UBarney opened a new pull request, #19411:
URL: https://github.com/apache/datafusion/pull/19411
## Which issue does this PR close?
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- Closes #17635.
## Rationale for this change
This PR introduces a Perfect Hash Join optimization by using an array-based
direct mapping(`ArrayMap`) instead of a HashMap.
The array-based approach significantly outperforms the standard Hash Join
when the build-side keys are **_dense_** (i.e., the ratio of `count / (max -
min)` is high) or when the key range `(max - min)` is sufficiently **small**.
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The following results from the hj.rs benchmark suite. The benchmark was
executed with the optimization enabled by setting
`DATAFUSION_EXECUTION_PERFECT_HASH_JOIN_MIN_KEY_DENSITY=0.8`
```
┏━━━━━━━━━━━━━━┳━━━━━━━━━━━┳━━━━━━━━━━━━┳━━━━━━━━━━━━━━━┓
┃ Query ┃ base ┃ perfect_hj ┃ Change ┃
┡━━━━━━━━━━━━━━╇━━━━━━━━━━━╇━━━━━━━━━━━━╇━━━━━━━━━━━━━━━┩
│ QQuery 1 │ 5.15 ms │ 4.57 ms │ +1.13x faster │
│ QQuery 2 │ 132.34 ms │ 90.67 ms │ +1.46x faster │
│ QQuery 3 │ 117.28 ms │ 68.26 ms │ +1.72x faster │
│ QQuery 4 │ 142.72 ms │ 94.52 ms │ +1.51x faster │
│ QQuery 5 │ 177.38 ms │ 145.15 ms │ +1.22x faster │
│ QQuery 6 │ 77.69 ms │ 79.75 ms │ no change │
└──────────────┴───────────┴────────────┴───────────────┘
┏━━━━━━━━━━━━━━━━━━━━━━━━━━━┳━━━━━━━━━━┓
┃ Benchmark Summary ┃ ┃
┡━━━━━━━━━━━━━━━━━━━━━━━━━━━╇━━━━━━━━━━┩
│ Total Time (base) │ 652.57ms │
│ Total Time (perfect_hj) │ 482.92ms │
│ Average Time (base) │ 108.76ms │
│ Average Time (perfect_hj) │ 80.49ms │
│ Queries Faster │ 5 │
│ Queries Slower │ 0 │
│ Queries with No Change │ 1 │
│ Queries with Failure │ 0 │
└───────────────────────────┴──────────┘
```
The following results from tpch-sf10
```
┏━━━━━━━━━━━━━━┳━━━━━━━━━━━━┳━━━━━━━━━━━━┳━━━━━━━━━━━━━━━┓
┃ Query ┃ base ┃ perfect_hj ┃ Change ┃
┡━━━━━━━━━━━━━━╇━━━━━━━━━━━━╇━━━━━━━━━━━━╇━━━━━━━━━━━━━━━┩
│ QQuery 1 │ 739.66 ms │ 743.84 ms │ no change │
│ QQuery 2 │ 315.94 ms │ 317.53 ms │ no change │
│ QQuery 3 │ 655.79 ms │ 669.24 ms │ no change │
│ QQuery 4 │ 215.48 ms │ 218.79 ms │ no change │
│ QQuery 5 │ 1131.42 ms │ 1146.03 ms │ no change │
│ QQuery 6 │ 202.32 ms │ 190.83 ms │ +1.06x faster │
│ QQuery 7 │ 1734.06 ms │ 1710.50 ms │ no change │
│ QQuery 8 │ 1185.05 ms │ 1173.90 ms │ no change │
│ QQuery 9 │ 2036.76 ms │ 1994.30 ms │ no change │
│ QQuery 10 │ 907.32 ms │ 893.20 ms │ no change │
│ QQuery 11 │ 306.63 ms │ 275.46 ms │ +1.11x faster │
│ QQuery 12 │ 404.00 ms │ 381.95 ms │ +1.06x faster │
│ QQuery 13 │ 531.67 ms │ 498.58 ms │ +1.07x faster │
│ QQuery 14 │ 317.63 ms │ 303.04 ms │ no change │
│ QQuery 15 │ 602.24 ms │ 572.18 ms │ no change │
│ QQuery 16 │ 200.00 ms │ 201.68 ms │ no change │
│ QQuery 17 │ 1848.67 ms │ 1790.60 ms │ no change │
│ QQuery 18 │ 2130.63 ms │ 2179.84 ms │ no change │
│ QQuery 19 │ 501.81 ms │ 529.85 ms │ 1.06x slower │
│ QQuery 20 │ 637.91 ms │ 661.72 ms │ no change │
│ QQuery 21 │ 1882.43 ms │ 1917.10 ms │ no change │
│ QQuery 22 │ 130.68 ms │ 141.76 ms │ 1.08x slower │
└──────────────┴────────────┴────────────┴───────────────┘
┏━━━━━━━━━━━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━┓
┃ Benchmark Summary ┃ ┃
┡━━━━━━━━━━━━━━━━━━━━━━━━━━━╇━━━━━━━━━━━━┩
│ Total Time (base) │ 18618.10ms │
│ Total Time (perfect_hj) │ 18511.93ms │
│ Average Time (base) │ 846.28ms │
│ Average Time (perfect_hj) │ 841.45ms │
│ Queries Faster │ 4 │
│ Queries Slower │ 2 │
│ Queries with No Change │ 16 │
│ Queries with Failure │ 0 │
└───────────────────────────┴────────────┘
```
## What changes are included in this PR?
- During the `collect_left_input` (build) phase, we now conditionally use an
`ArrayMap` instead of a standard `JoinHashMapType`. This optimization is
triggered only when the following conditions are met:
- The join is on a single integer join key.
- The key distribution is sufficiently dense or the key range (max - min)
is small enough to justify an array-based allocation.
- The `ArrayMap` works by storing the minimum key as an offset and using a
Vec to directly map a key `k` to its build-side index via `data[k- offset]`.
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## Are these changes tested?
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## Are there any user-facing changes?
Yes, this PR introduces two new session configuration parameters to control
the behavior of the Perfect Hash Join optimization:
- `perfect_hash_join_small_build_threshold`: This parameter defines the
maximum key range (max_key - min_key) for the build side to be considered
"small." If the key range is below this threshold, the array-based join will be
triggered regardless of key density.
- `perfect_hash_join_min_key_density`: This parameter sets the minimum
density (row_count / key_range) required to enable the perfect hash join
optimization for larger key ranges
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