1. How can we optimize the query performance during the selection refinement process?

To optimize the query performance during the selection refinement process, you can follow these steps:

1. Indexing: Indexes can significantly improve query performance. Make sure that all columns used in WHERE, ORDER BY, GROUP BY, and JOIN clauses are indexed.

2. Query Simplification: Simplify your queries as much as possible. Avoid using subqueries and instead use joins if possible.

3. Use EXPLAIN PLAN: The EXPLAIN PLAN statement displays the execution plan that the database optimizer would choose, given the available indexes. This can help you understand how your query will be executed.

4. Limit the Amount of Data: Use the LIMIT clause to restrict the amount of data that needs to be read from the database.

5. Avoid Using Wildcards at the Start of Predicates: The use of a wildcard at the start of a predicate (e.g., LIKE '%string') can greatly slow down a query, as it prevents the use of indexes.

6. Use Appropriate Data Types: Using appropriate data types for your columns can also improve performance.

7. Optimize Like Statements: If you have to use a LIKE statement, try to optimize it by avoiding leading wildcards.

8. Avoid Functions in Predicates: Using functions in the WHERE clause can result in full table scans.

9. Partition Large Tables: If you're dealing with very large tables, consider partitioning them. This can greatly improve query performance.

10. Keep Statistics Up to Date: The database optimizer uses statistics about the distribution of data in your tables to create the most efficient query plan. If these statistics are out of date, the optimizer may choose a less-than-optimal query plan.