feat: perf small_union and small_symmetric_difference

[KKould]

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Summary

small_union and small_symmetric_difference functions were rewritten for in-place calculations to avoid frequent creation of HybridBitmap.

main

Benchmark	Size	Fastest	Slowest	Median	Mean	Samples	Iters
bitmap_or_agg	100000	8.182 ms	8.462 ms	8.311 ms	8.316 ms	61	61
	1000000	82.87 ms	83.85 ms	83.49 ms	83.38 ms	6	6
bitmap_union	100000	7.907 ms	8.47 ms	7.995 ms	8.066 ms	62	62
	1000000	79.89 ms	80.81 ms	80.38 ms	80.34 ms	7	7
bitmap_union_disjoint	100000	7.094 ms	7.483 ms	7.209 ms	7.222 ms	70	70
	1000000	84.33 ms	85.63 ms	84.88 ms	84.92 ms	6	6
bitmap_xor_agg	100000	38.36 ms	39.99 ms	38.59 ms	38.65 ms	13	13
	1000000	474.3 ms	475.3 ms	474.8 ms	474.8 ms	2	2
bitmap_xor_agg_overlap	100000	6.687 ms	6.982 ms	6.813 ms	6.826 ms	74	74
	1000000	68.16 ms	69.17 ms	68.61 ms	68.64 ms	8	8

pr

Benchmark	Size	Fastest	Slowest	Median	Mean	Samples	Iters
bitmap_or_agg	100000	5.816 ms	6.177 ms	5.946 ms	5.954 ms	84	84
	1000000	59.36 ms	59.97 ms	59.67 ms	59.69 ms	9	9
bitmap_union	100000	5.805 ms	6.079 ms	5.923 ms	5.926 ms	85	85
	1000000	59.21 ms	59.98 ms	59.35 ms	59.44 ms	9	9
bitmap_union_disjoint	100000	6.932 ms	7.303 ms	7.093 ms	7.095 ms	71	71
	1000000	81.9 ms	82.79 ms	82.4 ms	82.35 ms	7	7
bitmap_xor_agg	100000	37.38 ms	37.93 ms	37.64 ms	37.65 ms	14	14
	1000000	469.9 ms	475 ms	472.5 ms	472.5 ms	2	2
bitmap_xor_agg_overlap	100000	5.163 ms	5.407 ms	5.261 ms	5.267 ms	95	95
	1000000	52.85 ms	53.88 ms	53.35 ms	53.34 ms	10	10

Tests

• Unit Test
• Logic Test
• Benchmark Test
• No Test - Explain why

Type of change

• Bug Fix (non-breaking change which fixes an issue)
• New Feature (non-breaking change which adds functionality)
• Breaking Change (fix or feature that could cause existing functionality not to work as expected)
• Documentation Update
• Refactoring
• Performance Improvement
• Other (please describe):

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This change is 

[KKould]

@codex review

[Copilot]

Pull request overview

This PR improves performance of bitmap operations by optimizing small_union and small_symmetric_difference functions to perform in-place calculations instead of creating new HybridBitmap instances. Benchmark results show performance improvements of approximately 25-30% for bitmap union operations and 20-25% for XOR operations on overlapping data.

Key changes:

• Refactored small_union and small_symmetric_difference to use in-place algorithms that work backwards through the data
• Optimized decode_small_bitmap to use ptr::read_unaligned for better deserialization performance
• Added comprehensive benchmark tests for various bitmap aggregate functions

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/common/io/src/bitmap.rs	Refactored small_union and small_symmetric_difference to in-place operations; optimized decode_small_bitmap; updated tests; added performance comments
src/query/functions/benches/bench.rs	Reorganized imports into bitmap module; added helper functions and comprehensive benchmark tests for bitmap operations

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