beliefer opened a new pull request #29800:
URL: https://github.com/apache/spark/pull/29800
### What changes were proposed in this pull request?
Spark SQL supports some window function like `NTH_VALUE`, `FIRST_VALUE` and
`LAST_VALUE`.
If we specify window frame like `UNBOUNDED PRECEDING AND CURRENT ROW` or
`UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING`, we can elimate some calculations.
For example: if we execute the SQL show below:
```
SELECT NTH_VALUE(col,
2) OVER(ORDER BY rank UNBOUNDED PRECEDING
AND CURRENT ROW)
FROM tab;
```
The output for row number greater than 1, return the fixed value. otherwise,
return null. So we just calculate the value once and notice whether the row
number less than 2.
`UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING` is simpler.
### Why are the changes needed?
Improve the performance for `NTH_VALUE`, `FIRST_VALUE` and `LAST_VALUE`.
### Does this PR introduce _any_ user-facing change?
'No'.
### How was this patch tested?
Jenkins test.
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