Lost-Update-Problem in Concurrent schedules

In a schedule, if update performed by transaction T1 on data item ‘X’ gets overwritten by the update performed by transaction T2 on same data item ‘X’, then we say that update of T1 is lost to the update of T2.

This problem is known as Lost-Update-Problem in concurrent schedules.


with X = 50 and Y =50 (initial values)

T1 T2
read(x) (T1I1)
x=x+10 (T1I2)
read(x) (T2I1)
x=x+20 (T2I2)
write(x) (T1I3)
read(y) (T1I4)
write(x) (T2I2)
commit (T2I2)
y=y+10 (T1I5)
write(y) (T1I6)
commit (T1I7)

Explanation: Continue reading

Serial Schedules, Concurrent Schedules and Conflict Operations

A schedule is the representation of execution sequence for all the instructions of the transactions.

Schedules are categorized in two types:

  • Serial Schedules
  • Concurrent Schedules

Serial Schedules:

A schedule is said to be serial if and only if all the instructions of all the transactions get executed non-preemptively as an unit.


Each serial schedule consists of a sequence of instructions from various transactions, where the instructions belonging to one single transaction appear together in that schedule. Continue reading

Concurrent Executions in Transaction

Transaction-processing systems usually allow multiple transactions to run concurrently. Allowing multiple transactions to update data concurrently causes several complications with consistency of the data.

Ensuring consistency in spite of concurrent execution of transactions requires extra work; it is far easier to insist that transactions run serially—that is, one at a time, each starting only after the previous one has completed.

However, there are two good reasons for allowing concurrency: Continue reading

Difference between Normalization and Normal Forms


Normalization is the systematic process applied on the relations to reduce the degree of redundancy.

Normalization is defined as systematic because it always gives guarantee for following properties –

  • Lossless decomposition.
  • Dependency preservation.

Continue reading

DBMS Notes for GATE 2019

Types of Functional Dependencies in Normalization


Functional Dependency:

In Relational database, Functional dependency is denoted as X -> Y where X:Determinant and Y: Dependent. So, as per the concept the value of Y gets determined by the value of X.

If value of X gets duplicated, then in those rows value of Y shall also gets duplicated correspondingly.

If the determinant X value is unique (different) then the dependent Y could have any value meaning:

  • for same X , value of Y should be same.

  • for different X, value of Y could be same or different.

Continue reading……..