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Big Data Analytics Articles
Page 118 of 135
Explain cartesian product in relational algebra (DBMS)?
Query is a question or requesting information. Query language is a language which is used to retrieve information from a database.Query language is divided into two types −Procedural languageNon-procedural languageProcedural languageInformation is retrieved from the database by specifying the sequence of operations to be performed.For Example − Relational algebra.Structure Query language (SQL) is based on relational algebra.Relational algebra consists of a set of operations that take one or two relations as an input and produces a new relation as output.Types of Relational Algebra operationsThe different types of relational algebra operations are as follows −Select operationProject operationRename operationUnion operationIntersection operationDifference operationCartesian ...
Read MoreWhat is an optimistic concurrency control in DBMS?
All data items are updated at the end of the transaction, at the end, if any data item is found inconsistent with respect to the value in, then the transaction is rolled back.Check for conflicts at the end of the transaction. No checking while the transaction is executing. Checks are all made at once, so low transaction execution overhead. Updates are not applied until end-transaction. They are applied to local copies in a transaction space.PhasesThe optimistic concurrency control has three phases, which are explained below −Read PhaseVarious data items are read and stored in temporary variables (local copies). All operations ...
Read MoreExplain about the Time stamp ordering protocol in DBMS
The timestamp-ordering protocol ensures serializability among transactions in their conflicting read and write operations. This is the responsibility of the protocol system that the conflicting pair of tasks should be executed according to the timestamp values of the transactions.A conflict occurs when an older transaction tries to read/write a value already read or written by a younger transaction. Read or write proceeds only if the last update on that data item was carried out by an older transaction.Otherwise, the transaction requesting read/write is restarted and gives a new timestamp. Here no locks are used so no deadlock.The timestamp of transaction ...
Read MoreExplain serial execution or transaction with an example(DBMS)
There are three possible ways in which a transaction can be executed. These are as follows −Serial execution − In serial execution, the second transaction can begin its execution only after the first transaction has completed. This is possible on a uniprocessor system.Parallel execution − In parallel execution, two transactions can start their execution at exactly the same instant of time. For this, we require more than one processor.Concurrent execution − In concurrent execution, execution of the second process can begin even before the process has completed its execution.Let us consider two transactions T1 and T2. In concurrent execution, the ...
Read MoreExplain schedules in 2PL with multiple granularities locking and schedules under tree(DBMS)
Locking and unlocking of the database should be done in such a way that there is no inconsistency, deadlock and no starvation.2PL locking protocolEvery transaction will lock and unlock the data item in two different phases.Growing Phase − All the locks are issued in this phase. No locks are released, after all changes to data-items are committed and then the second phase (shrinking phase) starts.Shrinking phase − No locks are issued in this phase, all the changes to data-items are noted (stored) and then locks are released.Consider a tree structure database as follows −A -> B -> CLet us now ...
Read MoreWhat is a materialized view in DBMS?
A materialized view is a view whose contents are computed and stored. Materialized view is also a logical virtual table, but in this case the result of the query is stored in the table or the disk. The performance of the materialized view is better than normal view since the data is stored in the disk.It's also called indexed views since the table created after the query is indexed and can be accessed faster and efficiently.ExampleConsider the view given below −Create view branchloan(branch-name, total-loan) as select branch-name , sum(amount) from loan groupby branch-name;Materializing the above view would be especially useful ...
Read MoreCheck the view serializability for the given schedules(DBMS)
A schedule has view-serializability if it is viewed as equivalent to a serial schedule. A schedule is view serializable if the following three rules are satisfied −Rule 1 − If Ti reads data initially, after this Tj writes the same data, in the given schedule. This sequence must be followed in the transaction combination (read write operation).Rule 2 − If Ti writes data initially, after this Tj reads the same data, in the given schedule. This sequence must be followed in the transaction combination (write read operation).Rule 3 − If Ti writes data, after this Tj writes the data finally. ...
Read MoreWhen do we say the schedule is conflict equivalent(DBMS)?
Two schedules are said to be conflict equivalent if the order of any two conflicting operations are the same in both the schedules.Also, a concurrent schedule S is conflict equivalent to a serial schedule S’, if we can obtain S’ out of S by swapping the order of execution of non-conflicting instructions.Example 1Even if the schedule S1 keeps the database in the consistent state, we cannot convert it into a serial schedule and hence we conclude that the schedule is not conflict equivalent to any of the serial schedules.So, instead of considering only the read and write operation, we will ...
Read MoreWhat do you mean by schedule in DBMS?
A transaction must satisfy the ACID properties of DBMS, namelyAtomicity − Execute all or none transaction.Consistency − The DB should be consistent before and after transaction execution.Isolation − Transaction must be executed in isolated form.Durability − Roll backing should always be possible in Case of any failure.A schedule is defined as an execution sequence of transactions. A schedule maintains the order of the operation in each individual transaction. A schedule is the arrangement of transaction operations. A schedule may contain a set of transactions.We already know that a transaction is a set of operations. To run transactions concurrently, we arrange ...
Read MoreWhat is an expression tree in DBMS?
Expression tree is also called a query tree. It demonstrates an evaluation plan.ExampleConsider an evaluation plan for the given query −Select name from customer, account where customer.name=account.name and account.balance>2000;There are two evaluation plans −Πcustomer.name(σcustomer.name=account.name^ account.balance>2000(customerXaccount)Πcustomer.name(σcustomer.name=account.name(customerXσ account.balance>2000(account)The expression trees for the above evaluation plans are as follows −Cost evaluator evaluates the cost of different evaluation plans and chooses the evaluation plan with lowest cost. Disk access time, CPU time, number of operations, number of tuples, size of tuples are considered for cost calculations.
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