SQL Server Locks Question: Download Questions PDF
Explain the various types of concurrency problem?
Answer:
Concurrency problem:
Lost updates: this occurs when two or more transactions are trying to update same row without being aware of each other. The last update overwrites updates made by the other transactions, which results in lost data.
Uncommitted dependency (Dirty read): This occurs when a second transaction selects a row that is being updated by another transaction. This second transaction is reading data that may not have been committed.
Inconsistent Analysis (Nonrepeatable Read): This occurs when a second transaction accesses the same row several times and reads different data each time. It is similar to dirty read. However, here it reads committed data, but different data each time.
Phantom Reads: This occurs when a range of rows which is being read by a transaction is deleted or updated. The transaction's first read of the range of rows shows a row that no longer exists in the second or succeeding read, as a result of a deletion by a different transaction.
Lost updates: this occurs when two or more transactions are trying to update same row without being aware of each other. The last update overwrites updates made by the other transactions, which results in lost data.
Uncommitted dependency (Dirty read): This occurs when a second transaction selects a row that is being updated by another transaction. This second transaction is reading data that may not have been committed.
Inconsistent Analysis (Nonrepeatable Read): This occurs when a second transaction accesses the same row several times and reads different data each time. It is similar to dirty read. However, here it reads committed data, but different data each time.
Phantom Reads: This occurs when a range of rows which is being read by a transaction is deleted or updated. The transaction's first read of the range of rows shows a row that no longer exists in the second or succeeding read, as a result of a deletion by a different transaction.
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