Mryange opened a new pull request, #69923:
URL: https://github.com/apache/doris/pull/69923
### What problem does this PR solve?
Issue Number: N/A
Problem Summary: Single-tablet OLAP queries can still introduce a gather
exchange before scalar aggregation. Root cause: the scan distribution does not
advertise that the complete input can execute in one storage-bound instance.
Add a storage-gather scan alternative while retaining the natural distribution
for join planning, and preserve its placement constraints through sorting and
fragment-merging boundaries.
The optimization requires the Nereids distribute planner, effective
parallelism of one, and query cache disabled. Sampling and special scan modes
retain the existing path.
### Release note
Add `enable_single_tablet_gather` (default true) to eliminate unnecessary
gather exchanges for eligible single-tablet OLAP queries.
### Check List (For Author)
- Test:
- [x] Unit Test: 9 focused FE tests passed before the final
legacy-planner gate was added.
- [x] Regression test: `single_tablet_gather` generated with
`-forceGenOut` and rerun successfully before that gate was added.
- [x] Manual test: FE build/Checkstyle and single-instance profile
checks passed before that gate was added; the scalar query used one fragment
without an exchange. Random distribution and selected rollup checks also passed.
- Final legacy-planner gate and its added assertion were
source-reviewed; build/tests have not been rerun for that adjustment. Multi-BE
execution remains untested.
- Behavior changed:
- [x] Yes. Eligible single-tablet queries can execute without a gather
exchange.
- Does this need documentation?
- [x] No. The session variable describes the supported scope.
### Check List (For Reviewer who merge this PR)
- [ ] Confirm the release note
- [ ] Confirm test cases
- [ ] Confirm document
- [ ] Add branch pick label
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