Following the comprehensive planning and assessment phase, the execution phase involves the actual migration of databases from Microsoft SQL Server to the chosen SQL solution. This phase is critical and requires meticulous attention to detail to ensure data integrity and minimize downtime.

1. Environment Preparation

• Finalize the Target Environment: Ensure the target SQL database environment is fully set up, including the installation and configuration of the database server, any required extensions, and security settings.
• Provision Adequate Resources: Based on your assessments, ensure the target environment has the necessary computational resources, storage, and networking capabilities to handle the migrated databases and their workloads.

2. Data Migration Strategy

• Choose a Migration Method: Depending on the size of your databases and your downtime tolerance, decide whether a direct migration, dump and load, or replication-based migration is most appropriate.
• Schema Conversion: Use available tools or manual processes to convert database schemas from SQL Server to the format required by the target database. Address any incompatibilities in data types, indexes, and constraints.
• Data Type Mappings: Carefully map SQL Server data types to the equivalent types in the target SQL solution, paying special attention to differences that could affect data accuracy or performance.

3. Migrating Business Logic

• Stored Procedures and Functions: Convert T-SQL code to the target SQL dialect. This may require rewriting code to accommodate differences in SQL implementations.
• SSIS Packages Conversion: For migrating SSIS packages, evaluate if the target SQL solution offers similar ETL capabilities or if third-party ETL tools are needed. Reimplement the data integration and transformation logic accordingly.
• Jobs and Scheduled Tasks: Recreate SQL Server Agent jobs in the target system. This might involve using native schedulers, cron jobs in Linux, or third-party scheduling tools.

4. Testing and Validation

• Data Validation: After migrating data, perform comprehensive validation to ensure completeness and accuracy. Compare record counts, checksums, and sample data between the source and target databases.
• Performance Testing: Conduct performance tests to identify any queries or operations that do not meet expected performance levels. Optimize indexes, queries, and configurations based on the findings.
• Application Integration Testing: Test all applications and services that interact with the database for proper functionality, performance, and stability. Ensure all connections and integrations with the new SQL solution are correctly configured.

5. Migration Execution

• Dry Runs: Perform multiple dry run migrations in the test environment to refine the process, reduce the potential for errors, and estimate the required downtime for the final cutover.
• Data Migration: Execute the migration according to the chosen method, closely monitoring the process for any issues or errors.
• Cutover Planning: Plan the cutover to the new system carefully, including a detailed timeline, responsibilities, communication plans, and a rollback strategy in case of unforeseen problems.

6. Monitoring and Troubleshooting

• Monitor the Migration Process: Continuously monitor the migration for performance bottlenecks, errors, and other issues. Use logs and monitoring tools to get real-time insights.
• Troubleshoot Issues: Address any issues as they arise during the migration. Having experts on hand who are familiar with both SQL Server and the target SQL solution can be invaluable for quick problem resolution.

7. Finalizing the Migration

• Post-Migration Cleanup: After the migration is complete, perform any necessary cleanup tasks, such as removing obsolete data, decommissioning old servers, and updating documentation.
• Performance Optimization: Fine-tune the target environment based on the operational data and performance metrics collected since the migration.

This phase is about turning the plans and preparations into action. The success of this phase relies heavily on thorough testing, effective problem-solving, and clear communication among all stakeholders. In the final part, we will discuss post-migration activities, including long-term maintenance, performance tuning, and scaling strategies.