3. How do you create an index on a table in a relational database, and what is the significance of indexing in terms of query performance?

Creating an index on a table in a relational database involves a few steps. Here's a general guide:

1. Identify the column(s) you want to index: This is typically based on the queries you run most often. For example, if you frequently search for users by their last name, you might want to index the "last_name" column.

2. Use the "CREATE INDEX" SQL command: The syntax for this command is "CREATE INDEX index_name ON table_name (column_name)". For example, to create an index on the "last_name" column in a "users" table, you would run "CREATE INDEX index_last_name ON users (last_name)".

3. Run the command: You can do this in your database management system's command line interface, or in a SQL script.

The significance of indexing in terms of query performance is that it can greatly speed up data retrieval. Without an index, the database management system must go through every row in the table to find the ones that satisfy the query conditions (a "full table scan"). With an index, it can find the rows much more quickly, similar to how an index in a book helps you find information more quickly than if you had to read the entire book.

However, indexes also have some drawbacks. They take up space, and they can slow down write operations (INSERT, UPDATE, DELETE), because the database has to update the index every time the data changes. Therefore, it's important to use indexes judiciously and monitor their impact on your database's performance.