If it is not allowed to rollback a TRUNCATE statement then how is it possible to use it in a transaction? [duplicate]

Answer 1

MS SQL:

The concept that a Truncate statement cannot be rolled back is fairly commonly misunderstood.

When you execute a Truncate statement, it does not get logged in the log file as it is a DDL statement.

So if you Truncate a table, you cannot Roll Back to a point in time before the truncate.

However, in a Transaction, Rollback is permitted and functions just as any other rollback would.

The rollback referred to is not transactional rollback.

In essence: Anywhere before a COMMIT is issued, you can rollback a TRUNCATE statement. After a COMMIT, you cannot rollback the data even by using the log file. Unrecoverable.

Answer 2

In Oracle, the TRUNCATE statement terminates the current transaction before it executes (with a COMMIT, I believe). Then, because it is treated like a DDL statement, the TRUNCATE executes without possibility of rollback. And then a new transaction is started.

As I understand it, the behaviour of other DDL statements in Oracle is somewhat similar.

Some DBMS provide transactional DDL statements - and very useful it is, too.

Answer 3

It is a fairly common SQL Server belief that Truncate Cannot Be Rolled Back Because It Is Not Logged. However the code in your question just verifies that this is in fact a myth.

TRUNCATE does entail much less logging than DELETE as it only logs the page deallocatons and some metadata updates to system tables rather than recording the entire contents of the deleted rows to the log (See a detailed breakdown of logging here).

SQL Server continues to lock the deallocated pages until the transaction commits preventing their re-allocation so ROLLBACK of a TRUNCATE simply needs to reverse these page deallocations and system table updates that are recorded in the log.