perf: Add bulk NULL-aware string builders, use in `lower` and `upper` by neilconway · Pull Request #21789 · apache/datafusion

neilconway · 2026-04-22T16:42:01Z

Which issue does this PR close?

Part of Cleanup / overhaul StringViewArrayBuilder and related types #21684

Rationale for this change

Introduce three new string array builders with bulk null tracking:

StringArrayBuilder (Utf8)
LargeStringArrayBuilder (LargeUtf8)
StringViewArrayBuilder (Utf8View)

Each builder has the following API:

append_value(&str) -- add a non-NULL value (row)
append_placeholder() -- add a NULL row placeholder
finish(Option) -- finish the build, specify NULLs

These are the counterpart of Arrow's GenericStringBuilder / StringViewBuilder but it
skips per-row NULL buffer maintenance, which lets callers compute the NULL buffer in
bulk when possible.

This PR also switches case_conversion to use the new APIs, which is used to
implement lower, upper, and the Spark equivalents. This improves lower / upper
performance by 3-15% on microbenchmarks. More UDFs (~10) will be converted to use
this API in future PRs.

What changes are included in this PR?

Add new builders
Add unit tests
Adopt builders in case_conversion

Are these changes tested?

Yes.

Are there any user-facing changes?

No.

Rename the three builders in `datafusion/functions/src/strings.rs` to make their special-purpose nature explicit: - `StringArrayBuilder` -> `ConcatStringBuilder` - `LargeStringArrayBuilder` -> `ConcatLargeStringBuilder` - `StringViewArrayBuilder` -> `ConcatStringViewBuilder` These builders are used only by `concat` and `concat_ws`; their `write::<CHECK_VALID>(col: &ColumnarValueRef, i)` + `append_offset()` API is shaped around accumulating multiple input fragments into one output row and is not a general-purpose string-builder replacement. The `Concat` prefix makes that clear and frees the previous names for a follow-up change that introduces general-purpose bulk-nulls builders with a simpler `append_value(&str)` API.

Introduce three new string array builders with bulk null tracking: - `StringArrayBuilder` (Utf8) - `LargeStringArrayBuilder` (LargeUtf8) - `StringViewArrayBuilder` (Utf8View) Each builder has the following API: - append_value(&str) -- add a non-NULL value (row) - append_placeholder() -- add a NULL row placeholder - finish(Option<NullBuffer>) -- finish the build, specify NULLs These are the counterpart of Arrow's `GenericStringBuilder` / `StringViewBuilder` but it skips per-row NULL buffer maintenance, which lets callers compute the NULL buffer in bulk when possible. This PR also switches `case_conversion` to use the new APIs, which is used to implement `lower`, `upper`, and the Spark equivalents. This improves `lower` / `upper` performance by 3-15% on microbenchmarks. More UDFs (~10) will be converted to use this API in future PRs.

…w-builder-api # Conflicts: # datafusion/functions/src/strings.rs

EeshanBembi

Nice builder abstraction — the null-deferred pattern is clean and removes the per-row null-buffer write overhead. A few non-blocking observations below.

EeshanBembi

Nit: consider exposing an append_null() alias alongside append_placeholder(). Arrow's own builders all use append_null(), and downstream contributors reaching for this builder will naturally look for that name first. Both names can coexist.

neilconway · 2026-04-23T13:00:40Z

Thanks for the review @EeshanBembi !

Nit: consider exposing an append_null() alias alongside append_placeholder().

I don't agree with adding an alias; I'd prefer to pick one name or the other. append_null is what StringBuilder in Arrow does, but the semantics are a little different there -- in that case, append_null is all you need to do to append a null value. In this case, calling append_placeholder isn't sufficient to add a NULL row; you also need to pass a NULL bitmap with the corresponding bit set. I think using a name other than append_null makes this a bit more clear.

…w-builder-api

…w-builder-api # Conflicts: # datafusion/functions/src/string/common.rs

neilconway · 2026-04-24T19:17:40Z

cc @mbutrovich , in case you have a free moment to assist in the next step of the quest to optimize NULL handling 😊

neilconway · 2026-04-24T19:36:46Z

Benchmarks:

  lower_all_values_are_ascii: 1024                                  5.4µs  -> 5.3µs   -1.9%
  lower_all_values_are_ascii: 4096                                 21.4µs  -> 21.1µs  -1.4%
  lower_all_values_are_ascii: 8192                                 41.7µs  -> 41.3µs  -1.0%

  ascii_sv 4096 len10  nulls0   mixed=f                           160.2µs  -> 151.4µs -5.5%
  ascii_sv 4096 len10  nulls0   mixed=t                           179.7µs  -> 181.1µs +0.8%
  ascii_sv 4096 len10  nulls0.1 mixed=f                           164.6µs  -> 141.7µs -13.9%
  ascii_sv 4096 len10  nulls0.1 mixed=t                           181.7µs  -> 172.8µs -4.9%
  ascii_sv 4096 len64  nulls0   mixed=f                           149.0µs  -> 154.1µs +3.4%
  ascii_sv 4096 len64  nulls0   mixed=t                           187.8µs  -> 194.8µs +3.7%
  ascii_sv 4096 len64  nulls0.1 mixed=f                           147.6µs  -> 145.4µs -1.5%
  ascii_sv 4096 len64  nulls0.1 mixed=t                           189.2µs  -> 178.2µs -5.8%
  ascii_sv 4096 len128 nulls0   mixed=f                           188.7µs  -> 191.2µs +1.3%
  ascii_sv 4096 len128 nulls0   mixed=t                           204.6µs  -> 202.6µs -1.0%
  ascii_sv 4096 len128 nulls0.1 mixed=f                           190.2µs  -> 172.8µs -9.1%
  ascii_sv 4096 len128 nulls0.1 mixed=t                           201.7µs  -> 195.0µs -3.3%
  ascii_sv 8192 len10  nulls0   mixed=f                           319.4µs  -> 303.8µs -4.9%
  ascii_sv 8192 len10  nulls0   mixed=t                           366.3µs  -> 363.6µs -0.7%
  ascii_sv 8192 len10  nulls0.1 mixed=f                           334.8µs  -> 284.6µs -15.0%
  ascii_sv 8192 len10  nulls0.1 mixed=t                           362.1µs  -> 339.0µs -6.4%
  ascii_sv 8192 len64  nulls0   mixed=f                           296.6µs  -> 309.1µs +4.2%
  ascii_sv 8192 len64  nulls0   mixed=t                           384.6µs  -> 384.0µs -0.2%
  ascii_sv 8192 len64  nulls0.1 mixed=f                           299.7µs  -> 290.6µs -3.0%
  ascii_sv 8192 len64  nulls0.1 mixed=t                           378.9µs  -> 355.4µs -6.2%
  ascii_sv 8192 len128 nulls0   mixed=f                           394.9µs  -> 415.3µs +5.2%
  ascii_sv 8192 len128 nulls0   mixed=t                           408.4µs  -> 407.4µs -0.2%
  ascii_sv 8192 len128 nulls0.1 mixed=f                           419.0µs  -> 350.9µs -16.3%
  ascii_sv 8192 len128 nulls0.1 mixed=t                           409.6µs  -> 388.3µs -5.2%

  nonascii_sv 4096 len10  nulls0   mixed=f                        372.8µs  -> 377.0µs +1.1%
  nonascii_sv 4096 len10  nulls0   mixed=t                        375.2µs  -> 372.6µs -0.7%
  nonascii_sv 4096 len10  nulls0.1 mixed=f                        345.5µs  -> 357.9µs +3.6%
  nonascii_sv 4096 len10  nulls0.1 mixed=t                        346.0µs  -> 340.7µs -1.5%
  nonascii_sv 4096 len64  nulls0   mixed=f                        358.9µs  -> 370.5µs +3.2%
  nonascii_sv 4096 len64  nulls0   mixed=t                        382.0µs  -> 386.4µs +1.2%
  nonascii_sv 4096 len64  nulls0.1 mixed=f                        353.7µs  -> 349.7µs -1.1%
  nonascii_sv 4096 len64  nulls0.1 mixed=t                        363.7µs  -> 340.3µs -6.4%
  nonascii_sv 4096 len128 nulls0   mixed=f                        371.6µs  -> 393.8µs +6.0%
  nonascii_sv 4096 len128 nulls0   mixed=t                        376.2µs  -> 387.4µs +3.0%
  nonascii_sv 4096 len128 nulls0.1 mixed=f                        349.6µs  -> 349.0µs -0.2%
  nonascii_sv 4096 len128 nulls0.1 mixed=t                        363.4µs  -> 341.1µs -6.1%
  nonascii_sv 8192 len10  nulls0   mixed=f                        738.5µs  -> 771.2µs +4.4%
  nonascii_sv 8192 len10  nulls0   mixed=t                        759.2µs  -> 760.1µs +0.1%
  nonascii_sv 8192 len10  nulls0.1 mixed=f                        705.8µs  -> 700.4µs -0.8%
  nonascii_sv 8192 len10  nulls0.1 mixed=t                        710.9µs  -> 695.0µs -2.2%
  nonascii_sv 8192 len64  nulls0   mixed=f                        781.1µs  -> 759.5µs -2.8%
  nonascii_sv 8192 len64  nulls0   mixed=t                        746.3µs  -> 791.6µs +6.1%
  nonascii_sv 8192 len64  nulls0.1 mixed=f                        726.4µs  -> 696.9µs -4.1%
  nonascii_sv 8192 len64  nulls0.1 mixed=t                        733.7µs  -> 690.0µs -6.0%
  nonascii_sv 8192 len128 nulls0   mixed=f                        746.5µs  -> 769.6µs +3.1%
  nonascii_sv 8192 len128 nulls0   mixed=t                        742.5µs  -> 764.1µs +2.9%
  nonascii_sv 8192 len128 nulls0.1 mixed=f                        745.2µs  -> 717.4µs -3.7%
  nonascii_sv 8192 len128 nulls0.1 mixed=t                        707.5µs  -> 699.9µs -1.1%

  lower_the_first_value_is_nonascii: 1024                          45.6µs  -> 42.6µs  -6.6%
  lower_the_first_value_is_nonascii: 4096                         183.6µs  -> 172.7µs -5.9%
  lower_the_first_value_is_nonascii: 8192                         372.1µs  -> 356.5µs -4.2%
  lower_the_middle_value_is_nonascii: 1024                         45.8µs  -> 43.2µs  -5.7%
  lower_the_middle_value_is_nonascii: 4096                        187.0µs  -> 175.7µs -6.0%
  lower_the_middle_value_is_nonascii: 8192                        373.2µs  -> 360.0µs -3.5%

mbutrovich

Thanks @neilconway! One minor request: it might be worth adding an all-placeholders test (every row null) to exercise the placeholder_count == null_count boundary. The existing tests all mix values and placeholders.

neilconway · 2026-04-24T21:35:13Z

@mbutrovich Thanks, good idea! Done.

mbutrovich

Thanks as always for the performance improvements, @neilconway!

neilconway added 2 commits April 22, 2026 07:45

github-actions Bot added the functions Changes to functions implementation label Apr 22, 2026

neilconway mentioned this pull request Apr 22, 2026

chore: Rename concat-specific string builders, make pub(crate) #21695

Merged

neilconway added 2 commits April 22, 2026 12:43

cargo fmt

2c9bdfc

Merge remote-tracking branch 'origin/main' into neilc/perf-string-vie…

11ef3da

…w-builder-api # Conflicts: # datafusion/functions/src/strings.rs

neilconway changed the title ~~perf: Add new string builders that support bulk NULLs, use in lower and upper~~ perf: Add bulk NULL-aware string builders, use in lower and upper Apr 22, 2026

EeshanBembi reviewed Apr 23, 2026

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Comment thread datafusion/functions/src/strings.rs

Comment thread datafusion/functions/src/string/common.rs

EeshanBembi reviewed Apr 23, 2026

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neilconway added 7 commits April 23, 2026 10:17

Improve comments, per review

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Check that placeholder count is consistent with null buffer

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Merge remote-tracking branch 'origin/main' into neilc/perf-string-vie…

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…w-builder-api

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Merge remote-tracking branch 'origin/main' into neilc/perf-string-vie…

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…w-builder-api # Conflicts: # datafusion/functions/src/string/common.rs

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mbutrovich added this pull request to the merge queue Apr 24, 2026

Merged via the queue into apache:main with commit 794f30e Apr 24, 2026
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perf: Add bulk NULL-aware string builders, use in `lower` and `upper`#21789

perf: Add bulk NULL-aware string builders, use in `lower` and `upper`#21789
mbutrovich merged 12 commits intoapache:mainfrom
neilconway:neilc/perf-string-view-builder-api

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