Define Dynamic Database? Explain Phantom Problem By Considering Suitable Example?

A dynamic database is a type of database system in which the database schema can be modified while the system is in use. This allows for more flexibility and adaptability compared to static databases, which have a fixed schema that cannot be changed without significant effort and downtime.

One problem that can arise in dynamic databases is the phantom problem, which occurs when a transaction reads a set of records that satisfy a certain condition, and another transaction inserts or updates records that also satisfy that condition before the first transaction completes. This can result in the first transaction seeing extra records that were not present when it started, creating a phantom set of records.

For example, suppose we have a dynamic database of student records, with two tables: 'students' and 'enrollments'. The 'students' table has columns for the student's ID, name, and major. The 'enrollments' table has columns for the student's ID, the course ID, and the semester.

Now, consider two transactions that are running concurrently. The first transaction executes the following query:

sql
SELECT * FROM enrollments WHERE semester = 'Spring 2022';

This query returns a set of records for all the enrollments in the Spring 2022 semester. The second transaction then inserts a new record into the 'enrollments' table for a student who is taking a new course in the Spring 2022 semester:

sql
INSERT INTO enrollments VALUES (123, 'CS101', 'Spring 2022');

If the first transaction has not yet completed, it will now see an extra record that was not present when it started, creating a phantom set of records.

To prevent the phantom problem, database systems use various concurrency control methods, such as locking or multiversion concurrency control, to ensure that transactions do not interfere with each other in unexpected ways.