jayzhan211 opened a new issue, #26470:
URL: https://github.com/apache/datafusion/issues/26470
### Describe the bug
When a sliding window frame (one that does not start at `UNBOUNDED
PRECEDING`) contains only NULL values, `bit_xor` returns `0` instead of `NULL`.
This only happens after the frame has already seen a non-NULL value. The same
aggregate over a whole partition returns `NULL` correctly.
### To Reproduce
```sql
SELECT x,
bit_xor(x) OVER (ORDER BY i ROWS BETWEEN CURRENT ROW AND CURRENT ROW) AS
sliding,
bit_xor(x) OVER (PARTITION BY i) AS whole
FROM (VALUES (1, 5), (2, NULL), (3, 6)) t(i, x)
ORDER BY i;
```
```
+------+---------+-------+
| x | sliding | whole |
+------+---------+-------+
| 5 | 5 | 5 |
| NULL | 0 | NULL |
| 6 | 6 | 6 |
+------+---------+-------+
```
### Expected behavior
`sliding` is `NULL` on the second row, as in `whole`. DuckDB returns `NULL`
there.
### Additional context
`BitXorAccumulator` keeps an `Option` that becomes `Some` on the first
non-NULL value. `retract_batch` XORs the leaving values back out, but the state
never goes back to `None`. So once every non-NULL value has left the frame,
`evaluate` returns `Some(0)`:
https://github.com/apache/datafusion/blob/3470bd0fbc/datafusion/functions-aggregate/src/bit_and_or_xor.rs#L455-L466
One way to fix it is to count the non-NULL values in the frame and return
NULL when the count is zero. `avg` had the same kind of problem in #22139.
### Willingness to contribute
Feel free to take it!
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