(improvement) (Cython only) deserializers: use direct PyUnicode_DecodeUTF8/ASCII fr… (tens hundreds of us improvement, x2.5-x3 speedup)

[mykaul]

(improvement) (Cython only) deserializers: use direct PyUnicode_DecodeUTF8/ASCII from C buffer pointer

Replace the two-step to_bytes(buf).decode('utf8') pattern in DesUTF8Type and DesAsciiType with direct CPython C API calls (PyUnicode_DecodeUTF8 and PyUnicode_DecodeASCII). This eliminates an intermediate bytes object allocation per text cell — the old code created a Python bytes object from the C buffer pointer via to_bytes(buf), then immediately decoded it to str and discarded the bytes.

Text (UTF8Type/VarcharType) is the most common CQL column type, so this optimization applies to the majority of cells in typical workloads.

Benchmark results

Measured on the same machine, back-to-back (Python 3.14.3, Cython 3.2.4, GCC 15.2.1, --benchmark-disable-gc). Median times reported.

Scenario	Before (master)	After (this PR)	Speedup
UTF8 1row x 1col short (11B)	1,158 ns	454 ns	2.55x
UTF8 1row x 10col short	3,262 ns	1,505 ns	2.17x
UTF8 100rows x 5col medium (46B)	129,349 ns	36,270 ns	3.57x
UTF8 1000rows x 5col medium	1,131,159 ns	322,309 ns	3.51x
UTF8 100rows x 5col long (200B)	121,866 ns	40,662 ns	3.00x
UTF8 100rows x 5col multibyte	275,148 ns	113,751 ns	2.42x
ASCII 100rows x 5col medium	86,118 ns	36,401 ns	2.37x
ASCII 1000rows x 5col medium	832,972 ns	348,660 ns	2.39x
Mixed 100rows 3text+2int	82,798 ns	27,792 ns	2.98x

All existing unit tests pass (15 Cython tests, 9 deserializer-specific including invalid-input error propagation tests).

Review comments addressed

All 3 Copilot review comments have been addressed:

1. Benchmark file renamed from test_utf8_decode_benchmark.py to utf8_decode_benchmark.py (no test_ prefix, won't be auto-collected) and pytest.importorskip guards added.
2. Correctness tests moved to tests/unit/cython/test_deserializers.py (decoupled from benchmark dependency).
3. cythontest decorator adopted instead of hand-rolled try/except ImportError, consistent with the rest of tests/unit/cython/ and respecting VERIFY_CYTHON=True CI mode.

Pre-review checklist

• I have split my patch into logically separate commits.
• All commit messages clearly explain what they change and why.
• I added relevant tests for new features and bug fixes.
• All commits compile, pass static checks and pass test.
• PR description sums up the changes and reasons why they should be introduced.
• I have provided docstrings for the public items that I want to introduce.
• I have adjusted the documentation in ./docs/source/.
• I added appropriate Fixes: annotations to PR description.

[Copilot]

Pull request overview

Optimizes text deserialization performance in the Cython row parser by decoding UTF-8/ASCII directly from the underlying buffer, and adds a benchmark module intended to measure the improvement.

Changes:

• Replace to_bytes(buf).decode(...) with PyUnicode_DecodeASCII/UTF8(buf.ptr, buf.size, NULL) in Cython deserializers.
• Add a new pytest-based benchmark suite for UTF-8/ASCII decode and pipeline parsing.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
cassandra/deserializers.pyx	Uses CPython unicode decode APIs to avoid intermediate bytes allocations during ASCII/UTF-8 deserialization.
benchmarks/test_utf8_decode_benchmark.py	Adds end-to-end and microbenchmarks plus correctness checks for the new decode approach.

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[Copilot]

Pull request overview

This PR optimizes Cython-based UTF-8 and ASCII text deserialization by decoding directly from the underlying C buffer (avoiding an intermediate bytes allocation), and adds correctness tests plus a benchmark to validate/measure the change.

Changes:

• Switch DesUTF8Type/DesAsciiType to use PyUnicode_DecodeUTF8 / PyUnicode_DecodeASCII on Buffer.ptr/Buffer.size.
• Add unit tests covering common and edge-case text decode scenarios (empty, ASCII, multibyte, long strings, NULLs, multiple rows).
• Add a pytest-benchmark benchmark file to compare end-to-end Cython row parsing before/after the optimization.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
cassandra/deserializers.pyx	Replace to_bytes(...).decode(...) with direct CPython Unicode decoding calls for UTF-8/ASCII deserializers.
tests/unit/cython/test_deserializers.py	New correctness-focused unit tests for the optimized Cython decoding path.
benchmarks/utf8_decode_benchmark.py	New benchmark suite to measure the performance impact of the UTF-8/ASCII decode optimization.

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