alamb commented on code in PR #12302:
URL: https://github.com/apache/datafusion/pull/12302#discussion_r1743625178
##########
datafusion/physical-plan/src/sorts/merge.rs:
##########
@@ -156,12 +164,22 @@ impl<C: CursorValues> SortPreservingMergeStream<C> {
}
// try to initialize the loser tree
if self.loser_tree.is_empty() {
- // Ensure all non-exhausted streams have a cursor from which
- // rows can be pulled
- for i in 0..self.streams.partitions() {
- if let Err(e) = ready!(self.maybe_poll_stream(cx, i)) {
- self.aborted = true;
- return Poll::Ready(Some(Err(e)));
+ // Ensure all non-exhausted streams have a cursor from which rows
can be pulled
+ let remaining_partitions = self.uninitiated_partitions.clone();
+ for i in remaining_partitions {
+ match self.maybe_poll_stream(cx, i) {
+ Poll::Ready(Err(e)) => {
+ self.aborted = true;
+ return Poll::Ready(Some(Err(e)));
+ }
+ Poll::Pending => {
+ self.uninitiated_partitions.rotate_left(1);
+ cx.waker().wake_by_ref();
Review Comment:
I did some research -- see
https://github.com/synnada-ai/datafusion-upstream/pull/34/files#r1743621057
I think calling `wake_by_ref` effectively tells tokio to schedule this poll
loop again after handling other tasks, which makes sense to me (as I am not
sure how else we would signal to tokio that the merge is ready to go)
But I share your concern that this will cause some sort of performance issue
##########
datafusion/physical-plan/src/sorts/merge.rs:
##########
@@ -156,12 +164,22 @@ impl<C: CursorValues> SortPreservingMergeStream<C> {
}
// try to initialize the loser tree
if self.loser_tree.is_empty() {
- // Ensure all non-exhausted streams have a cursor from which
- // rows can be pulled
- for i in 0..self.streams.partitions() {
- if let Err(e) = ready!(self.maybe_poll_stream(cx, i)) {
Review Comment:
I coded up what I had in mind here:
https://github.com/synnada-ai/datafusion-upstream/pull/34
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