Database Systems: Schedules and Serial Schedules

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Questions and Answers

What is a schedule in database concurrency control?

A data structure to store database objects

A sequence of operations in a transaction

A mechanism to control concurrent access

A history of computations showing all the events of interest (correct)

What is the primary concern of transaction scheduling?

Ensuring data security

Maintaining database consistency (correct)

Improving system performance

Reducing the number of transactions

What is a serial schedule?

A schedule that uses locking mechanisms

A schedule that does not interleave the actions of different transactions (correct)

A schedule that allows concurrent access

A schedule that interleaves the actions of different transactions

What is the primary characteristic of a serializable schedule?

It is equivalent to a serial schedule Signup and view all the answers

What is the consequence of a bad concurrent schedule?

Data inconsistency Signup and view all the answers

What operations are considered in serializability analysis?

Read, Write Signup and view all the answers

What is true about two view equivalent schedules?

They have the same effect on the database. Signup and view all the answers

What is a necessary condition for a schedule to be recoverable?

For each pair of transactions, the commit operation of T2 appears before the commit operation of T1. Signup and view all the answers

What is the consequence of two transactions accessing the same database items in an interleaved way?

The schedule may not be serializable. Signup and view all the answers

What is a blind write?

A write operation on a data item without having a read operation on the same item beforehand. Signup and view all the answers

What is the consequence of a transaction failing after committing?

We cannot abort the committed transaction. Signup and view all the answers

Why should data items be accessed in a mutually exclusive way?

To prevent data inconsistency. Signup and view all the answers

What is a necessary condition for two instructions l1 and l2 to be in conflict?

l1 and l2 are on the same data item and at least one of them is a write operation Signup and view all the answers

What does it mean for two schedules S and S’ to be conflict equivalent?

S and S’ can be transformed into each other by a series of nonconflicting instructions Signup and view all the answers

What is a conflict serializable schedule?

A schedule that is conflict equivalent to a serial schedule Signup and view all the answers

What is a condition for two schedules S and S’ to be considered conflict equivalent?

For each data item Q, if transaction T1 reads the initial value of Q in schedule S, then transaction T1 must also do the same in S’ Signup and view all the answers

What is not a condition for two instructions l1 and l2 to be conflict?

l1 and l2 are in the same transaction Signup and view all the answers

What is true about a conflict serializable schedule?

Not every schedule is conflict serializable Signup and view all the answers

Study Notes

Schedules

A schedule is a history of computations showing all the events of interest.
A schedule or history of T1...Tn has operations of T1…Tn.

Event of Interest

Events of interest: Read, Write, Commit, Abort in Ti
Abbreviations: ri, wi, ci, ai

Serial Schedule

A serial schedule is a schedule that does not interleave the actions of different transactions.
Transactions are wholly before or after others.
Serial schedules are correct (assuming each transaction is correct).

Concurrent Schedule

A concurrent schedule is good if it is equivalent to a serial schedule.
A bad concurrent schedule may lead to an inconsistent state.

Equivalent Schedules

Equivalent schedules: For any database state, the effect (on the set of objects in the database) of executing the first schedule is identical to the effect of executing the second schedule.

Serializable Schedule

A serializable schedule is a schedule whose effect on any consistent database instance is guaranteed to be identical to that of some complete serial schedule over the set of committed transactions.
Only consider read and write operations in serializability analysis.

View Equivalence

Two schedules are view equivalent if for each data item, the item is read and updated by the same sequence of transactions in both schedules.
A schedule S is view serializable if it is view equivalent to a serial schedule.

Conflict Serializability

Conflict Serializable => View Serializable
Every conflict serializable schedule is view serializable but vice versa is not true.
If a schedule S is view serializable but not conflict serializable S contains a blind write.

Recoverability

A schedule S is recoverable if for each pair of transactions T1 and T2 in S such that T1 reads a data item previously written by T2, the commit operation of T2 appears before the commit operation of T1.

Concurrency Problems

This problem occurs when two transactions that access the same database items have their operations interleaved in a way that makes the value of database items incorrect.
This problem occurs when one transaction updates a database item and then the transaction fails for some reason.

Data Item Access

Idea: Data items should be accessed in a mutually exclusive way.

Instruction Equivalence

Instructions l1 and l2 are equivalent if they have the same effect.
l1 = op(X), l2 = op(Y), X not equal to Y. => Yes
l1 = read(Q), l2 = read(Q). => Yes
l1 = read(Q), l2 = write(Q). => No
l1 = write(Q), l2 = read(Q). => No
l1 = write(Q), l2 = write(Q). => No

Conflict

l1 and l2 are conflict if:
- l1 and l2 are in different transactions.
- l1 and l2 are on the same data item.
- At least one of them is a write operation.

Conflict Equivalence

If a schedule S can be transferred into a schedule S’ by a series of nonconflicting instructions, then S and S’ are conflict equivalent schedules.
A schedule S is a conflict serializable if it is conflict equivalent to a serial schedule.

Considerations for Schedules

For each data item Q if transaction T1 reads the initial value of Q in schedule S, then transaction T1 must also do the same in S’.
For each data item, the same transaction reads the item both in S and S’.
For each data item Q if transaction T1 executes read in schedule S and if that value was produced by write(Q) executed by transaction T2, then the similar case must be in S’.
For each data item, the sequence of updates made by the transactions are same both in S and S’.
For each data item Q if (any) transaction performs the final write operation in schedule S, then it must do the same in S’.

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Description

This quiz covers schedules in database systems, including the concept of serial schedules and the properties of correct schedules.