alamb commented on a change in pull request #7958:
URL: https://github.com/apache/arrow/pull/7958#discussion_r470174139
##########
File path: rust/datafusion/src/execution/physical_plan/limit.rs
##########
@@ -81,39 +82,40 @@ struct LimitPartition {
impl Partition for LimitPartition {
fn execute(&self) -> Result<Arc<Mutex<dyn RecordBatchReader + Send +
Sync>>> {
- // collect up to "limit" rows on each partition
- let threads: Vec<JoinHandle<Result<Vec<RecordBatch>>>> = self
+ // apply limit in parallel across all input partitions
+ let local_limit = self
.partitions
.iter()
.map(|p| {
- let p = p.clone();
- let limit = self.limit;
- thread::spawn(move || {
- let it = p.execute()?;
- collect_with_limit(it, limit)
- })
+ Arc::new(LocalLimitExec::new(
+ p.clone(),
+ self.schema.clone(),
+ self.limit,
+ )) as Arc<dyn Partition>
})
.collect();
- // combine the results from each thread, up to the limit
+ // limit needs to collapse inputs down to a single partition
+ let merge = MergeExec::new(self.schema.clone(), local_limit);
+ // MergeExec has a single partition
+ let it = merge.partitions()?[0].execute()?;
Review comment:
I wonder if we should actually assert / check here that MergeExec does
indeed produce only a single partition? Perhaps by adding an
`assert_eq!(partitions.len(), 1)` type check; I am thinking of the case in the
future if/when someone changes `MergeExec` to work differently and produce
multiple partitions or something and then spends time tracking down some tricky
wrong results bug
##########
File path: rust/datafusion/src/execution/physical_plan/limit.rs
##########
@@ -81,39 +82,40 @@ struct LimitPartition {
impl Partition for LimitPartition {
fn execute(&self) -> Result<Arc<Mutex<dyn RecordBatchReader + Send +
Sync>>> {
- // collect up to "limit" rows on each partition
- let threads: Vec<JoinHandle<Result<Vec<RecordBatch>>>> = self
+ // apply limit in parallel across all input partitions
+ let local_limit = self
.partitions
.iter()
.map(|p| {
- let p = p.clone();
- let limit = self.limit;
- thread::spawn(move || {
- let it = p.execute()?;
- collect_with_limit(it, limit)
- })
+ Arc::new(LocalLimitExec::new(
Review comment:
This "pick the top N from each partition and then pick the top N from
the merged result" is an example of operator pushdown -- it is cool, and I
wonder if there would be some way to represent this in the planner (rather than
execution time).
I am thinking, for example, of pushing the limit down past `Projection`s for
example.
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