
         // Construct the result batch for unmatched right rows using a utility function
         match self.process_right_unmatched() {
             Ok(Some(batch)) => {
@@ -1205,7 +1235,7 @@ impl NestedLoopJoinStream {
         // should be with the expected schema for this operator
         if !self.handled_empty_output {
             let zero_count = Count::new();
-            if *self.join_metrics.baseline.output_rows() == zero_count {
+            if *self.metrics.join_metrics.baseline.output_rows() == zero_count {
                 let empty_batch = RecordBatch::new_empty(Arc::clone(&self.output_schema));
                 self.handled_empty_output = true;
                 return Poll::Ready(Some(Ok(empty_batch)));
@@ -1455,7 +1485,11 @@ impl NestedLoopJoinStream {
             if let Some(batch) = self.output_buffer.next_completed_batch() {
                 // HACK: this is not part of `BaselineMetrics` yet, so update it
                 // manually
-                self.join_metrics.output_batches.add(1);
+                self.metrics.join_metrics.output_batches.add(1);
+
+                // Update output rows for selectivity metric
+                let output_rows = batch.num_rows();
+                self.metrics.selectivity.add_part(output_rows);
 
                 return Some(Poll::Ready(Some(Ok(batch))));
             }