[GitHub] [arrow] Dandandan commented on a change in pull request #9036: ARROW-11053: [Rust] [DataFusion] Optimize joins with dynamic capacity for output batches

[GitBox <gi...@apache.org>]

Dandandan commented on a change in pull request #9036:
URL: https://github.com/apache/arrow/pull/9036#discussion_r549556756



##########
File path: rust/datafusion/src/physical_plan/hash_join.rs
##########
@@ -531,14 +561,45 @@ impl Stream for HashJoinStream {
         self.right
             .poll_next_unpin(cx)
             .map(|maybe_batch| match maybe_batch {
-                Some(Ok(batch)) => Some(build_batch(
-                    &batch,
-                    &self.left_data,
-                    &self.on_right,
-                    &self.join_type,
-                    &self.schema,
-                )),
-                other => other,
+                Some(Ok(batch)) => {
+                    let start = Instant::now();
+                    let capacity = if self.num_output_batches == 0 {

Review comment:
       In https://github.com/apache/arrow/pull/9032 it was benchmarked to be a bit faster currently to initialize with capacity 0 than to use the correct capacity upfront. I think that is more something curious and hopefully will change, but that might be the case here too? What is the performance if we initialize with 0 everywhere?




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