clintropolis opened a new pull request #10219:
URL: https://github.com/apache/druid/pull/10219
### Description
This PR adds a computed `isNullable` property to `ColumnCapabilities` in
service of allowing segment metadata queries to accurately report which columns
have null values via the updated `ColumnAnalysis`.
In a follow-up PR, with this information in place, `RowSignature` can be
expanded to also track the nullable columns, allowing the SQL planner to more
correctly plan a query based on whether or not a column has nulls instead of
tying it to `druid.generic.useDefaultValueForNull`.
`INFORMATION_SCHEMA.COLUMNS` will also correctly set `IS_NULLABLE`. This change
is moderately disruptive to tests however, so I decided to split it out to keep
things easy to review.
Other follow-up work to do after this PR is modifying the query engines to
take advantage of this new information by examining the column capabilities
instead of using the global value of `druid.generic.useDefaultValueForNull`,
for example to decide if `NullableNumericAggregator` needs to be employed for
numeric primitive aggregations to handle null or not.
Longer term, I would like to remove/repurpose
`druid.generic.useDefaultValueForNull` and make Druid always be in SQL
compatible mode, and at ingestion time just specify on the dimension specs
which columns can and cannot have null values and what the default value should
be (like other databases). I expect a setting will remain once the dust is
settled on how to handle `null` and `''` for legacy string columns created in
default mode, but I think it should be possible to transition everywhere else
to just always operate in SQL compatible null handling mode with little
disruption.
<hr>
This PR has:
- [x] 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.
- [ ] added Javadocs for most classes and all non-trivial methods. Linked
related entities via Javadoc links.
- [ ] added comments explaining the "why" and the intent of the code
wherever would not be obvious for an unfamiliar reader.
- [x] 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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