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Chapter 18: Problem 18

How does index locking resolve the phantom problem?

Short Answer

Expert verified
Index locking can resolve the phantom problem in databases by locking the range of keys in the index that is used by a transaction. This prevents other transactions from inserting new 'phantom' rows that match the index keys used by the original transaction, ensuring a consistent view of data by restricting concurrent modifications.

Step by step solution

01

Describe the Phantom Problem

In a database, the phantom problem is a scenario where a transaction reads a set of rows from the database that match a certain condition, and when the same transaction attempts to read the rows again (using the same condition), it discovers additional rows that weren't there previously. These new rows are known as 'phantoms'.
02

Explain Index Locking

Index locking is a technique used in databases to maintain consistency and prevent conflicts among concurrent transactions. When a transaction needs to access a data item, it is required to obtain a lock on it. The particular data item remains inaccessible to other transactions until the lock is released. This ensures that no other transaction can interfere with the data item until the initial transaction has completed.
03

Connect Index Locking with the Phantom Problem

Index locking helps resolve the phantom problem by locking not only the actual data rows accessed by a transaction but also the range of keys in the index used to acquire these rows. By doing so, it prevents other transactions from inserting new rows matching the index keys used by the initial transaction, thus preventing the appearance of phantom rows. After an index range is locked, no other transaction can insert, update, or delete any row within this range until the transaction that holds the lock completes. In this way, index locking ensures the consistent state of a transaction, resolving the phantom problem.

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Most popular questions from this chapter

When are latches used?

What are some variations of the two-phase locking protocol? Why is strict or rigorous two-phase locking often preferred?

Describe the wait-die and wound-wait protocols for deadlock prevention.

The MGL protocol states that a transaction \(\mathrm{T}\) can unlock a node \(\mathrm{N}\), only if none of the children of node \(\mathrm{N}\) are still locked by transaction \(\mathrm{T}\). Show that without this condition, the MGL protocol would be incorrect.

Compare binary locks to exclusive/shared locks. Why is the latter type of locks preferable?
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