Podcast
Questions and Answers
What condition must be met for two schedules to be considered view equivalent?
What condition must be met for two schedules to be considered view equivalent?
- Both schedules must have the same set of transactions (correct)
- The final write operation in one schedule must be different from the other
- The initial value reads and write operations must be different
- Both schedules must have different sets of transactions
In view serializability, what must a schedule be view equivalent to?
In view serializability, what must a schedule be view equivalent to?
- A parallel schedule
- A conflicting schedule
- An inconsistent schedule
- A serial schedule (correct)
What is the relationship between conflict serializability and view serializability?
What is the relationship between conflict serializability and view serializability?
- Conflict serializable schedules are always inconsistent
- Conflict serializable schedules are never view serializable
- Every view serializable schedule is also conflict serializable (correct)
- View serializability has no relation to conflict serializability
Which of the following is a condition for view equivalence between two schedules?
Which of the following is a condition for view equivalence between two schedules?
What distinguishes view equivalence in schedules?
What distinguishes view equivalence in schedules?
In view serializability, what must a schedule be equivalent to?
In view serializability, what must a schedule be equivalent to?
What type of schedule is both conflict and view serializable?
What type of schedule is both conflict and view serializable?
What characteristic must every conflict serializable schedule also have?
What characteristic must every conflict serializable schedule also have?
What must a transaction do in both schedules for them to be view equivalent?
What must a transaction do in both schedules for them to be view equivalent?
If a schedule is view equivalent to a parallel schedule, what property does it fail to achieve?
If a schedule is view equivalent to a parallel schedule, what property does it fail to achieve?
Flashcards
Lost Update Problem
Lost Update Problem
Occurs when two transactions read the same data, modify it, and then commit their changes, resulting in one modification being lost.
Temporary Update Problem
Temporary Update Problem
Happens when a transaction modifies a value, but before committing, another transaction reads the partially updated value, leading to inaccuracies.
Incorrect Summary Problem
Incorrect Summary Problem
Arise when transactions that summarize data, like calculating a total, run concurrently, resulting in an inaccurate summary due to uncommitted changes.
Atomicity
Atomicity
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Consistency
Consistency
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Isolation
Isolation
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Durability
Durability
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Validation-Based Protocol
Validation-Based Protocol
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Snapshot Isolation
Snapshot Isolation
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Snapshot Isolation Motivation
Snapshot Isolation Motivation
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Study Notes
Concurrent Execution Issues
- Lost Update Problem: Occurs when two transactions read the same data and then update it concurrently, leading to loss of updates from one transaction.
- Temporary Update Problem: Results when a transaction updates a value, but before it commits, another transaction reads that intermediate value, leading to inconsistencies.
- Incorrect Summary Problem: Arises when transactions that summarize data execute concurrently, resulting in an inaccurate summary due to uncommitted changes.
ACID Properties of Transactions
- Atomicity: Ensures that all operations of a transaction are completed; if not, none are reflected in the database.
- Consistency: Ensures that transactions, when executed in isolation, preserve the integrity and consistency of the database.
- Isolation: Multiple concurrent transactions must operate independently; intermediate results must remain hidden from other transactions.
- Durability: Guarantees that once a transaction is successfully completed, its changes persist, even in the event of a system failure.
- Also known as optimistic concurrency control, where transactions hope for successful validation before committing.
Validation-Based Protocol
- Each transaction ( T_i ) is associated with three timestamps:
- Start(( T_i )): Marks the beginning of the transaction.
- Validation(( T_i )): Indicates when the transaction begins its validation phase.
- Finish(( T_i )): Designates when the transaction completes the write phase.
- The serializability order is based on the Validation timestamp to enhance concurrency.
- Effective when the likelihood of transaction conflicts is low, reducing the need for transaction rollbacks.
Snapshot Isolation
- Motivation: To address concurrency conflicts between decision support queries, which read large data volumes, and OLTP transactions that make small updates.
- Solution: Provides a logical "snapshot" of the database state to read-only transactions while read-write transactions function under standard locking mechanisms.
- Utilizes multiversion 2-phase locking to manage concurrency effectively.
- A challenge remains in determining whether a transaction is read-only to apply appropriate isolation measures.
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Description
Test your knowledge on concurrent execution issues in database systems based on the content from the 7th edition of 'Database System Concepts' by Silberschatz, Korth, and Sudarshan. Questions cover topics like the lost update problem, temporary update problem, and incorrect summary problem.
