What is Phantom read?
A Phantom read occurs when one user is repeating a read operation on the same records, but has new records in the results set: READ UNCOMMITTED. Also called a Dirty read. When this isolation level is used, a transaction can read uncommitted data that later may be rolled back.
What is a phantom problem explain your answer with the example?
The so-called phantom problem occurs within a transaction when the same query produces different sets of rows at different times. For example, if a SELECT is executed twice, but returns a row the second time that was not returned the first time, the row is a “phantom” row.
How do you solve a phantom read?
In the above example, to fix the phantom read concurrency problem let set the transaction isolation level of Transaction 1 to serializable. The Serializable Transaction Isolation Level places a range lock on the rows returns by the transaction based on the condition.
What is Phantom read in SQL?
Phantom Reads: Occurs when, during a transaction, new rows are added (or deleted) by another transaction to the records being read. New rows can be added by other transactions, so you end up getting different number of rows by the same query in current transaction.
How do I stop phantom read?
PHANTOM reads can be prevented by using SERIALIZABLE isolation level, the highest level. This level acquires RANGE locks thus preventing READ, Modification and INSERT operation on other transaction until the first transaction gets completed.
What is dirty read in SQL?
Dirty Reads A dirty read occurs when a transaction reads data that has not yet been committed. For example, suppose transaction 1 updates a row. Transaction 2 reads the updated row before transaction 1 commits the update. If transaction 1 reexecutes the statement that reads the rows, it gets a different set of rows.
Does Read committed lock?
The key difference between SQL Server locking read committed and locking repeatable read (which also takes shared locks when reading data) is that read committed releases the shared lock as soon as possible, whereas repeatable read holds these locks to the end of the enclosing transaction.
What is a dirty read in SQL?
A dirty read occurs when one transaction is permitted to read data that is being modified by another transaction that is running concurrently but which has not yet committed itself. If the transaction that modifies the data commits itself, the dirty read problem doesn’t occur.
Why Nolock is bad?
NOLOCK is often exploited as a magic way to speed up database reads, but I try to avoid using it whever possible. The result set can contain rows that have not yet been committed, that are often later rolled back. An error or Result set can be empty, be missing rows or display the same row multiple times.
Which is an example of the 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. In the above example, once transaction 2 reads the variable X, transaction 1 deletes the variable X without transaction 1’s knowledge.
How to fix the phantom read concurrency problem?
When does phantom read occur in SQL Server?
This generally happens when a second transaction inserts some new rows in between the first and second query execution of the first transaction that matches the WHERE clause of the query executed by the first transaction. Let’s understand this with an example. We are going to use the following Employees table for this example.
What’s the difference between a phantom read and a non repeatable read?
A non-repeatable read occurs, when during the course of a transaction, a row is retrieved twice and the values within the row differ between reads. and. A phantom read occurs when, in the course of a transaction, two identical queries are executed, and the collection of rows returned by the second query is different from the first.