Improve Select and Retrieve performance

[devin-ai-integration]

Summary

Reduces per-call and per-row allocations and overhead in the Select/Retrieve hot paths.

Allocation & overhead reductions (initial pass)

• Fix Hash.Compute thread safety: Replaced static mutable fields (_a, _b, _c) with local variables passed by ref. The old implementation was not thread-safe — concurrent callers would corrupt shared state, producing incorrect hash values and cache misses.
• new Param[] {} → Array.Empty<Param>(): Avoids allocating a throwaway empty array on every cached Retrieve call (both sync and async paths).
• GuessOperationType: indexed for loop instead of operation.Any(char.IsWhiteSpace): Avoids delegate allocation per call.
• GetQueryKey optimization: Changed parameter from IEnumerable<Param> to IList<Param>, replaced LINQ chain (OrderBy/Select/ToDelimitedString) with a manual StringBuilder loop, and skips sorting when there's only one parameter.
• Trimmed redundant Log.Capture calls in cache hit/miss paths to reduce string interpolation overhead.

Deeper per-row optimizations (latest revision)

• Bypass Map dispatch in ConvertDataReader: The generic Map<TSource, TResult> method performs MappingFactory.IsIndexer (3 type checks) + additional is pattern matching on every row, even though the source is always IDataRecord. Now calls FastIndexerMapper<IDataRecord, T>.Map / FastIndexerMapperWithTypeCoercion<IDataRecord, T>.Map directly.
• Pre-cache Mapper.PropertyMapper delegates for multi-map secondary types: Avoids per-row Tuple<Type,Type,bool,bool> allocation + ConcurrentDictionary.GetOrAdd lookup in Mapper.CreateDelegate for each secondary type.
• Pre-resolve mapper delegate per result set in ConvertDataReaderMultiResult: Same optimization for multi-result queries.
• Cache primary key column names per type: New PrimaryKeyColumnCache avoids per-row LINQ (Where/Select/OrderBy/ToArray) in CreateIdentity.
• Pre-allocate args array before per-row loop: Avoids new object[types.Count] allocation on every row in multi-map scenarios.
• Cache config.AutoTypeCoercion and types[resultIndex] in locals: Avoids repeated property access and list indexer lookups per row.
• Replace LINQ ToList() with array allocation for reflectedTypes in multi-result path.
• Pre-size HashSet with FieldCount capacity in GetColumns (netstandard2.1+/net8.0 only due to API availability).

All 230 unit tests pass.

Review & Testing Checklist for Human

• ⚠️ Verify FastIndexerMapper dispatch is correct — The code now bypasses Map<TSource, TResult> and calls FastIndexerMapper<IDataRecord, T>.Map directly. Verify this is semantically equivalent for all entity types (interfaces, concrete classes, with/without type coercion).
• ⚠️ Verify secondary mapper pre-caching — For multi-map queries, Mapper.CreateDelegate(typeof(IDataRecord), types[i], true, autoTypeCoercion) is called once before the loop. The original code went through Map(object, Type, bool) which did IsIndexer + GetIndexerType. Confirm the pre-cached delegate produces identical results.
• Verify Log.CaptureEnd balance — The diff removes Log.CaptureBegin calls but keeps some Log.CaptureEnd calls on the cache-hit return paths. This may cause mismatched begin/end in the logging stack.
• Confirm cache key format change is acceptable — GetQueryKey output format changed slightly (no trailing "/" when params are empty). Existing in-memory L1 cache keys will no longer match after upgrade, causing a one-time cache miss on first call.
• Run the Nemo.Benchmark project against a database to measure actual throughput improvement on Select/Retrieve paths (e.g., dotnet run -c Release in tests/Nemo.Benchmark/).

Notes

• Requested by: @stepaside
• Link to Devin run: https://app.devin.ai/sessions/9fa2287058044176a3ee99a349e2d59c

[devin-ai-integration]

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