Change variable name for left_smaller_than_right

src/daft-local-execution/src/pipeline.rs

@@ -329,11 +329,12 @@ pub fn physical_plan_to_pipeline(
             let left_schema = left.schema();
             let right_schema = right.schema();
 
-            // TODO(desmond): Stats should always be materialized by this point. We should fix this on the Daft Connect side.
-            // If stats are materialized, use them to determine the build and probe sides.
+            // To determine whether to use the left or right side of a join for building a probe table, we consider:
+            // 1. Cardinality of the sides. Probe tables should be built on the smaller side.
+            // 2. Join type. Different join types have different requirements for which side can build the probe table.
             let left_stats_state = left.get_stats_state();
             let right_stats_state = right.get_stats_state();
-            let left_smaller_than_right = match (left_stats_state, right_stats_state) {
+            let build_on_left = match (left_stats_state, right_stats_state) {
                 (StatsState::Materialized(left_stats), StatsState::Materialized(right_stats)) => {
                     left_stats.approx_stats.upper_bound_bytes
                         <= right_stats.approx_stats.upper_bound_bytes
@@ -365,8 +366,8 @@ pub fn physical_plan_to_pipeline(
             // 2. Stream rows on the left until all rows have been seen.
             // 3. Finally, emit all unmatched rows from the right.
             let build_on_left = match join_type {
-                JoinType::Inner => left_smaller_than_right,
-                JoinType::Outer => left_smaller_than_right,
+                JoinType::Inner => build_on_left,
+                JoinType::Outer => build_on_left,
                 // For left outer joins, we build on right so we can stream the left side.
                 JoinType::Left => false,
                 // For right outer joins, we build on left so we can stream the right side.