![]() ![]() The SQL Server RCSI implementation takes advantage of this to provide transactions with a point-in-time view of committed data, where that point in time is the moment the current statement began execution (not the moment any containing transaction started). There is nothing in the SQL standard that requires the data read by a read committed transaction to be the most-recently committed data. Note carefully though that this is not the same as saying that code will behave the same under RCSI as when using the locking implementation of read committed, in fact this is quite generally not the case. When this is enabled, transactions requesting read committed isolation automatically use the RCSI implementation no changes to existing T-SQL code is required to use RCSI. If the database option READ_COMMITTED_SNAPSHOT is ON, SQL Server uses a row-versioning implementation of the read committed isolation level. As it happens, both physical implementations of read committed isolation in SQL Server can experience non-repeatable reads and phantom rows, though the precise details are quite different. The standard also says that read committed transactions might experience the concurrency phenomena known as non-repeatable reads and phantoms (though they are not actually required to do so). ![]() Another way to express this requirement is to say a read committed transaction must only encounter committed data. The SQL standard requires that a transaction operating at the read committed isolation level not experience any dirty reads. While both implementations meet the requirements laid down in the SQL standard for read committed isolation behaviours, RCSI has quite different physical behaviours from the locking implementation we looked at in the previous post in this series. SQL Server provides two physical implementations of the read committed isolation level defined by the SQL standard, locking read committed and read committed snapshot isolation ( RCSI).
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