neilconway opened a new issue, #25210:
URL: https://github.com/apache/datafusion/issues/25210
### Describe the bug
`map[key]` decides which entries belong to a row by considering only the
map's offsets, not the validity buffer. However, the Arrow spec allows an
element of a map or list to be marked invalid/NULL in the validity buffer, but
still to have a non-empty offset range (such an element should be considered
NULL).
One way to get this in practice is via `nullif`, which is implemented by
modifying the input's validity buffer and not modifying offsets.
### To Reproduce
```sql
CREATE TABLE t AS
SELECT MAP {'a': 1, 'b': 2} AS m, MAP {'a': 1, 'b': 2} AS n
UNION ALL
SELECT MAP {'a': 3, 'b': 4}, MAP {'x': 9};
SELECT NULLIF(m, n) AS nulled, NULLIF(m, n)['b'] AS value FROM t;
```
Produces
```
+--------------+-------+
| nulled | value |
+--------------+-------+
| NULL | 2 |
| {a: 3, b: 4} | 4 |
+--------------+-------+
```
### Expected behavior
```
+--------------+-------+
| nulled | value |
+--------------+-------+
| NULL | NULL |
| {a: 3, b: 4} | 4 |
+--------------+-------+
```
### Additional context
_No response_
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