clintropolis opened a new pull request, #16723:
URL: https://github.com/apache/druid/pull/16723
### Description
Fixes of #16690, but tries to instead fix the attempted behavior of coercing
`[]` into `[null]` for `UnnestDimensionCursor` to be less disruptive while we
determine what the correct behavior for unnest on `[]` for a multi-value string
actually is. For regular arrays, `[]` is skipped per the standard, but for MVDs
it looks like the `UnnestDimensionCursor` was trying to match group-by
behavior. Idk if matching group-by behavior is correct, but the way it was
doing it was totally wrong if `null` wasn't id 0 in the dictionary (or if the
dictionary had no nulls at all). This PR fixes that by using the idLookup of
the underlying dimension selector to lookup the null value id, and if present,
using that, and if not, creating a synthetic nullId at position 0 and
offsetting all of the real dictionary ids by 1 as we have done in some other
places for other reasons (e.g. adapters for default value mode when segments
were written in SQL compatible null handling mode).
<hr>
This PR has:
- [ ] been self-reviewed.
- [ ] using the [concurrency
checklist](https://github.com/apache/druid/blob/master/dev/code-review/concurrency.md)
(Remove this item if the PR doesn't have any relation to concurrency.)
- [ ] added documentation for new or modified features or behaviors.
- [ ] a release note entry in the PR description.
- [ ] added Javadocs for most classes and all non-trivial methods. Linked
related entities via Javadoc links.
- [ ] added or updated version, license, or notice information in
[licenses.yaml](https://github.com/apache/druid/blob/master/dev/license.md)
- [ ] added comments explaining the "why" and the intent of the code
wherever would not be obvious for an unfamiliar reader.
- [ ] added unit tests or modified existing tests to cover new code paths,
ensuring the threshold for [code
coverage](https://github.com/apache/druid/blob/master/dev/code-review/code-coverage.md)
is met.
- [ ] added integration tests.
- [ ] been tested in a test Druid cluster.
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