jonahgao commented on PR #14046:
URL: https://github.com/apache/datafusion/pull/14046#issuecomment-2579269061
> Why haven’t we been displaying `null` values as `NULL` so far? What was
the original reasoning or intention behind this decision?
I guess it's to follow PostgreSQL CLI. PostgreSQL CLI does not display null
values as well, but it always displays null elements of arrays as NULL.
```sh
$ psql
psql (16.6 (Ubuntu 16.6-0ubuntu0.24.04.1))
Type "help" for help.
psql=> select null, array[null, 1, null];
?column? | array
----------+---------------
| {NULL,1,NULL}
(1 row)
psql=> \pset null mynull
Null display is "mynull".
psql=> select null, array[null, 1, null];
?column? | array
----------+---------------
mynull | {NULL,1,NULL}
(1 row)
```
I think we can follow PostgreSQL more thoroughly, or always display null,
regardless of whether it is a list element.
The latter can be achieved by setting
[FormatOptions](https://github.com/apache/arrow-rs/blob/d0260fcffa07a4cb8650cc290ab29027a3a8e65c/arrow-cast/src/display.rs#L62).
The fix should not involve the DataFusion core; it should be related to
DataFusion CLI and sqllogictests.
Some displays of DataFusion CLI are not user-friendly, for example
```sh
DataFusion CLI v44.0.0
> select array[null, null, null,1,null];
+------------------------------------------+
| make_array(NULL,NULL,NULL,Int64(1),NULL) |
+------------------------------------------+
| [, , , 1, ] |
+------------------------------------------+
1 row(s) fetched.
Elapsed 0.006 seconds.
```
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