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How does transaction isolation work?
When a transaction uses this isolation level, a SELECT query (without a FOR UPDATE/SHARE clause) sees only data committed before the query began; it never sees either uncommitted data or changes committed during query execution by concurrent transactions.
When one transaction reads a changed record that has not been committed to the database then what anomaly will be used there?
Dirty Read – A Dirty read is the situation when a transaction reads a data that has not yet been committed. For example, Let’s say transaction 1 updates a row and leaves it uncommitted, meanwhile, Transaction 2 reads the updated row.
How do you serialize a transaction?
A transaction is said to be Serializable if it is equivalent to serial schedule. Consider the two cases below. We can see that irrespective of the order of the execution, the transaction results in same result. Hence they can be scheduled serially and hence these transactions are Serializable.
What is phantom read problem?
The phantom read problem occurs when a transaction reads a variable once but when it tries to read that same variable again, an error occurs saying that the variable does not exist. Example: Thus, when transaction 2 tries to read X, it is not able to it.
Can a statement be read at the READ UNCOMMITTED level?
Specifies that statements can read rows that have been modified by other transactions but not yet committed. Transactions running at the READ UNCOMMITTED level do not issue shared locks to prevent other transactions from modifying data read by the current transaction.
Can a read only transaction be started in a database?
Starting a transaction to perform a read-only operation adds to the overhead of the processing thread and can cause shared read locks on the database (depending on what type of database you are using and what the isolation level is set to).
What’s the difference between READ COMMITTED and UNCOMMITTED isolation?
In addition to that, for several popular products, read committed isolation is the lowest isolation level available; their implementations of read uncommitted isolation are exactly the same as read committed.
Can a database engine read only committed data?
First, the SQL standard requirement to read only committed data does not necessarily mean that the committed data read by a transaction will be the most-recently committed data. A database engine is allowed to read a committed version of a row from any point in the past, and still comply with the SQL standard definition.