Add investigation summary with action plan

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

Author: Copilot

INVESTIGATION_SUMMARY.md

@@ -0,0 +1,111 @@
+# Functional Tests Investigation Summary
+
+## Overview
+
+This investigation analyzed all functional test failures against MariaDB 11.6.2 to identify root causes and create an action plan for fixes.
+
+## Quick Facts
+
+- **Investigation Date:** December 3, 2025
+- **Database Tested:** MariaDB 11.6.2-MariaDB-ubu2404
+- **Test Duration:** 7.33 minutes
+- **Total Tests:** 30,173
+- **Pass Rate:** 65.5% (19,760 passed)
+- **Failure Rate:** 32.7% (9,874 failed)
+
+## Key Findings
+
+### Critical Issue: RETURNING Clause Incompatibility
+
+**99% of failures** trace back to MariaDB not supporting the `RETURNING` clause used by MySQL 8.0+.
+
+**Error Example:**
+```
+MySqlException: You have an error in your SQL syntax; check the manual 
+that corresponds to your MariaDB server version for the right syntax 
+to use near 'RETURNING 1'
+```
+
+**Solution:** Implement MariaDB-specific SQL generation using `SELECT ROW_COUNT()` or `LAST_INSERT_ID()` instead of `RETURNING`.
+
+### High Priority: Type Mapping Issues
+
+32+ tests fail due to `LEAST()` function not having proper type mapping:
+```
+InvalidOperationException: Expression 'LEAST(@p, 1)' in the SQL tree 
+does not have a type mapping assigned.
+```
+
+**Solution:** Register LEAST function with appropriate type mapper in MySqlQuerySqlGenerator.
+
+## Investigation Reports
+
+1. **[functional_tests_failure_analysis_mariadb.md](functional_tests_failure_analysis_mariadb.md)**
+   - Complete analysis with all error categories
+   - Detailed statistics and breakdowns
+   - Technical recommendations
+   - Testing strategy
+
+2. **[test_failure_examples_mariadb.md](test_failure_examples_mariadb.md)**
+   - Specific failing test examples by category
+   - Actual error messages
+   - Sample test names
+
+3. **[failing_tests_checklist.md](failing_tests_checklist.md)**
+   - Prioritized checklist for tracking fixes
+   - Organized by priority level
+   - Progress tracking template
+
+## Recommended Action Plan
+
+### Phase 1: Fix RETURNING Clause (Critical)
+1. Detect MariaDB server type in SQL generators
+2. Replace RETURNING clause with ROW_COUNT()/LAST_INSERT_ID()
+3. Update MySqlUpdateSqlGenerator and related components
+4. **Expected Impact:** Should fix ~95% of failures
+
+### Phase 2: Fix Type Mapping (High)
+1. Add proper type mapping for LEAST() function
+2. Update MySqlQuerySqlGenerator
+3. **Expected Impact:** Should fix 32+ tests
+
+### Phase 3: Re-evaluate & Clean Up
+1. Re-run full test suite after Phase 1 & 2
+2. Update test baselines for MariaDB-specific SQL
+3. Address any remaining edge cases
+
+## Test Environment Details
+
+```
+Database: MariaDB 11.6.2-MariaDB-ubu2404
+Platform: Ubuntu Linux
+.NET SDK: 10.0.100
+Connection: localhost:3306
+SQL Mode: STRICT_TRANS_TABLES,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION
+Max Connections: 512
+```
+
+## Running the Tests
+
+```bash
+# Start MariaDB
+docker run --name mariadb -e MYSQL_ROOT_PASSWORD=Password12! -p 127.0.0.1:3306:3306 -d mariadb:11.6.2
+
+# Build and run tests
+dotnet build -c Debug
+dotnet test test/EFCore.MySql.FunctionalTests -c Debug --no-build --logger "console;verbosity=detailed"
+```
+
+**Note:** Output is ~970K lines. Grep for "Failed" to see failures.
+
+## Next Steps
+
+1. Review the comprehensive analysis report
+2. Prioritize fixes based on impact
+3. Implement RETURNING clause detection/replacement
+4. Add LEAST function type mapping
+5. Re-run tests and validate improvements
+
+---
+
+**Investigation completed successfully. All findings documented and ready for action.**