Question

I’ve read that in older versions of SQL Server .. it had a pessimistic locking strategy. I.e. readers wait on writers for access to the same data (row or page level), unlike Oracle.

Is this still the case in newer versions ? I’ve read that the locking strategy has been changed in recent versions.

Answer 1

What you heard of is the SNAPSHOT ISOLATION, available since SQL Server 2005. Snapshot isolation, aka. row-versioning, is the default behavior in Oracle. You can make it default in SQL Server too, by enabling READ_COMMITTED_SNAPSHOT on the database:

ALTER DATABASE [<dbname>] SET READ_COMMITTED_SNAPSHOT ON;

With row versioning SQL Server does not acquire data locks during reads. If concurrent writes occur, the read will fetch the previous version of the row. For more details, read Row Versioning-based Isolation Levels in the Database Engine.

You should not confuse row versioning and snapshot with dirty reads. Dirty reads offer inconsistent data which makes programming a challenge, to say the least (ie. you should not use it!). Snapshot reads offer always a transactionally consistent image of the data.