cecemei opened a new pull request, #19613:
URL: https://github.com/apache/druid/pull/19613
### Description
Debugging why `matchAggregateProjection` returns null is difficult today
because there are 14 distinct rejection sites spread across the matching
pipeline, and there's no visibility into which one fired without a debugger or
global DEBUG logging.
This PR adds a `projectionTrace` query context flag. When set to true,
rejection reasons are logged at INFO level so they're visible without changing
log4j config. When unset, the same messages are emitted at DEBUG level.
Usage:
```
{
"context": { "projectionTrace": true }
}
```
Rejection messages include the projection name and the specific reason
(e.g., incompatible ordering, missing column, granularity mismatch), making it
straightforward to identify which projection was rejected and why.
<hr>
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This PR has:
- [ ] been self-reviewed.
- [ ] using the [concurrency
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- [ ] 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
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- [ ] 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
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- [ ] 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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