Updating a table in sql server 2016
To understand better the locking in SQL Server, it is important to understand that locking is designed to ensure the integrity of the data in the database, as it forces every SQL Server transaction to pass the ACID test.
ACID test consists of 4 requirements that every transaction have to pass successfully: SQL Server locking is the essential part of the isolation requirement and it serves to lock the objects affected by a transaction.
However, a shared lock can be imposed by several transactions at the same time over the same page or row and in that way several transactions can the ability for data reading since the reading process itself will not affect anyhow the actual page or row data.
In addition, a shared lock will allow write operations, but no DDL changes will be allowed Update lock (U) – this lock is similar to an exclusive lock but is designed to be more flexible in a way.
Essentially, locks are in-memory structures which have owners, types, and the hash of the resource that it should protect.
A lock as an in-memory structure is 96 bytes in size.
Without escalation, locks could require a significant amount of memory resources.
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.
After the transaction acquires an SIU lock on the table, the query with the PAGELOCK hint will acquire the shared (S) lock while the update query will acquire intent update (IU) lock Update with intent exclusive (UIX) – when update lock (U) and intent exclusive (IX) locks are acquired at lower hierarchy resources in the table simultaneously, the update with intent exclusive lock will be acquired at the table level as a consequence Schema locks (Sch) – The SQL Server database engine recognizes two types of the schema locks: Schema modification lock (Sch-M) and Schema stability lock (Sch-S) Bulk Update locks (BU) – this lock is designed to be used by bulk import operations when issued with a TABLOCK argument/hint.
When a bulk update lock is acquired, other processes will not be able to access a table during the bulk load execution.
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.