fix: Concatenate chunk bitmaps in a single pass - #223
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concatManyColumns concatenates the data in one VB.concat, but the
nullable path folds the per-chunk bitmaps pairwise and carries the
running length by concatenating the data vectors again at every step:
concatBms ((b1, v1) : (b2, v2) : rest') =
let merged = go b1 (VB.length v1) b2 (VB.length v2)
in concatBms ((merged, v1 <> v2) : rest')
That reinstates the accumulator recopy the function exists to avoid.
Allocation grows 15x for 4x the rows where linear would be 4x.
The second case is an oracle rather than a repro: uneven chunks that
straddle byte boundaries must keep every value and null position, which
is the property the new bitmap writes have to preserve.
The nullable path folded the per-chunk bitmaps pairwise and concatenated the data vectors on every step just to carry a running length, so it recopied the accumulator exactly the way the function's one-pass design sets out to avoid. Take the lengths up front and splice every chunk's bits into one preallocated destination with concatValidity, the byte-level splice the Bitmap module already has. For 160 chunks of 10k nullable rows: allocation growth drops from 15x to 4x per 4x the rows, and forcing the bitmap from 449ms to 7.9ms.
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The nullable path of
concatManyColumnsfolded the per-chunk bitmaps pairwise and re-concatenated the data vectors at each step just to carry a running length, so it recopied the accumulator — the thing the function exists to avoid. UseconcatValidityinstead.160 chunks of 10k nullable rows: forcing the bitmap goes from 449ms to 7.9ms. The test pins values and null positions across uneven chunks and checks allocation grows linearly.