As you have seen there is a really strong need for Update Locks in a relational database, because otherwise it would yield to deadlock situations and decreased concurrency.I hope that you now have a better understanding of Update Locks, and how they are used in SQL Server. But the problem with that approach is limited concurrency, because in the mean time no other SELECT query can read the data that is currently exclusively locked.

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Sql lock table while updating video

The table lock prevents conflicting DDL operations that would override data changes in a current transaction. These DML operations require table locks to reserve DML access to the table on behalf of a transaction and to prevent DDL operations that would conflict with the transaction.

You can explicitly obtain a table lock using the Locks in DML Operations Oracle Database automatically obtains row-level and table-level locks on behalf of DML operations.

Let’s try to imagine what happens when multiple UPDATE query plans acquire Shared Locks in the first phase of the plan, and afterwards convert these Shared Locks to Exclusive Locks when the data is finally changed in the third phase of the query plan: That approach would lead to a traditional deadlock situation in a relational database: That’s one of the main reasons why implementers of relational database engines have introduced Update Locks to avoid that specific deadlock situation.

An Update Lock is only compatible with a Shared Lock, but isn’t compatible with another Update or Exclusive Lock.

DML locks prevent destructive interference of simultaneous conflicting DML or DDL operations.

DML statements automatically acquire locks at both the table level and the row level.

, is to guarantee the integrity of data being accessed concurrently by multiple users.

For example, a DML lock can prevent multiple customers from buying the last copy of a book available from an online bookseller.

The row lock exists until the transaction commits or rolls back.