An atomic statement is a statement that is executed completely or not at all. For example, an UPDATE statement that updates thousands of rows might encounter an error after updating many rows. If the statement does not complete, all changed rows revert back to their original state. The UPDATE statement is atomic.
All non-compound SQL statements are atomic. You can make a compound statement atomic by adding the keyword ATOMIC after the BEGIN keyword.
BEGIN ATOMIC UPDATE Employees SET ManagerID = 501 WHERE EmployeeID = 467; UPDATE Employees SET BirthDate = 'bad_data'; END
In this example, the two update statements are part of an atomic compound statement. They must either succeed or fail as one. The first update statement would succeed. The second one causes a data conversion error since the value being assigned to the BirthDate column cannot be converted to a date.
The atomic compound statement fails and the effect of both UPDATE statements is undone. Even if the currently executing transaction is eventually committed, neither statement in the atomic compound statement takes effect.
If an atomic compound statement succeeds, the changes made within the compound statement take effect only if the currently executing transaction is committed. In the case when an atomic compound statement succeeds but the transaction in which it occurs gets rolled back, the atomic compound statement also gets rolled back. A savepoint is established at the start of the atomic compound statement. Any errors within the statement result in a rollback to that savepoint.
When an atomic compound statement is executed in autocommit (unchained) mode, the commit mode changes to manual (chained) until statement execution is complete. In manual mode, DML statements executed within the atomic compound statement do not cause an immediate COMMIT or ROLLBACK. If the atomic compound statement completes successfully, a COMMIT statement is executed; otherwise, a ROLLBACK statement is executed. For more information about autocommit behavior, see Setting autocommit or manual commit mode and Controlling autocommit behavior.
You cannot use COMMIT and ROLLBACK and some ROLLBACK TO SAVEPOINT statements within an atomic compound statement. See Transactions and savepoints in procedures and triggers.
There is a case where some, but not all, statements within an atomic compound statement are executed. This happens when an exception handler within the compound statement deals with an error.
For more information, see Using exception handlers in procedures and triggers.
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