[branch-4.0](variant) Skip full footer scan when constructing VariantStatsCaculator (#62819)#63113
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csun5285 wants to merge 2 commits intoapache:branch-4.0from
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[branch-4.0](variant) Skip full footer scan when constructing VariantStatsCaculator (#62819)#63113csun5285 wants to merge 2 commits intoapache:branch-4.0from
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…culator (apache#62819) In **vertical compaction**, `SegmentWriter::init()` is called multiple times against the same writer (key columns first, then each value-column group). The footer **accumulates** across calls, so every additional `init()` re-scans an ever-larger footer — including entries from prior `init()`s that the current `column_ids` cannot address. | | Before | After | |---|---|---| | Single construction | O(footer accumulated size) | O(this init's column_ids size) | | N vertical-compaction `init()` calls | **O(N²)** | **O(N)** | Snapshot `_footer.columns_size()` **before** `_create_writers` appends new entries and pass it to `VariantStatsCaculator` as `footer_column_offset`. The constructor only walks `[offset, end)` — its own slice. When performing compaction with 10,000 columns in the variant, this method accounts for around 8% of CPU usage. After changing it to O(N), this section has disappeared from the flame graph. (cherry picked from commit ffefcae)
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…rized:: in variant_stats_calculator_test
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pick from master #62819
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