Support PreHashAgg
Open
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type/enhancement
- Dominant language
- C++
- Stars
- 1k
- Forks
- 423
- Avg merge
- 1d 15h
- Merged PRs (30d)
- 24
Description
Enhancement
For two-staged HashAgg, we can support PreHashAgg the first stage HashAgg. It means when the NDV is large(a.k.a. most hash keys are different), there is no need to do hash agg computation, just do pass through and send it to the second stage HashAgg.
This can improve performance when NDV is large.
Contributor guide
No contributing guide indexed for this repository
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Start by locating the two-stage HashAgg implementation and the first-stage execution path in TiFlash. Read how HashAgg handles high-NDV inputs and how results are passed to the second stage. Done means PreHashAgg is supported for this case and the large-NDV performance improvement is demonstrated.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- cpp
- Domain
- databases
- Issue type
- Feature
- Difficulty
- 5/5
- Estimated time
- Over a week
- Activity status
- Stale
- Clarity
- Needs clarification
- Newbie friendliness
- 25/100