HappenLee opened a new pull request, #66942:
URL: https://github.com/apache/doris/pull/66942
### What problem does this PR solve?
Issue Number: None
Related PR: None
Problem Summary:
Existing `<aggregate>_state` plus `<aggregate>_union` constructs and
serializes one aggregate state for every input row before merging those states.
For queries with tens of millions of raw rows, this per-row state
materialization can erase much of the pre-aggregation benefit.
This PR adds a generic `<aggregate>_combine(...)` combinator. It feeds raw
arguments directly into the nested aggregate function and emits one compatible
`AggState` per group. For example, `avg_combine(x)` is aggregation-equivalent
to `avg_union(avg_state(x))`, while avoiding a serialized AVG state for every
source row. The resulting state remains consumable by the existing `_union` and
`_merge` combinators.
The BE wrapper preserves the nested aggregate state layout and delegates raw
accumulation, partial serialization, merge/deserialization, nullable handling,
and fixed-length serialized column creation. Nereids resolves and translates
the generic combinator and supports MV roll-up through the matching `_union`
function. Zero-argument aggregates, DISTINCT, and aggregate ORDER BY
expressions are rejected for now.
### Release note
Add the experimental `<aggregate>_combine(...)` combinator for directly
producing reusable aggregate states.
### Check List (For Author)
- Test
- [x] Regression test
- [x] Unit Test
- [ ] Manual test (add detailed scripts or steps below)
- [ ] No need to test or manual test. Explain why:
Regression coverage: `test_agg_state_avg`, including nullable input,
all-null input, grouped partial aggregation, and compatibility with
`avg_union(avg_state(...))`.
Unit coverage: `AggregateStateCombineTest` and `CombineCombinatorTest`. Full
BE and FE builds also passed; clang-tidy reported no warnings for the changed
BE files.
- Behavior changed:
- [ ] No.
- [x] Yes. Adds a new aggregate-state combinator.
- Does this need documentation?
- [x] No. The function is covered by the existing experimental AggState
feature.
- [ ] Yes.
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