Automatic Locks in DML Operations

The purpose of a DML lock, also called a data lock, 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. 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. In the sections that follow, the acronym in parentheses after each type of lock or lock mode is the abbreviation used in the Locks Monitor of Oracle Enterprise Manager. Enterprise Manager might display "TM" for any table lock, rather than indicate the mode of table lock (such as RS or SRX).

The types of row and table locks are summarized here. For a more complete discussion of the types of row and table locks, see Oracle Database Concepts.

Row Locks (TX) A row lock, also called a TX lock, is a lock on a single row of a table. A transaction acquires a row lock for each row modified by one of the following statements: INSERT, UPDATE, DELETE, MERGE, and SELECT ... FOR UPDATE. The row lock exists until the transaction commits or rolls back.

When a transaction obtains a row lock for a row, the transaction also acquires a table lock for the table in which the row resides. The table lock prevents conflicting DDL operations that would override data changes in a current transaction.

Table Locks (TM) A transaction automatically acquires a table lock (TM lock) when a table is modified with the following statements: INSERT, UPDATE, DELETE, MERGE, and SELECT ... FOR UPDATE. 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 LOCK TABLE statement, as described in "Manual Data Locking".

A table lock can be held in any of the following modes:

• A row share lock (RS), also called a subshare table lock (SS), indicates that the transaction holding the lock on the table has locked some rows in the table and intends to update them, as is the case in a SELECT ... FOR UPDATE statement. An SS lock is the least restrictive mode of table lock, offering the highest degree of concurrency for a table.

• A row exclusive lock (RX), also called a subexclusive table lock (SX), indicates that the transaction holding the lock has made updates to rows in the table. An SX lock by itself allows other transactions to insert, update, merge into, or delete other rows in the table concurrently. Therefore, SX locks allow multiple transactions to obtain simultaneous SX and SS locks for the same table.

• A share table lock (S) held by one transaction allows other transactions to query the table but allows updates only if a share table lock is held by only a single transaction. Multiple transactions may hold a share table lock concurrently, so holding this lock is not sufficient to ensure that a transaction can modify the table.

• A share row exclusive table lock (SRX), also called a share-subexclusive table lock (SSX), is more restrictive than a share table lock. Only one transaction at a time can acquire an SSX lock on a given table. An SSX lock held by a transaction allows other transactions to query or lock specific rows using SELECT ... FOR UPDATE, but not to update the table.

• An exclusive table lock (X) is the most restrictive mode of table lock, allowing the transaction that holds the lock exclusive write access to the table. Only one transaction can obtain an X lock for a table.

Locks in DML Operations

Oracle Database automatically obtains row-level and table-level locks on behalf of DML operations. The type of operation determines the locking behavior. Table B-1 summarizes the information in this section.

Locks When Rows Are Queried

A query can be explicit, as in the SELECT statement, or implicit, as in most INSERT, MERGE, UPDATE, and DELETE statements. The only DML statement that does not necessarily include a query component is an INSERT statement with a VALUES clause. Because queries only read data, they are the SQL statements least likely to interfere with other SQL statements.

The following characteristics apply to a query without the FOR UPDATE clause:

• The query acquires no data locks. Therefore, other transactions can query and update a table being queried, including the specific rows being queried. Because queries without the FOR UPDATE clause do not acquire any data locks to block other operations, such queries are often referred to as nonblocking queries.

• The query does not have to wait for any data locks to be released. Therefore, the query can always proceed. An exception to this rule is that queries may have to wait for data locks in some very specific cases of pending distributed transactions.

Locks When Rows Are Modified

Some databases use a lock manager to maintain a list of locks in memory. Oracle Database, in contrast, stores lock information in the data block that contains the locked row. Each row lock affects only a single row.

Oracle Database uses a queuing mechanism for acquisition of row locks. If a transaction requires a row lock, and if the row is not already locked, then the transaction acquires a lock in the row's data block. The transaction itself has an entry in the interested transaction list (ITL) section of the block header. Each row modified by this transaction points to a copy of the transaction ID stored in the ITL. Thus, 100 rows in the same block modified by a single transaction require 100 row locks, but all 100 rows reference a single transaction ID.

When a transaction ends, the transaction ID remains in the ITL section of the data block header. If a new transaction wants to modify a row, then it uses the transaction ID to determine whether the lock is active. If the lock is active, then the session of the new transaction asks to be notified when the lock is released; otherwise, the new transaction acquires the lock.

The characteristics of INSERT, UPDATE, DELETE, and SELECT ... FOR UPDATE statements are as follows:

• A transaction containing a DML statement acquires exclusive row locks on the rows modified by the statement. Therefore, other transactions cannot update or delete the locked rows until the locking transaction either commits or rolls back.

• In addition to these row locks, a transaction containing a DML statement that modifies data also requires at least a subexclusive table lock (SX) on the table that contains the affected rows. If the transaction already holds an S, SRX, or X table lock for the table, which are more restrictive than an SX lock, then the SX lock is not needed and is not acquired. If the containing transaction already holds only an SS lock, however, then Oracle Database automatically converts the SS lock to an SX lock.

• A transaction that contains a DML statement does not require row locks on any rows selected by a subquery or an implicit query.

  In the following sample UPDATE statement, the SELECT statement in parentheses is a subquery, whereas the WHERE a > 5 clause is an implicit query:

  UPDATE t SET x = ( SELECT y FROM t2 WHERE t2.z = t.z ) WHERE a > 5;
  

  A subquery or implicit query inside a DML statement is guaranteed to be consistent as of the start of the query and does not see the effects of the DML statement of which it forms a part.

• A query in a transaction can see the changes made by previous DML statements in the same transaction, but not the uncommitted changes of other transactions.