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In the world of database management, understanding how SQL Server handles and caches execution plans can significantly optimize your database performance. Execution plans represent the actual steps your SQL Server 2019, MySQL 8.0, or DB2 11.5 will take to execute a particular SQL command. Once a plan is created, it's stored in the plan cache for future use. This mechanism saves resources and improves performance by reusing the same plan for identical queries.

SQL Server uses a cost-based query optimizer. This means it tries to formulate the least expensive execution plan for a given query, considering factors like CPU time and I/O operations. When a query is submitted, SQL Server will first check the plan cache to see if an execution plan for the same query already exists. If it does, it will reuse that plan. If it doesn't, the query optimizer is called in to create a new execution plan.

-- T-SQL code for plan cache overview
SELECT usecounts, cacheobjtype, objtype, text 
FROM sys.dm_exec_cached_plans 
CROSS APPLY sys.dm_exec_sql_text(plan_handle) 
ORDER BY usecounts DESC;

In MySQL 8.0, the concept of execution plan caching is quite similar. The MySQL optimizer uses a cost-based model and will try to reuse execution plans whenever possible. The 'EXPLAIN' statement can be used to view the execution plan for a query.

-- MySQL code to view execution plan
EXPLAIN SELECT * FROM employees WHERE emp_no = 10001;

For DB2 11.5, the database engine also uses a cost-based optimizer and caches execution plans. The 'EXPLAIN PLAN FOR' statement can be used to view the execution plan for a particular query.

-- DB2 code to view execution plan
EXPLAIN PLAN FOR SELECT * FROM employees WHERE emp_no = 10001;

In the realm of cloud databases, Azure SQL and Azure Synapse follow similar execution plan caching principles. Azure SQL uses the same model as SQL Server, and Azure Synapse, being an analytical service, optimizes the execution plans for OLAP operations. Both services provide comprehensive views and tools for inspecting the execution plans and their performance.

In conclusion, understanding and leveraging execution plan caching can be a powerful tool in optimizing your SQL performance. Whether you're using SQL Server 2019, MySQL 8.0, DB2 11.5, Azure SQL, or Azure Synapse, being aware of how your database engine handles execution plans and caches can make a significant difference in your database's efficiency and performance.