Add comprehensive fix summary documentation

Author: MPCoreDeveloper

QUERYCOMPILER_FIX_SUMMARY.md

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+# QueryCompiler Fix Summary
+
+## Problem
+The test `CompiledQuery_1000RepeatedSelects_CompletesUnder8ms` was failing with:
+```
+1000 compiled queries with cached plans should complete in <2000ms; took 3591ms
+```
+
+## Root Cause Analysis
+
+### Investigation Steps
+1. Used debugger to confirm `stmt.CompiledPlan` was `null`
+2. Added diagnostic logging to `QueryCompiler.Compile()` 
+3. Discovered the compilation was failing with:
+   ```
+   InvalidOperationException: The binary operator GreaterThan is not defined for the types 'System.Object' and 'System.Object'.
+   ```
+
+### Root Cause
+When compiling WHERE clauses like `value > 500`, the QueryCompiler was attempting to create `Expression.GreaterThan(left, right)` where:
+- `left` = Dictionary value (type: `object`)
+- `right` = Literal constant (type: `int`)
+
+The original code tried to convert both to the same type, but when both ended up as `object` type, the `Expression.GreaterThan()` method failed because LINQ expression trees cannot create comparison operators for `object` types.
+
+## Solution
+
+### Changes Made to `src\SharpCoreDB\Services\QueryCompiler.cs`
+
+1. **Use IComparable for object-typed comparisons** (new method `CompareUsingIComparable`):
+   - When either operand is `object` type, use `IComparable.CompareTo()` instead of direct comparison operators
+   - Example: `left.CompareTo(right) > 0` instead of `left > right`
+
+2. **Handle numeric type mismatches** (new method `GetCommonNumericType`):
+   - Finds the widest common numeric type for two operands
+   - Precedence: decimal > double > float > long > int > short > byte
+   - Prevents cast exceptions when comparing Int32 with Decimal, etc.
+
+3. **Refactored `ConvertBinaryExpression`**:
+   - Prioritize `IComparable` for object-typed values
+   - Use `GetCommonNumericType()` for strongly-typed numeric comparisons
+   - Fall back to `IComparable` if no common type exists
+
+### Code Structure
+```csharp
+if (left.Type == typeof(object) || right.Type == typeof(object))
+{
+    return CompareUsingIComparable(left, right, op);
+}
+
+if (left.Type != right.Type)
+{
+    var commonType = GetCommonNumericType(left.Type, right.Type);
+    if (commonType != null)
+    {
+        // Convert to common type
+    }
+    else
+    {
+        return CompareUsingIComparable(left, right, op);
+    }
+}
+```
+
+## Results
+
+### Performance Improvement
+- **Before**: 3,591ms+ for 1000 queries (FAILED)
+- **After**: 137-1466ms for 1000 queries (PASSED)
+- **Improvement**: ~2.5-26x faster (queries now use compiled expression trees)
+
+### Test Results
+- ✅ All 9 CompiledQueryTests passing (1 skipped by design)
+- ✅ Compilation now succeeds for queries with WHERE clauses
+- ✅ Handles complex WHERE clauses with multiple conditions
+- ✅ Properly converts between numeric types (Int32, Decimal, etc.)
+
+## Files Modified
+1. `src\SharpCoreDB\Services\QueryCompiler.cs`
+   - Fixed `ConvertBinaryExpression()` method
+   - Added `CompareUsingIComparable()` method
+   - Added `GetCommonNumericType()` method
+
+2. `tests\SharpCoreDB.Tests\CompiledQueryTests.cs`
+   - Removed workaround skip logic
+   - Test now runs and passes successfully
+
+## Technical Details
+
+### Why IComparable?
+- `IComparable.CompareTo()` is implemented by all comparable types (Int32, Decimal, String, etc.)
+- Returns: -1 (less), 0 (equal), or 1 (greater)
+- Works with runtime-typed values without requiring compile-time type information
+- Handles boxing/unboxing automatically
+
+### Expression Tree Pattern
+```csharp
+// Convert to IComparable
+var left = Expression.Convert(objectValue, typeof(IComparable));
+var right = objectValue2; // Keep as object for CompareTo parameter
+
+// Call CompareTo
+var compareCall = Expression.Call(left, compareToMethod, right);
+
+// Compare result with 0
+var zero = Expression.Constant(0);
+var result = Expression.GreaterThan(compareCall, zero); // compareTo > 0
+```
+
+## Future Considerations
+- Consider caching compiled WHERE filters to avoid recompilation
+- Could optimize by detecting numeric types at parse time
+- May want to add support for custom IComparer implementations
+- Consider adding telemetry to track compilation success rate
+
+## Related Issues
+- Fixes the "141/200 mystery" where compiled queries were falling back to SqlParser
+- Improves performance of repeated SELECT queries by 2.5-26x
+- Enables true compiled query execution using expression trees