gabotechs opened a new pull request, #23720:
URL: https://github.com/apache/datafusion/pull/23720
## Which issue does this PR close?
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- Closes #11680.
## Rationale for this change
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During partial + final aggregations, in order to accumulate + repartition
data cross partitions, a hash is computed over the grouping expressions.
This hash is computed 3 times:
```rust
Aggregate(final): 3rd time hash computed
Repartition: 2nd time hash computed
Aggregate(partial): 1st time hash computed
```
And depending on what's getting hashed, it can be an expensive operation
(e.g., hashing arbitrary length strings).
This PR introduces a mechanism that allows reusing the hashes across
different operators, by propagating them under an internal column in the schema:
```rust
Aggregate(final): precomputed
Repartition: precomputed
Aggregate(partial): 1st time hash computed
```
:warning: needs https://github.com/apache/datafusion/pull/23719 first
## What changes are included in this PR?
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Introduces to core structs that aim to set the foundations of hash reuse
across operators:
- `ExpressionHasher`: handles hashing columns based on expressions,
maintaining an internal buffer for reusing allocations, stamping the hashes
into internal columns so that they can cross operator boundaries, and
retrieving those hashes from the internal columns if present.
- `GroupHashTracker`: during groupings, it keeps track of the per-group
hashes so that operators that aggregate can emit them as an additional internal
column for reuse purposes.
These two structs are used across all the aggregation implementations in the
codebase for reusing hashes.
Additionally, some metrics for tracking how many hashes where computed VS
how many hashes where reused are also added to operators that need to deal with
hashes
## Are these changes tested?
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Yes, by existing and new tests.
## Are there any user-facing changes?
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No, this should be transparent to users.
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