Second pass of hot-path optimization on parse_tuple_payload. Two changes
to converters.py:
1. Split decode() into public + internal. Added _decode_base(base_tc,
raw, encoding) that takes an already-base-typed code and skips the
redundant base_type() call. Public decode() is now a one-line
wrapper. parse_tuple_payload's 4 call sites swapped to use
_decode_base directly. _fastpath.py's external decode() caller is
unaffected.
2. Pre-compiled struct.Struct unpackers. The fixed-width integer/float
decoders (_decode_smallint, _decode_int, _decode_bigint,
_decode_smfloat, _decode_float, _decode_date) switched from per-call
struct.unpack(fmt, raw) to module-level bound methods like
_UNPACK_INT = struct.Struct("!i").unpack. Format-string parsed once
at module load. Measured 37% faster than per-call struct.unpack on
CPython 3.13 micro.
Performance vs Phase 23 baseline:
* decode_int: 173 ns -> 139 ns (-20%)
* decode_bigint: 188 ns -> 150 ns (-20%)
* parse_tuple_5cols: 2047 ns -> 1592 ns (-22%)
* 1k-row SELECT: 1255 us -> 989 us (-21%)
Cumulative vs original Phase 21 baseline:
* decode_int: 230 ns -> 139 ns (-40%)
* parse_tuple_5cols: 2796 ns -> 1592 ns (-43%)
* 1k-row SELECT: 1477 us -> 989 us (-33%)
Real-world fetch ceiling: 358K rows/sec -> ~620K rows/sec.
Margaret Hamilton review surfaced one HIGH-severity finding addressed
before tagging:
* H: The no-collision guarantee that makes _decode_base safe is
structural but undocumented (all DECODERS keys are ≤ 0xFF, all flag
bits are ≥ 0x100, so flagged inputs cannot coincidentally match).
Added load-bearing INVARIANT comment at DECODERS dict explaining
the constraint and what to do if violated. Cross-referenced from
_decode_base's docstring for bidirectional traceability.
baseline.json refreshed; all 224 integration tests pass; ruff clean.
Ryan Malloy
dfa60ea501