Add functional issues investigation summary and roadmap

Created comprehensive summary document for the 375 actual functional issues (non-baseline mismatches):

Categories:
- Sequence contains no elements: 175 tests (18.9%)
- ComplexJSON operations: 95 tests (10.3%)
- Composing expression errors: 47 tests (5.1%)
- Other functional issues: 58 tests (6.3%)

Document provides:
- Root cause analysis for each category
- Investigation approach
- Time estimates (8-20 hours for functional fixes)
- Phased roadmap for addressing issues
- Comparison to baseline updates (18-45 hours for assertions only)

This prioritizes actual code bugs over test assertion updates.

Co-authored-by: renemadsen <76994+renemadsen@users.noreply.github.com>

Author: Copilot

FUNCTIONAL_ISSUES_SUMMARY.md

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+# Functional Issues Investigation Summary
+
+## Overview
+
+After fixing the two critical root causes (RETURNING clause and LEAST/GREATEST type mapping), **375 tests (40.5% of remaining 925 failures) have actual functional issues** requiring code-level fixes, not just test baseline updates.
+
+## Issue Categories
+
+### 1. "Sequence Contains No Elements" - 175 Tests (18.9%)
+
+**Nature:** Query execution returns empty result sets where data is expected.
+
+**Possible Causes:**
+- Query translation differences between MySQL and MariaDB
+- Missing data in test fixtures for MariaDB
+- WHERE clause conditions that filter out all rows on MariaDB
+- Aggregate operations (First, Single, etc.) failing on empty sets
+
+**Investigation Needed:**
+- Run subset of failing tests to capture actual error context
+- Determine if issue is data-related or query translation
+- Check if MariaDB-specific SQL syntax differences affect results
+
+### 2. ComplexJSON Operations - 95 Tests (10.3%)
+
+**Nature:** JSON operations failing on MariaDB 11.6.2.
+
+**Known Issues:**
+- JSON bulk updates
+- JSON projections
+- JSON set operations
+- Potential differences in JSON function support between MySQL and MariaDB
+
+**Investigation Needed:**
+- Verify MariaDB 11.6.2 JSON function compatibility
+- Check if JSON_TABLE, JSON_EXTRACT, etc. behave differently
+- Determine if JSON type mapping needs MariaDB-specific handling
+
+### 3. Composing Expression Errors - 47 Tests (5.1%)
+
+**Nature:** Query translation failures when composing complex expressions.
+
+**Likely Causes:**
+- Nested query operations not translating correctly
+- Complex LINQ expressions not supported by query translator
+- Expression visitor issues with MariaDB-specific SQL
+
+**Investigation Needed:**
+- Identify specific expression patterns that fail
+- Check if EF Core 10 changes affect expression composition
+- Determine if MySQL-specific optimizations don't work on MariaDB
+
+### 4. Other Functional Issues - 58 Tests (6.3%)
+
+**Nature:** Miscellaneous failures not fitting other categories.
+
+**Potential Issues:**
+- LEFT JOIN not supported in certain contexts
+- Type conversion differences
+- Collation/character set issues
+- Stored procedure compatibility
+
+## Investigation Approach
+
+### Phase 1: Data Analysis (2-3 hours)
+1. Run sample tests from each category
+2. Capture full error context and stack traces
+3. Identify common patterns within each category
+4. Determine root causes for each category
+
+### Phase 2: Targeted Fixes (Variable - depends on findings)
+1. **Sequence Contains No Elements**: 
+   - If data issue: Update test fixtures
+   - If query issue: Fix query translation
+   
+2. **ComplexJSON**: 
+   - Check MariaDB JSON compatibility
+   - Update JSON function mappings if needed
+   
+3. **Composing Expressions**: 
+   - Fix expression visitor logic
+   - Add MariaDB-specific handling
+   
+4. **Other Issues**: 
+   - Address individually based on root cause
+
+### Phase 3: Verification (1-2 hours)
+1. Re-run affected test subsets
+2. Verify fixes don't break other tests
+3. Update documentation
+
+## Time Estimates
+
+- **Investigation**: 2-3 hours
+- **Fixes**: 5-15 hours (depends on complexity)
+- **Testing**: 1-2 hours
+- **Total**: 8-20 hours
+
+Compare to baseline updates: 18-45 hours for test assertions only.
+
+## Current Status
+
+### ✅ COMPLETED
+- RETURNING clause compatibility fix
+- LEAST/GREATEST type mapping fix
+- 90.6% failure reduction achieved
+- Comprehensive analysis and categorization
+
+### 🔄 IN PROGRESS
+- Functional issue investigation
+
+### ⏳ PENDING
+- Functional issue fixes (this document provides roadmap)
+- SQL baseline updates (550 tests - can be deferred)
+
+## Recommendations
+
+1. **Priority**: Fix functional issues first (actual bugs)
+2. **Defer**: SQL baseline updates (test assertions only)
+3. **Incremental**: Address functional issues by category
+4. **Verify**: Each fix with targeted test subset
+
+## Next Steps
+
+1. Run sample tests from "Sequence contains no elements" category
+2. Capture and analyze error patterns
+3. Implement targeted fixes
+4. Repeat for other categories
+
+This provides a more valuable improvement than updating 550 test baselines that don't affect production functionality.