SQL READ COMMITTED SHARED LOCKS UPDATE

Because different versions of data blocks can exist simultaneously, transactions can read the version of data committed at the point in time required by a query and return results that are consistent to a single point in time. In this way, the database can present a view of data to multiple concurrent users, with each view consistent to a point in time. Oracle Database maintains data consistency by using a multiversion consistency model and various types of locks and transactions. In short, real-world considerations usually require a compromise between perfect transaction isolation and performance. Complete isolation of concurrently running transactions could mean that one transaction cannot perform an insertion into a table being queried by another transaction. While this degree of isolation between transactions is generally desirable, running many applications in serializable mode can seriously compromise application throughput. A serializable transaction operates in an environment that makes it appear as if no other users were modifying data in the database. To describe consistent transaction behavior when transactions run concurrently, database researchers have defined a transaction isolation model called serializability. Therefore, a multiuser database must provide the following:ĭata concurrency, which ensures that users can access data at the same timeĭata consistency, which ensures that each user sees a consistent view of the data, including visible changes made by the user's own transactions and committed transactions of other users Transactions executing simultaneously must produce meaningful and consistent results.

SQL READ COMMITTED SHARED LOCKS UPDATE

However, in a multiuser database, statements within multiple simultaneous transactions can update the same data.

In a single-user database, a user can modify data without concern for other users modifying the same data at the same time.

Introduction to Data Concurrency and Consistency

Sql read committed shared locks