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Are there Phantoms in MySQL?
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 deal with phantom problems?
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 record?
Phantom record:- When a new record is inserted by some transaction T, that satisfies the condition, a set of records accessed by another Transaction . At this time, transaction followed by transaction T, it is new one and it is not included for equivalent serial order.
What is a dirty read in database?
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.
What are phantom limbs?
Definition. Phantom limb syndrome is the feeling of sensations in a limb that has been removed. There may be feelings in the limb as if it were still attached to their body. This is because the brain continues to get messages from nerves that used to “feel” for the missing limb.
When is a row a phantom in MySQL?
/ 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 does InnoDB prevent phantom rows in MySQL?
To prevent phantoms, InnoDB uses an algorithm called next-key locking that combines index-row locking with gap locking. InnoDB performs row-level locking in such a way that when it searches or scans a table index, it sets shared or exclusive locks on the index records it encounters.
Is there a free version of MySQL for phantom reads?
Sql Server offers another isolation level, snapshot, that does what MySql InnoDb and Postgres does in repeatable read (which is a lock-free, version-based implementation of repeatable read without phantom reads, but is not serializable). Sql Server Express is free although you do need a Windows machine.
Why do phantom reads occur in SQL Server?
Phantom reads can occur because not range-locks exist, then an example is (pseudocode): The important thing here is the transactions syncronization, you can use sync points. To complement Dani’s good answer, you could use Microsoft Sql Server to show that behavior to your students.