perf: optimize regexp_match for literal pattern usage (20% faster)

[andygrove]

Which issue does this PR close?

N/A

Rationale for this change

Optimize existing expressions

What changes are included in this PR?

Pass a literal regexp pattern/flags to arrow's kernel as scalar Datums so the regex is compiled once per batch, instead of expanding the literal to a full array and forcing a per-row HashMap<String, Regex> cache lookup (plus a per-row format! allocation when flags are present).

Are these changes tested?

Existing tests + new unit tests.

Benchmark (criterion):

• regexp_match_1000 literal pattern utf8view: 21.394% faster (base 249069ns -> cand 195784ns)
• regexp_match_1000 pattern array: 2.996% faster (base 190020ns -> cand 184327ns)
• regexp_match_1000 literal pattern: 23.964% faster (base 261795ns -> cand 199059ns)
• regexp_match_1000 literal pattern and flags: 37.227% faster (base 280047ns -> cand 175794ns)

Full criterion output:

regexp_match_1000 literal pattern
                        time:   [198.39 µs 198.66 µs 199.03 µs]
                        change: [−24.244% −23.964% −23.600%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 9 outliers among 100 measurements (9.00%)
  1 (1.00%) low mild
  4 (4.00%) high mild
  4 (4.00%) high severe

regexp_match_1000 literal pattern and flags
                        time:   [175.59 µs 175.68 µs 175.81 µs]
                        change: [−37.342% −37.227% −37.124%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 2 outliers among 100 measurements (2.00%)
  1 (1.00%) high mild
  1 (1.00%) high severe

regexp_match_1000 literal pattern utf8view
                        time:   [195.75 µs 195.85 µs 195.98 µs]
                        change: [−21.511% −21.394% −21.298%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 4 outliers among 100 measurements (4.00%)
  3 (3.00%) high mild
  1 (1.00%) high severe

regexp_match_1000 pattern array
                        time:   [184.11 µs 184.22 µs 184.37 µs]
                        change: [−3.1007% −2.9962% −2.8581%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 5 outliers among 100 measurements (5.00%)
  1 (1.00%) high mild
  4 (4.00%) high severe

Are there any user-facing changes?

[comphead]

Thanks @andygrove

[codecov-commenter]

Codecov Report

❌ Patch coverage is 92.68293% with 6 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@3a29d6b). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
datafusion/functions/src/regex/regexpmatch.rs	92.68%	2 Missing and 4 partials ⚠️

Additional details and impacted files

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