## What changes were proposed in this pull request?
Time windowing in Spark currently performs an Expand + Filter, because there is no way to guarantee the amount of windows a timestamp will fall in, in the general case. However, for tumbling windows, a record is guaranteed to fall into a single bucket. In this case, doubling the number of records with Expand is wasteful, and can be improved by using a simple Projection instead.
Benchmarks show that we get an order of magnitude performance improvement after this patch.
## How was this patch tested?
Existing unit tests. Benchmarked using the following code:
```scala
import org.apache.spark.sql.functions._
spark.time {
spark.range(numRecords)
.select(from_unixtime((current_timestamp().cast("long") * 1000 + 'id / 1000) / 1000) as 'time)
.select(window('time, "10 seconds"))
.count()
}
```
Setup:
- 1 c3.2xlarge worker (8 cores)
1 B rows ran in 287 seconds after this optimization. I didn't wait for it to finish without the optimization. Shows about 5x improvement for large number of records.
Author: Burak Yavuz <brkyvz@gmail.com>
Closes#18364 from brkyvz/opt-tumble.
5282bae040
