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What is conflict serializability in SQL ?

Buchiredddypalli Koushik

Overview

Serializability is bascially used to ensure that transactions are executing in a correct way without any interruption.

  • Serializability allows the database to be in a consistent state.

  • Serializability is generally used in the transactions.

Serializability is of two types:

  • Conflict serializability
  • View serializability

Conflict serializability occurs when two instructions conflict for a single data item.

There will be no conflict if we read the data, but there will be a conflict if we write the data.

Instructions O i and O j are the transactions of Ti and Tj respectively. Then there exists a conflict if some item Q accessed by both O i and O j. At least one of these instructions writes Q.

  1. O i = read(Q), O j = read(Q). O i and O j don’t conflict.

  2. O i = read(Q), O j = write(Q). They conflict.

  3. O i = write(Q), O j = read(Q). They conflict.

  4. O i = write(Q), O j = write(Q). They conflict.

If a schedule S can be converted into a schedule S ´ by a series of swaps of non-conflicting instructions, we say that S and S ´ conflict equivalent.

S

T1

T2

Read(A)


Write(A)



Read(A)


Write(A)



Read(B)


Write(B)





Read(B)


Write(B)

S'

T1

T2

Read(A)


Write(A)


Read(B)


Write(B)





Read(A)


Write(A)


Read(B)


Write(B)

  • Here S and S’ are conflict equivalent.

  • The order of each pair of conflicting operations in S and S` is the same.

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Buchiredddypalli Koushik
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