What precisely do you mean by "query optimization?"

please improe this writing: There is a big difference between time it takes to perform each of the queries. After adding primary key to each table, the time to perform each of the queries was faster than what it was before. Creating a primary key help identify unique data. Primary key in a table is a crucial column used in queries, optimized for fast performance with an associated index. The primary key index is the most efficient access path for a table. They are created after a table is created, optimizing search speed. Once constructed, indexes are internal structures that the user cannot actively access. If the internal query optimization procedure concludes that using an index will increase search efficiency, then it will be employed. When SQL queries are internally ready for execution, they automatically undergo optimization. The best way to run each query is determined by the optimization process, which may or may not include using a relevant index. Read and write speeds are…

A real-world SQL database sample should be provided and explained.