The "concise" option is not terribly concise in my experience. Here's an example:
> Q: "What is the difference between serializable and snapshot isolation?" (with concise enabled)
> A: "According to geeksforgeeks.org, both snapshot isolation and serializable isolation levels are considered the highest levels of transaction isolation in DBMS. These levels are used to avoid concurrency issues such as dirty read, phantom read, and non-repeatable read. Both isolation levels avoid these issues, but in different ways.
As stated in techcommunity.microsoft.com, both snapshot and serializable isolation levels provide a read-consistent view of the database to all transactions. In either of these isolation levels, a transaction can only read data that has been committed. Moreover, a transaction can read the same data multiple times without ever observing any concurrent transactions making changes to this data. The unexpected read committed and repeatable read results that are possible in other isolation levels are not possible in serializable or snapshot isolation levels.
In serializable isolation level, SQL Server acquires key range locks and holds them until the end of the transaction. A key range lock ensures that, once a transaction reads data, no other transaction can alter that data - not even to insert phantom rows - until the transaction holding the lock completes. In snapshot isolation level, SQL Server does not acquire any locks. Thus, it is possible for a concurrent transaction to modify data that a second transaction has already read. The second transaction simply does not observe the changes and continues to read an old copy of the data.
Serializable isolation level relies on pessimistic concurrency control. It guarantees consistency by assuming that two transactions might try to update the same data and uses locks to ensure that they do not but at a cost of reduced concurrency - one transaction must wait for the other to complete and two transactions can deadlock. Snapshot isolation level relies on optimistic concurrency control. It allows transactions to proceed without locks and with maximum concurrency, but may need to fail and rollback a transaction if two transactions attempt to modify the same data at the same time. According to techcommunity.microsoft.com, it is clear that there are differences in the level of concurrency that can be achieved and in the failures (deadlocks vs. update conflicts) that are possible with the serializable and snapshot isolation levels.
As explained in learn.microsoft.com, snapshot isolation must be enabled by setting the ALLOW_SNAPSHOT_ISOLATION ON database option before it is used in transactions. This activates the mechanism for storing row versions in the temporary database (tempdb). You must enable snapshot isolation in each database that uses it with the Transact-SQL ALTER DATABASE statement. This is different from traditional isolation levels of READ COMMITTED, REPEATABLE READ, SERIALIZABLE, and READ UNCOMMITTED, which require no configuration. The READ_COMMITTED_SNAPSHOT ON option allows access to versioned rows under the default READ COMMITTED isolation level. If the READ_COMMITTED_SNAPSHOT option is set to OFF, you must explicitly set the Snapshot isolation level for each session in order to access versioned rows.
In conclusion, both snapshot and serializable isolation levels are the highest levels of transaction isolation in DBMS, but they avoid concurrency issues in different ways. Serializable isolation level uses pessimistic concurrency control, while snapshot isolation level uses optimistic concurrency control. Serializable isolation level guarantees consistency, but it is achieved at the cost of reduced concurrency, while snapshot isolation level provides maximum concurrency but may need to fail and rollback a transaction if two transactions attempt to modify the same data at the same time. To use snapshot isolation, you must enable it with the Transact-SQL ALTER DATABASE statement."
When I feed this into ChatGPT 4 and ask for a more concise summary, I get this, which is closer to what I'd expect from Phind with the "concise" option enabled:
> "Snapshot and serializable isolation levels are the highest levels of transaction isolation in DBMS, preventing concurrency issues like dirty read, phantom read, and non-repeatable read. Serializable isolation relies on pessimistic concurrency control, using locks to ensure consistency, which may reduce concurrency and cause deadlocks. Snapshot isolation uses optimistic concurrency control, allowing maximum concurrency but may require rolling back transactions if conflicts arise. Snapshot isolation must be enabled with the Transact-SQL ALTER DATABASE statement before use."
Expert mode combined with Concise mode tends to work best. The default mode is more verbose and doesn't listen to the concise flag as closely as Expert mode. This is definitely something we're working on.
> Q: "What is the difference between serializable and snapshot isolation?" (with concise enabled)
> A: "According to geeksforgeeks.org, both snapshot isolation and serializable isolation levels are considered the highest levels of transaction isolation in DBMS. These levels are used to avoid concurrency issues such as dirty read, phantom read, and non-repeatable read. Both isolation levels avoid these issues, but in different ways.
As stated in techcommunity.microsoft.com, both snapshot and serializable isolation levels provide a read-consistent view of the database to all transactions. In either of these isolation levels, a transaction can only read data that has been committed. Moreover, a transaction can read the same data multiple times without ever observing any concurrent transactions making changes to this data. The unexpected read committed and repeatable read results that are possible in other isolation levels are not possible in serializable or snapshot isolation levels.
In serializable isolation level, SQL Server acquires key range locks and holds them until the end of the transaction. A key range lock ensures that, once a transaction reads data, no other transaction can alter that data - not even to insert phantom rows - until the transaction holding the lock completes. In snapshot isolation level, SQL Server does not acquire any locks. Thus, it is possible for a concurrent transaction to modify data that a second transaction has already read. The second transaction simply does not observe the changes and continues to read an old copy of the data.
Serializable isolation level relies on pessimistic concurrency control. It guarantees consistency by assuming that two transactions might try to update the same data and uses locks to ensure that they do not but at a cost of reduced concurrency - one transaction must wait for the other to complete and two transactions can deadlock. Snapshot isolation level relies on optimistic concurrency control. It allows transactions to proceed without locks and with maximum concurrency, but may need to fail and rollback a transaction if two transactions attempt to modify the same data at the same time. According to techcommunity.microsoft.com, it is clear that there are differences in the level of concurrency that can be achieved and in the failures (deadlocks vs. update conflicts) that are possible with the serializable and snapshot isolation levels.
As explained in learn.microsoft.com, snapshot isolation must be enabled by setting the ALLOW_SNAPSHOT_ISOLATION ON database option before it is used in transactions. This activates the mechanism for storing row versions in the temporary database (tempdb). You must enable snapshot isolation in each database that uses it with the Transact-SQL ALTER DATABASE statement. This is different from traditional isolation levels of READ COMMITTED, REPEATABLE READ, SERIALIZABLE, and READ UNCOMMITTED, which require no configuration. The READ_COMMITTED_SNAPSHOT ON option allows access to versioned rows under the default READ COMMITTED isolation level. If the READ_COMMITTED_SNAPSHOT option is set to OFF, you must explicitly set the Snapshot isolation level for each session in order to access versioned rows.
In conclusion, both snapshot and serializable isolation levels are the highest levels of transaction isolation in DBMS, but they avoid concurrency issues in different ways. Serializable isolation level uses pessimistic concurrency control, while snapshot isolation level uses optimistic concurrency control. Serializable isolation level guarantees consistency, but it is achieved at the cost of reduced concurrency, while snapshot isolation level provides maximum concurrency but may need to fail and rollback a transaction if two transactions attempt to modify the same data at the same time. To use snapshot isolation, you must enable it with the Transact-SQL ALTER DATABASE statement."
When I feed this into ChatGPT 4 and ask for a more concise summary, I get this, which is closer to what I'd expect from Phind with the "concise" option enabled:
> "Snapshot and serializable isolation levels are the highest levels of transaction isolation in DBMS, preventing concurrency issues like dirty read, phantom read, and non-repeatable read. Serializable isolation relies on pessimistic concurrency control, using locks to ensure consistency, which may reduce concurrency and cause deadlocks. Snapshot isolation uses optimistic concurrency control, allowing maximum concurrency but may require rolling back transactions if conflicts arise. Snapshot isolation must be enabled with the Transact-SQL ALTER DATABASE statement before use."