peter-toth opened a new pull request, #42559:
URL: https://github.com/apache/spark/pull/42559
### What changes were proposed in this pull request?
This PR fixes `percentile_disc()` function as currently it returns inforrect
results in some cases. E.g.:
```
SELECT
percentile_disc(0.0) WITHIN GROUP (ORDER BY a) as p0,
percentile_disc(0.1) WITHIN GROUP (ORDER BY a) as p1,
percentile_disc(0.2) WITHIN GROUP (ORDER BY a) as p2,
percentile_disc(0.3) WITHIN GROUP (ORDER BY a) as p3,
percentile_disc(0.4) WITHIN GROUP (ORDER BY a) as p4,
percentile_disc(0.5) WITHIN GROUP (ORDER BY a) as p5,
percentile_disc(0.6) WITHIN GROUP (ORDER BY a) as p6,
percentile_disc(0.7) WITHIN GROUP (ORDER BY a) as p7,
percentile_disc(0.8) WITHIN GROUP (ORDER BY a) as p8,
percentile_disc(0.9) WITHIN GROUP (ORDER BY a) as p9,
percentile_disc(1.0) WITHIN GROUP (ORDER BY a) as p10
FROM VALUES (0), (1), (2), (3), (4) AS v(a)
```
currently returns:
```
+---+---+---+---+---+---+---+---+---+---+---+
| p0| p1| p2| p3| p4| p5| p6| p7| p8| p9|p10|
+---+---+---+---+---+---+---+---+---+---+---+
|0.0|0.0|0.0|1.0|1.0|2.0|2.0|2.0|3.0|3.0|4.0|
+---+---+---+---+---+---+---+---+---+---+---+
```
but after this PR it returns the correct:
```
+---+---+---+---+---+---+---+---+---+---+---+
| p0| p1| p2| p3| p4| p5| p6| p7| p8| p9|p10|
+---+---+---+---+---+---+---+---+---+---+---+
|0.0|0.0|0.0|1.0|1.0|2.0|2.0|3.0|3.0|4.0|4.0|
+---+---+---+---+---+---+---+---+---+---+---+
```
### Why are the changes needed?
Bugfix.
### Does this PR introduce _any_ user-facing change?
Yes, fixes a correctness bug.
### How was this patch tested?
Added new UTs.
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