waynexia opened a new pull request, #22145:
URL: https://github.com/apache/datafusion/pull/22145
## Which issue does this PR close?
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## Rationale for this change
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This is an (agent made) PoC branch for clickbench q4 `SELECT COUNT(DISTINCT
UserID) FROM hits;`. In my test environment it's about 25% faster than `main`.
The basic idea is to skip maintaining group-id when there is no requirement
of it, e.g., for group-by cases without any accumulator.
Other findings on this experiment:
- this approach seems only work for primitive types, for other cases, like
q5 `SELECT COUNT(DISTINCT SearchPhrase) FROM hits;` where the key is string
type, the group calculation is not the critical path
- I tried another approach for the same q4 query, where I change the group
id type from 64 bits to 32 bits, it also improves a lot. My suspection is the
changed memory layout of HashTable is the source of improvement. We can
consider this further for other cases where group-id is required.
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## Are there any user-facing changes?
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