[MPP/Optimizer] Join type selection is not accurate
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priority/P1
- Dominant language
- C++
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Description
Director Xu found tidb optimzer compares cop task and root task cost ; then select hash join in the circumstances of small data sets.
But when data set is large, as 10G ssb test, this problem would not appear.
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First steps
- Read the whole issue, then the project's contributing guide.
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Research direction
The issue names no files, tests, or entry points. Start by reproducing the small-data case described and trace how TiDB compares cop-task and root-task costs when selecting a hash join. Done means the optimizer chooses an appropriate join type for both small and large datasets, with regression coverage for the reported case.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- cpp
- Domain
- databases
- Issue type
- Bug
- Difficulty
- 5/5
- Estimated time
- Over a week
- Activity status
- Stale
- Clarity
- Needs clarification
- Newbie friendliness
- 25/100