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The shared lock on a database level is imposed to prevent dropping of the database or restoring a database backup over the database in use.
For example, when a SELECT statement is issued to read some data, a shared lock (S) will be imposed on the database level, an intent shared lock (IS) will be imposed on the table and on the page level, and a shared lock (S) on the row itself In case of a DML statement (i.e.
More details about this is available in Guidelines for Optimizing Bulk Import SQL Server has introduced the locking hierarchy that is applied when reading or changing of data is performed.
The lock hierarchy starts with the database at the highest hierarchy level and down via table and page to the row at the lowest level Essentially, there is always a shared lock on the database level that is imposed whenever a transaction is connected to a database.
While objects are locked, SQL Server will prevent other transactions from making any change of data stored in objects affected by the imposed lock.
Once the lock is released by committing the changes or by rolling back changes to initial state, other transactions will be allowed to make required data changes.
Once the transaction that holds the update lock is ready to change the data, the update lock (U) will be transformed to an exclusive lock (X).
It is important to understand that update lock is asymmetrical in regards of shared locks.
Locking is the way that SQL Server manages transaction concurrency.An exclusive lock can be imposed to a page or row only if there is no other shared or exclusive lock imposed already on the target.This practically means that only one exclusive lock can be imposed to a page or row, and once imposed no other lock can be imposed on locked resources Shared lock (S) – this lock type, when imposed, will reserve a page or row to be available only for reading, which means that any other transaction will be prevented to modify the locked record as long as the lock is active.insert, update, delete) a shared lock (S) will be imposed on the database level, an intent exclusive lock (IX) or intent update lock (IU) will be imposed on the table and on the page level, and an exclusive or update lock (X or U) on the row Locks will always be acquired from the top to the bottom as in that way SQL Server is preventing a so-called Race condition to occur.Now that lock modes and lock hierarchy have been explained, let’s further elaborate on lock modes and how those translate to a lock hierarchy. At the row level, the following three lock modes can be applied: ✓ – Compatible ✗ – Incompatible A Schema lock (Sch) is also a table level lock as well, but it is not a data related lock To better understand the compatibility between these lock types, please refer to this table: In order to prevent a situation where locking is using too many resources, SQL Server has introduced the lock escalation feature.
However, a bulk update lock will not prevent another bulk load to be processed in parallel.