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, you can request any of the four standard transaction isolation levels.
But internally, there are only three distinct isolation levels, which correspond to the levels Read Committed, Repeatable Read, and Serializable.
In the case of , this means it is the updated version of the row that is locked and returned to the client.
Because of the above rule, it is possible for an updating command to see an inconsistent snapshot: it can see the effects of concurrent updating commands on the same rows it is trying to update, but it does not see effects of those commands on other rows in the database.
When an application receives this error message, it should abort the current transaction and retry the whole transaction from the beginning.
The second time through, the transaction will see the previously-committed change as part of its initial view of the database, so there is no logical conflict in using the new version of the row as the starting point for the new transaction's update.
in terms of searching for target rows: they will only find target rows that were committed as of the transaction start time.
In this case, the repeatable read transaction will wait for the first updating transaction to commit or roll back (if it is still in progress).
The behavior of the available isolation levels is detailed in the following subsections.
(However, the query does see the effects of previous updates executed within its own transaction, even though they are not yet committed.) This is a stronger guarantee than is required by the transaction see the same data, i.e., they do not see changes made by other transactions that committed after their own transaction started.
Applications using this level must be prepared to retry transactions due to serialization failures.
Applications that do complex queries and updates might require a more rigorously consistent view of the database than Read Committed mode provides.
The Repeatable Read isolation level only sees data committed before the transaction began; it never sees either uncommitted data or changes committed during transaction execution by concurrent transactions.
When you select the level Read Uncommitted you really get Read Committed, and phantom reads are not possible in the implementation of Repeatable Read, so the actual isolation level might be stricter than what you select.