Replace concat_elements_bytes BufferBuilders with Vec

[cakeni]

Which issue does this PR close?

• Part of [Epic] Replace OffsetBufferBuilder / BufferBuilder usage with Vec, when possible #10245.

Rationale for this change

Using Vec instead of BufferBuilder can benefit from Rust's optimized vector implementation. This updates the value and offset builders in concat_elements_bytes.

What changes are included in this PR?

• Replace the output value and offset BufferBuilder instances with capacity-matched vectors.
• Use extend_from_slice and push while preserving the existing offset calculations.
• Convert both vectors directly into the buffers used by ArrayDataBuilder.

Are these changes tested?

Yes. The following checks pass:

• cargo +stable-x86_64-pc-windows-gnu fmt --all -- --check
• cargo +stable-x86_64-pc-windows-gnu clippy -p arrow-string --all-targets --all-features --no-deps -- -D warnings
• cargo +stable-x86_64-pc-windows-gnu test -p arrow-string --all-features (182 unit tests and 10 doctests passed)

Are there any user-facing changes?

No.

[Jefffrey]

run benchmark concatenate_elements

[adriangbot]

🤖 Arrow criterion benchmark running (GKE) | trigger
Instance: c4a-highmem-16 (12 vCPU / 65 GiB) | Linux bench-c5263956732-1559-c7rhm 6.12.85+ #1 SMP Wed Jun 17 20:31:55 UTC 2026 aarch64 GNU/Linux

CPU Details (lscpu)

Architecture:                            aarch64
CPU op-mode(s):                          64-bit
Byte Order:                              Little Endian
CPU(s):                                  16
On-line CPU(s) list:                     0-15
Vendor ID:                               ARM
Model name:                              Neoverse-V2
Model:                                   1
Thread(s) per core:                      1
Core(s) per cluster:                     16
Socket(s):                               -
Cluster(s):                              1
Stepping:                                r0p1
BogoMIPS:                                2000.00
Flags:                                   fp asimd evtstrm aes pmull sha1 sha2 crc32 atomics fphp asimdhp cpuid asimdrdm jscvt fcma lrcpc dcpop sha3 sm3 sm4 asimddp sha512 sve asimdfhm dit uscat ilrcpc flagm sb paca pacg dcpodp sve2 sveaes svepmull svebitperm svesha3 svesm4 flagm2 frint svei8mm svebf16 i8mm bf16 dgh rng bti
L1d cache:                               1 MiB (16 instances)
L1i cache:                               1 MiB (16 instances)
L2 cache:                                32 MiB (16 instances)
L3 cache:                                80 MiB (1 instance)
NUMA node(s):                            1
NUMA node0 CPU(s):                       0-15
Vulnerability Gather data sampling:      Not affected
Vulnerability Indirect target selection: Not affected
Vulnerability Itlb multihit:             Not affected
Vulnerability L1tf:                      Not affected
Vulnerability Mds:                       Not affected
Vulnerability Meltdown:                  Not affected
Vulnerability Mmio stale data:           Not affected
Vulnerability Reg file data sampling:    Not affected
Vulnerability Retbleed:                  Not affected
Vulnerability Spec rstack overflow:      Not affected
Vulnerability Spec store bypass:         Mitigation; Speculative Store Bypass disabled via prctl
Vulnerability Spectre v1:                Mitigation; __user pointer sanitization
Vulnerability Spectre v2:                Mitigation; CSV2, BHB
Vulnerability Srbds:                     Not affected
Vulnerability Tsa:                       Not affected
Vulnerability Tsx async abort:           Not affected
Vulnerability Vmscape:                   Not affected

Comparing perf/arrow-string-concat-bytes-vecs (0b2f7df) to fc16607 (merge-base) diff

Run configuration

run benchmark concatenate_elements

BENCH_COMMAND=cargo bench --features=arrow,async,test_common,experimental,object_store --bench concatenate_elements
Results will be posted here when complete

---

File an issue against this benchmark runner

[adriangbot]

🤖 Arrow criterion benchmark completed (GKE) | trigger

Instance: c4a-highmem-16 (12 vCPU / 65 GiB)

Comparing perf/arrow-string-concat-bytes-vecs (0b2f7df) to fc16607 (merge-base) diff

Run configuration

run benchmark concatenate_elements

CPU Details (lscpu)

Architecture:                            aarch64
CPU op-mode(s):                          64-bit
Byte Order:                              Little Endian
CPU(s):                                  16
On-line CPU(s) list:                     0-15
Vendor ID:                               ARM
Model name:                              Neoverse-V2
Model:                                   1
Thread(s) per core:                      1
Core(s) per cluster:                     16
Socket(s):                               -
Cluster(s):                              1
Stepping:                                r0p1
BogoMIPS:                                2000.00
Flags:                                   fp asimd evtstrm aes pmull sha1 sha2 crc32 atomics fphp asimdhp cpuid asimdrdm jscvt fcma lrcpc dcpop sha3 sm3 sm4 asimddp sha512 sve asimdfhm dit uscat ilrcpc flagm sb paca pacg dcpodp sve2 sveaes svepmull svebitperm svesha3 svesm4 flagm2 frint svei8mm svebf16 i8mm bf16 dgh rng bti
L1d cache:                               1 MiB (16 instances)
L1i cache:                               1 MiB (16 instances)
L2 cache:                                32 MiB (16 instances)
L3 cache:                                80 MiB (1 instance)
NUMA node(s):                            1
NUMA node0 CPU(s):                       0-15
Vulnerability Gather data sampling:      Not affected
Vulnerability Indirect target selection: Not affected
Vulnerability Itlb multihit:             Not affected
Vulnerability L1tf:                      Not affected
Vulnerability Mds:                       Not affected
Vulnerability Meltdown:                  Not affected
Vulnerability Mmio stale data:           Not affected
Vulnerability Reg file data sampling:    Not affected
Vulnerability Retbleed:                  Not affected
Vulnerability Spec rstack overflow:      Not affected
Vulnerability Spec store bypass:         Mitigation; Speculative Store Bypass disabled via prctl
Vulnerability Spectre v1:                Mitigation; __user pointer sanitization
Vulnerability Spectre v2:                Mitigation; CSV2, BHB
Vulnerability Srbds:                     Not affected
Vulnerability Tsa:                       Not affected
Vulnerability Tsx async abort:           Not affected
Vulnerability Vmscape:                   Not affected

Details

group                                                          main                                   perf_arrow-string-concat-bytes-vecs
-----                                                          ----                                   -----------------------------------
concat str 1024                                                1.05     12.2±0.39µs        ? ?/sec    1.00     11.6±0.40µs        ? ?/sec
concat str 8192                                                1.18   173.8±24.61µs        ? ?/sec    1.00   147.7±18.28µs        ? ?/sec
concat str nulls 1024                                          1.32      8.9±0.16µs        ? ?/sec    1.00      6.8±0.14µs        ? ?/sec
concat str nulls 8192 over 100 arrays                          1.14     88.2±3.12µs        ? ?/sec    1.00     77.5±2.23µs        ? ?/sec
concat utf8_view  max_str_len=128 null_density=0               1.00     89.7±0.90µs        ? ?/sec    1.04     93.1±1.08µs        ? ?/sec
concat utf8_view  max_str_len=128 null_density=0.2             1.00     81.5±0.32µs        ? ?/sec    1.01     82.0±0.58µs        ? ?/sec
concat utf8_view  max_str_len=20 null_density=0                1.03     55.3±0.02µs        ? ?/sec    1.00     53.8±0.03µs        ? ?/sec
concat utf8_view  max_str_len=20 null_density=0.2              1.01     61.5±0.07µs        ? ?/sec    1.00     61.2±0.10µs        ? ?/sec
concat utf8_view all_inline max_str_len=12 null_density=0      1.00     49.7±0.02µs        ? ?/sec    1.00     49.7±0.03µs        ? ?/sec
concat utf8_view all_inline max_str_len=12 null_density=0.2    1.00     58.3±0.08µs        ? ?/sec    1.00     58.4±0.07µs        ? ?/sec

Resource Usage

base (merge-base)

Metric	Value
Wall time	105.0s
Peak memory	26.9 MiB
Avg memory	17.4 MiB
CPU user	98.9s
CPU sys	0.0s
Peak spill	0 B

branch

Metric	Value
Wall time	95.0s
Peak memory	14.0 MiB
Avg memory	11.8 MiB
CPU user	92.1s
CPU sys	0.0s
Peak spill	0 B

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