How Oracle Locks Data

Locks are mechanisms that prevent destructive interaction between transactions accessing the same resource--either user objects such as tables and rows or system objects not visible to users, such as shared data structures in memory and data dictionary rows.

In all cases, Oracle automatically obtains necessary locks when executing SQL statements, so users need not be concerned with such details. Oracle automatically uses the lowest applicable level of restrictiveness to provide the highest degree of data concurrency yet also provide fail-safe data integrity. Oracle also allows the user to lock data manually.

See Also: "Types of Locks"

Transactions and Data Concurrency

Oracle provides data concurrency and integrity between transactions using its locking mechanisms. Because the locking mechanisms of Oracle are tied closely to transaction control, application designers need only define transactions properly, and Oracle automatically manages locking.

Keep in mind that Oracle locking is fully automatic and requires no user action. Implicit locking occurs for all SQL statements so that database users never need to lock any resource explicitly. Oracle's default locking mechanisms lock data at the lowest level of restrictiveness to guarantee data integrity while allowing the highest degree of data concurrency.

See Also: "Explicit (Manual) Data Locking"

Modes of Locking

Oracle uses two modes of locking in a multiuser database:

• Exclusive lock mode prevents the associates resource from being shared. This lock mode is obtained to modify data. The first transaction to lock a resource exclusively is the only transaction that can alter the resource until the exclusive lock is released.
• Share lock mode allows the associated resource to be shared, depending on the operations involved. Multiple users reading data can share the data, holding share locks to prevent concurrent access by a writer (who needs an exclusive lock). Several transactions can acquire share locks on the same resource.

Lock Duration

All locks acquired by statements within a transaction are held for the duration of the transaction, preventing destructive interference including dirty reads, lost updates, and destructive DDL operations from concurrent transactions. The changes made by the SQL statements of one transaction become visible only to other transactions that start after the first transaction is committed.

Oracle releases all locks acquired by the statements within a transaction when you either commit or roll back the transaction. Oracle also releases locks acquired after a savepoint when rolling back to the savepoint. However, only transactions not waiting for the previously locked resources can acquire locks on the now available resources. Waiting transactions will continue to wait until after the original transaction commits or rolls back completely.

Data Lock Conversion Versus Lock Escalation

A transaction holds exclusive row locks for all rows inserted, updated, or deleted within the transaction. Because row locks are acquired at the highest degree of restrictiveness, no lock conversion is required or performed.

Oracle automatically converts a table lock of lower restrictiveness to one of higher restrictiveness as appropriate. For example, assume that a transaction uses a SELECT statement with the FOR UPDATE clause to lock rows of a table. As a result, it acquires the exclusive row locks and a row share table lock for the table. If the transaction later updates one or more of the locked rows, the row share table lock is automatically converted to a row exclusive table lock.

Lock escalation occurs when numerous locks are held at one level of granularity (for example, rows) and a database raises the locks to a higher level of granularity (for example, table). For example, if a single user locks many rows in a table, some databases automatically escalate the user's row locks to a single table. The number of locks is reduced, but the restrictiveness of what is being locked is increased.

Oracle never escalates locks. Lock escalation greatly increases the likelihood of deadlocks. Imagine the situation where the system is trying to escalate locks on behalf of transaction T1 but cannot because of the locks held by transaction T2. A deadlock is created if transaction T2 also requires lock escalation of the same data before it can proceed.

See Also: "Table Locks (TM)"

Deadlocks

A deadlock can occur when two or more users are waiting for data locked by each other. Deadlocks prevent some transactions from continuing to work. Figure 20-3 illustrates two transactions in a deadlock.

In Figure 20-3, no problem exists at time point A, as each transaction has a row lock on the row it attempts to update. Each transaction proceeds without being terminated. However, each tries next to update the row currently held by the other transaction. Therefore, a deadlock results at time point B, because neither transaction can obtain the resource it needs to proceed or terminate. It is a deadlock because no matter how long each transaction waits, the conflicting locks are held.

Figure 20-3 Two Transactions in a Deadlock

Text description of the illustration cncpt068.gif

Deadlock Detection

Oracle automatically detects deadlock situations and resolves them by rolling back one of the statements involved in the deadlock, thereby releasing one set of the conflicting row locks. A corresponding message also is returned to the transaction that undergoes statement-level rollback. The statement rolled back is the one belonging to the transaction that detects the deadlock. Usually, the signalled transaction should be rolled back explicitly, but it can retry the rolled-back statement after waiting. 

Note: In distributed transactions, local deadlocks are detected by analyzing a "waits for" graph, and global deadlocks are detected by a time-out. Once detected, nondistributed and distributed deadlocks are handled by the database and application in the same way.

Deadlocks most often occur when transactions explicitly override the default locking of Oracle. Because Oracle itself does no lock escalation and does not use read locks for queries, but does use row-level locking (rather than page-level locking), deadlocks occur infrequently in Oracle.

See Also: "Explicit (Manual) Data Locking" for more information about manually acquiring locks

Avoid Deadlocks

Multitable deadlocks can usually be avoided if transactions accessing the same tables lock those tables in the same order, either through implicit or explicit locks. For example, all application developers might follow the rule that when both a master and detail table are updated, the master table is locked first and then the detail table. If such rules are properly designed and then followed in all applications, deadlocks are very unlikely to occur.

When you know you will require a sequence of locks for one transaction, consider acquiring the most exclusive (least compatible) lock first.

Types of Locks

Oracle automatically uses different types of locks to control concurrent access to data and to prevent destructive interaction between users. Oracle automatically locks a resource on behalf of a transaction to prevent other transactions from doing something also requiring exclusive access to the same resource. The lock is released automatically when some event occurs so that the transaction no longer requires the resource.

Throughout its operation, Oracle automatically acquires different types of locks at different levels of restrictiveness depending on the resource being locked and the operation being performed.

Oracle locks fall into one of three general categories.

Lock	Description
DML locks (data locks)	DML locks protect data. For example, table locks lock entire tables, row locks lock selected rows.
DDL locks (dictionary locks)	DDL locks protect the structure of schema objects--for example, the definitions of tables and views.
Internal locks and latches	Internal locks and latches protect internal database structures such as datafiles. Internal locks and latches are entirely automatic.

The following sections discuss DML locks, DDL locks, and internal locks.

DML Locks

The purpose of a DML (data) lock is to guarantee the integrity of data being accessed concurrently by multiple users. DML locks prevent destructive interference of simultaneous conflicting DML or DDL operations. For example, Oracle DML locks guarantee that a specific row in a table can be updated by only one transaction at a time and that a table cannot be dropped if an uncommitted transaction contains an insert into the table.

DML operations can acquire data locks at two different levels: for specific rows and for entire tables.

Note: The acronym in parentheses after each type of lock or lock mode is the abbreviation used in the Locks Monitor of Enterprise Manager. Enterprise Manager might display TM for any table lock, rather than indicate the mode of table lock (such as RS or SRX).

Row Locks (TX)

The only DML locks Oracle acquires automatically are row-level locks. There is no limit to the number of row locks held by a statement or transaction, and Oracle does not escalate locks from the row level to a coarser granularity. Row locking provides the finest grain locking possible and so provides the best possible concurrency and throughput.

The combination of multiversion concurrency control and row-level locking means that users contend for data only when accessing the same rows, specifically:

• Readers of data do not wait for writers of the same data rows.
• Writers of data do not wait for readers of the same data rows unless SELECT ... FOR UPDATE is used, which specifically requests a lock for the reader.
• Writers only wait for other writers if they attempt to update the same rows at the same time.

  Note: Readers of data may have to wait for writers of the same data blocks in some very special cases of pending distributed transactions.

A transaction acquires an exclusive DML lock for each individual row modified by one of the following statements: INSERT, UPDATE, DELETE, and SELECT with the FOR UPDATE clause.

A modified row is always locked exclusively so that other users cannot modify the row until the transaction holding the lock is committed or rolled back. However, if the transaction dies due to instance failure, block-level recovery makes a row available before the entire transaction is recovered. Row locks are always acquired automatically by Oracle as a result of the statements listed previously.

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

See Also: "DDL Locks"

Table Locks (TM)

A transaction acquires a table lock when a table is modified in the following DML statements: INSERT, UPDATE, DELETE, SELECT with the FOR UPDATE clause, and LOCK TABLE. These DML operations require table locks for two purposes: to reserve DML access to the table on behalf of a transaction and to prevent DDL operations that would conflict with the transaction. Any table lock prevents the acquisition of an exclusive DDL lock on the same table and thereby prevents DDL operations that require such locks. For example, a table cannot be altered or dropped if an uncommitted transaction holds a table lock for it.

A table lock can be held in any of several modes: row share (RS), row exclusive (RX), share (S), share row exclusive (SRX), and exclusive (X). The restrictiveness of a table lock's mode determines the modes in which other table locks on the same table can be obtained and held.

Table 20-3 shows the table lock modes that statements acquire and operations that those locks permit and prohibit.

Table 20-3 Summary of Table Locks  

SQL Statement	Mode of Table Lock	Lock Modes Permitted?
		RS	RX	S	SRX	X
SELECT...FROM table...	none	Y	Y	Y	Y	Y
INSERT INTO table ...	RX	Y	Y	N	N	N
UPDATE table ...	RX	Y*	Y*	N	N	N
DELETE FROM table ...	RX	Y*	Y*	N	N	N
SELECT ... FROM table    FOR UPDATE OF ...	RS	Y*	Y*	Y*	Y*	N
LOCK TABLE table IN    ROW SHARE MODE	RS	Y	Y	Y	Y	N
LOCK TABLE table IN    ROW EXCLUSIVE MODE	RX	Y	Y	N	N	N
LOCK TABLE table IN    SHARE MODE	S	Y	N	Y	N	N
LOCK TABLE table IN    SHARE ROW EXCLUSIVE    MODE	SRX	Y	N	N	N	N
LOCK TABLE table IN    EXCLUSIVE MODE	X	N	N	N	N	N
	RS: row share RX: row exclusive S: share SRX: share row exclusive X: exclusive		*Yes, if no conflicting row locks are held by another transaction. Otherwise, waits occur.

The following sections explain each mode of table lock, from least restrictive to most restrictive. They also describe the actions that cause the transaction to acquire a table lock in that mode and which actions are permitted and prohibited in other transactions by a lock in that mode.

See Also: "Explicit (Manual) Data Locking"

Row Share Table Locks (RS)

A row share table lock (also sometimes called a subshare table lock, SS) indicates that the transaction holding the lock on the table has locked rows in the table and intends to update them. A row share table lock is automatically acquired for a table when one of the following SQL statements is executed:

SELECT ... FROM table ... FOR UPDATE OF ... ;

LOCK TABLE table IN ROW SHARE MODE;

A row share table lock is the least restrictive mode of table lock, offering the highest degree of concurrency for a table.

Permitted Operations: A row share table lock held by a transaction allows other transactions to query, insert, update, delete, or lock rows concurrently in the same table. Therefore, other transactions can obtain simultaneous row share, row exclusive, share, and share row exclusive table locks for the same table.

Prohibited Operations: A row share table lock held by a transaction prevents other transactions from exclusive write access to the same table using only the following statement:

LOCK TABLE table IN EXCLUSIVE MODE;

Row Exclusive Table Locks (RX)

A row exclusive table lock (also called a subexclusive table lock, SX) generally indicates that the transaction holding the lock has made one or more updates to rows in the table. A row exclusive table lock is acquired automatically for a table modified by the following types of statements:

INSERT INTO table ... ;

UPDATE table ... ;

DELETE FROM table ... ;

LOCK TABLE table IN ROW EXCLUSIVE MODE;

A row exclusive table lock is slightly more restrictive than a row share table lock.

Permitted Operations: A row exclusive table lock held by a transaction allows other transactions to query, insert, update, delete, or lock rows concurrently in the same table. Therefore, row exclusive table locks allow multiple transactions to obtain simultaneous row exclusive and row share table locks for the same table.

Prohibited Operations: A row exclusive table lock held by a transaction prevents other transactions from manually locking the table for exclusive reading or writing. Therefore, other transactions cannot concurrently lock the table using the following statements:

LOCK TABLE table IN SHARE MODE;

LOCK TABLE table IN SHARE EXCLUSIVE MODE;

LOCK TABLE table IN EXCLUSIVE MODE;

Share Table Locks (S)

A share table lock is acquired automatically for the table specified in the following statement:

LOCK TABLE table IN SHARE MODE;

Permitted Operations: A share table lock held by a transaction allows other transactions only to query the table, to lock specific rows with SELECT ... FOR UPDATE, or to execute LOCK TABLE ... IN SHARE MODE statements successfully. No updates are allowed by other transactions. Multiple transactions can hold share table locks for the same table concurrently. In this case, no transaction can update the table (even if a transaction holds row locks as the result of a SELECT statement with the FOR UPDATE clause). Therefore, a transaction that has a share table lock can update the table only if no other transactions also have a share table lock on the same table.

Prohibited Operations: A share table lock held by a transaction prevents other transactions from modifying the same table and from executing the following statements:

LOCK TABLE table IN SHARE ROW EXCLUSIVE MODE;

LOCK TABLE table IN EXCLUSIVE MODE;

LOCK TABLE table IN ROW EXCLUSIVE MODE;

Share Row Exclusive Table Locks (SRX)

A share row exclusive table lock (also sometimes called a share-subexclusive table lock, SSX) is more restrictive than a share table lock. A share row exclusive table lock is acquired for a table as follows:

LOCK TABLE table IN SHARE ROW EXCLUSIVE MODE;

Permitted Operations: Only one transaction at a time can acquire a share row exclusive table lock on a given table. A share row exclusive table lock held by a transaction allows other transactions to query or lock specific rows using SELECT with the FOR UPDATE clause, but not to update the table.

Prohibited Operations: A share row exclusive table lock held by a transaction prevents other transactions from obtaining row exclusive table locks and modifying the same table. A share row exclusive table lock also prohibits other transactions from obtaining share, share row exclusive, and exclusive table locks, which prevents other transactions from executing the following statements:

LOCK TABLE table IN SHARE MODE;

LOCK TABLE table IN SHARE ROW EXCLUSIVE MODE;

LOCK TABLE table IN ROW EXCLUSIVE MODE;

LOCK TABLE table IN EXCLUSIVE MODE;

Exclusive Table Locks (X)

An exclusive table lock is the most restrictive mode of table lock, allowing the transaction that holds the lock exclusive write access to the table. An exclusive table lock is acquired for a table as follows:

LOCK TABLE table IN EXCLUSIVE MODE;

Permitted Operations: Only one transaction can obtain an exclusive table lock for a table. An exclusive table lock permits other transactions only to query the table.

Prohibited Operations: An exclusive table lock held by a transaction prohibits other transactions from performing any type of DML statement or placing any type of lock on the table.

DML Locks Automatically Acquired for DML Statements

The previous sections explained the different types of data locks, the modes in which they can be held, when they can be obtained, when they are obtained, and what they prohibit. The following sections summarize how Oracle automatically locks data on behalf of different DML operations.

Table 20-4 summarizes the information in the following sections.

Table 20-4 Locks Obtained By DML Statements

DML Statement	Row Locks?	Mode of Table Lock
SELECT ... FROM table
INSERT INTO table ...	X	RX
UPDATE table ...	X	RX
DELETE FROM table ...	X	RX
SELECT ... FROM table ...    FOR UPDATE OF ...	X	RS
LOCK TABLE table IN ...
ROW SHARE MODE		RS
ROW EXCLUSIVE MODE		RX
SHARE MODE		S
SHARE EXCLUSIVE MODE		SRX
EXCLUSIVE MODE		X
	X: exclusive RX: row exclusive	RS: row share S: share SRX: share row exclusive

Default Locking for Queries

Queries are the SQL statements least likely to interfere with other SQL statements because they only read data. INSERT, UPDATE, and DELETE statements can have implicit queries as part of the statement. Queries include the following kinds of statements:

SELECT

INSERT ... SELECT ... ;

UPDATE ... ;

DELETE ... ;

They do not include the following statement:

SELECT ... FOR UPDATE OF ... ;

The following characteristics are true of all queries that do not use the FOR UPDATE clause:

• A query acquires no data locks. Therefore, other transactions can query and update a table being queried, including the specific rows being queried. Because queries lacking FOR UPDATE clauses do not acquire any data locks to block other operations, such queries are often referred to in Oracle as nonblocking queries.
• A query does not have to wait for any data locks to be released; it can always proceed. (Queries may have to wait for data locks in some very specific cases of pending distributed transactions.)

Default Locking for INSERT, UPDATE, DELETE, and SELECT ... FOR UPDATE

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

• The transaction that contains a DML statement acquires exclusive row locks on the rows modified by the statement. Other transactions cannot update or delete the locked rows until the locking transaction either commits or rolls back.
• The transaction that contains a DML statement does not need to acquire row locks on any rows selected by a subquery or an implicit query, such as a query in a WHERE clause. A subquery or implicit query in 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 it is part of.
• A query in a transaction can see the changes made by previous DML statements in the same transaction, but cannot see the changes of other transactions begun after its own transaction.
• In addition to the necessary exclusive row locks, a transaction that contains a DML statement acquires at least a row exclusive table lock on the table that contains the affected rows. If the containing transaction already holds a share, share row exclusive, or exclusive table lock for that table, the row exclusive table lock is not acquired. If the containing transaction already holds a row share table lock, Oracle automatically converts this lock to a row exclusive table lock.

DDL Locks

A data dictionary lock (DDL) protects the definition of a schema object while that object is acted upon or referred to by an ongoing DDL operation. Recall that a DDL statement implicitly commits its transaction. For example, assume that a user creates a procedure. On behalf of the user's single-statement transaction, Oracle automatically acquires DDL locks for all schema objects referenced in the procedure definition. The DDL locks prevent objects referenced in the procedure from being altered or dropped before the procedure compilation is complete.

Oracle acquires a dictionary lock automatically on behalf of any DDL transaction requiring it. Users cannot explicitly request DDL locks. Only individual schema objects that are modified or referenced are locked during DDL operations. The whole data dictionary is never locked.

DDL locks fall into three categories: exclusive DDL locks, share DDL locks, and breakable parse locks.

Exclusive DDL Locks

Most DDL operations, except for those listed in the next section, "Share DDL Locks", require exclusive DDL locks for a resource to prevent destructive interference with other DDL operations that might modify or reference the same schema object. For example, a DROP TABLE operation is not allowed to drop a table while an ALTER TABLE operation is adding a column to it, and vice versa.

During the acquisition of an exclusive DDL lock, if another DDL lock is already held on the schema object by another operation, the acquisition waits until the older DDL lock is released and then proceeds.

DDL operations also acquire DML locks (data locks) on the schema object to be modified.

Share DDL Locks

Some DDL operations require share DDL locks for a resource to prevent destructive interference with conflicting DDL operations, but allow data concurrency for similar DDL operations. For example, when a CREATE PROCEDURE statement is executed, the containing transaction acquires share DDL locks for all referenced tables. Other transactions can concurrently create procedures that reference the same tables and therefore acquire concurrent share DDL locks on the same tables, but no transaction can acquire an exclusive DDL lock on any referenced table. No transaction can alter or drop a referenced table. As a result, a transaction that holds a share DDL lock is guaranteed that the definition of the referenced schema object will remain constant for the duration of the transaction.

A share DDL lock is acquired on a schema object for DDL statements that include the following statements: AUDIT, NOAUDIT, COMMENT, CREATE [OR REPLACE] VIEW/ PROCEDURE/PACKAGE/PACKAGE BODY/FUNCTION/ TRIGGER, CREATE SYNONYM, and CREATE TABLE (when the CLUSTER parameter is not included).

Breakable Parse Locks

A SQL statement (or PL/SQL program unit) in the shared pool holds a parse lock for each schema object it references. Parse locks are acquired so that the associated shared SQL area can be invalidated if a referenced object is altered or dropped. A parse lock does not disallow any DDL operation and can be broken to allow conflicting DDL operations, hence the name breakable parse lock.

A parse lock is acquired during the parse phase of SQL statement execution and held as long as the shared SQL area for that statement remains in the shared pool.

See Also: Chapter 15, "Dependencies Among Schema Objects"

Duration of DDL Locks

The duration of a DDL lock depends on its type. Exclusive and share DDL locks last for the duration of DDL statement execution and automatic commit. A parse lock persists as long as the associated SQL statement remains in the shared pool.

DDL Locks and Clusters

A DDL operation on a cluster acquires exclusive DDL locks on the cluster and on all tables and materialized views in the cluster. A DDL operation on a table or materialized view in a cluster acquires a share lock on the cluster, in addition to a share or exclusive DDL lock on the table or materialized view. The share DDL lock on the cluster prevents another operation from dropping the cluster while the first operation proceeds.

Latches and Internal Locks

Latches and internal locks protect internal database and memory structures. Both are inaccessible to users, because users have no need to control over their occurrence or duration. The following section helps to interpret the Enterprise Manager or SQL*Plus LOCKS and LATCHES monitors.

Latches

Latches are simple, low-level serialization mechanisms to protect shared data structures in the system global area (SGA). For example, latches protect the list of users currently accessing the database and protect the data structures describing the blocks in the buffer cache. A server or background process acquires a latch for a very short time while manipulating or looking at one of these structures. The implementation of latches is operating system dependent, particularly in regard to whether and how long a process will wait for a latch.

Internal Locks

Internal locks are higher-level, more complex mechanisms than latches and serve a variety of purposes.

Dictionary Cache Locks

These locks are of very short duration and are held on entries in dictionary caches while the entries are being modified or used. They guarantee that statements being parsed do not see inconsistent object definitions.

Dictionary cache locks can be shared or exclusive. Shared locks are released when the parse is complete. Exclusive locks are released when the DDL operation is complete.

File and Log Management Locks

These locks protect various files. For example, one lock protects the control file so that only one process at a time can change it. Another lock coordinates the use and archiving of the redo log files. Datafiles are locked to ensure that multiple instances mount a database in shared mode or that one instance mounts it in exclusive mode. Because file and log locks indicate the status of files, these locks are necessarily held for a long time.

Tablespace and Rollback Segment Locks

These locks protect tablespaces and rollback segments. For example, all instances accessing a database must agree on whether a tablespace is online or offline. Rollback segments are locked so that only one instance can write to a segment.

Explicit (Manual) Data Locking

Oracle always performs locking automatically to ensure data concurrency, data integrity, and statement-level read consistency. However, you can override the Oracle default locking mechanisms. Overriding the default locking is useful in situations such as these:

• Applications require transaction-level read consistency or repeatable reads. In other words, queries in them must produce consistent data for the duration of the transaction, not reflecting changes by other transactions. You can achieve transaction-level read consistency by using explicit locking, read-only transactions, serializable transactions, or by overriding default locking.
• Applications require that a transaction have exclusive access to a resource so that the transaction does not have to wait for other transactions to complete.

Oracle's automatic locking can be overridden at the transaction level or the session level.

At the transaction level, transactions that include the following SQL statements override Oracle's default locking:

• The SET TRANSACTION ISOLATION LEVEL statement
• The LOCK TABLE statement (which locks either a table or, when used with views, the underlying base tables)
• The SELECT ... FOR UPDATE statement

Locks acquired by these statements are released after the transaction commits or rolls back.

At the session level, a session can set the required transaction isolation level with the ALTER SESSION statement.

Note: If Oracle's default locking is overridden at any level, the database administrator or application developer should ensure that the overriding locking procedures operate correctly. The locking procedures must satisfy the following criteria: data integrity is guaranteed, data concurrency is acceptable, and deadlocks are not possible or are appropriately handled.

See Also: Oracle9i SQL Reference for detailed descriptions of the SQL statements LOCK TABLE and SELECT ... FOR UPDATE

Examples of Concurrency under Explicit Locking

The following illustration shows how Oracle maintains data concurrency, integrity, and consistency when LOCK TABLE and SELECT with the FOR UPDATE clause statements are used.

Note: For brevity, the message text for ORA-00054 ("resource busy and acquire with NOWAIT specified") is not included. User-entered text is in bold.

Transaction 1	Time Point	Transaction 2
LOCK TABLE scott.dept IN ROW SHARE MODE; Statement processed	1
	2	DROP TABLE scott.dept; DROP TABLE scott.dept * ORA-00054 (exclusive DDL lock not possible because of T1's table lock)
	3	LOCK TABLE scott.dept IN EXCLUSIVE MODE NOWAIT; ORA-00054
	4	SELECT LOC FROM scott.dept WHERE deptno = 20 FOR UPDATE OF loc; LOC - - - - - - - DALLAS 1 row selected
UPDATE scott.dept SET loc = 'NEW YORK' WHERE deptno = 20; (waits because T2 has locked same rows)	5
	6	ROLLBACK; (releases row locks)
1 row processed. ROLLBACK;	7
LOCK TABLE scott.dept IN ROW EXCLUSIVE MODE; Statement processed.	8
	9	LOCK TABLE scott.dept IN EXCLUSIVE MODE NOWAIT; ORA-00054
	10	LOCK TABLE scott.dept IN SHARE ROW EXCLUSIVE MODE NOWAIT; ORA-00054
	11	LOCK TABLE scott.dept IN SHARE ROW EXCLUSIVE MODE NOWAIT; ORA-00054
	12	UPDATE scott.dept SET loc = 'NEW YORK' WHERE deptno = 20; 1 row processed.
	13	ROLLBACK;
SELECT loc FROM scott.dept WHERE deptno = 20 FOR UPDATE OF loc; LOC - - - - - - DALLAS 1 row selected.	14
	15	UPDATE scott.dept SET loc = 'NEW YORK' WHERE deptno = 20; (waits because T1 has locked same rows)
ROLLBACK;	16
	17	1 row processed. (conflicting locks were released) ROLLBACK;
LOCK TABLE scott.dept IN SHARE MODE Statement processed	18
	19	LOCK TABLE scott.dept IN EXCLUSIVE MODE NOWAIT; ORA-00054
	20	LOCK TABLE scott.dept IN SHARE ROW EXCLUSIVE MODE NOWAIT; ORA-00054
	21	LOCK TABLE scott.dept IN SHARE MODE; Statement processed.
	22	SELECT loc FROM scott.dept WHERE deptno = 20; LOC - - - - - - DALLAS 1 row selected.
	23	SELECT loc FROM scott.dept WHERE deptno = 20 FOR UPDATE OF loc; LOC - - - - - - DALLAS 1 row selected.
	24	UPDATE scott.dept SET loc = 'NEW YORK' WHERE deptno = 20; (waits because T1 holds conflicting table lock)
ROLLBACK;	25
	26	1 row processed. (conflicting table lock released) ROLLBACK;
LOCK TABLE scott.dept IN SHARE ROW EXCLUSIVE MODE; Statement processed.	27
	28	LOCK TABLE scott.dept IN EXCLUSIVE MODE NOWAIT; ORA-00054
	29	LOCK TABLE scott.dept IN SHARE ROW EXCLUSIVE MODE NOWAIT; ORA-00054
	30	LOCK TABLE scott.dept IN SHARE MODE NOWAIT; ORA-00054
	31	LOCK TABLE scott.dept IN ROW EXCLUSIVE MODE NOWAIT; ORA-00054
	32	LOCK TABLE scott.dept IN SHARE MODE NOWAIT; ORA-00054
	33	SELECT loc FROM scott.dept WHERE deptno = 20; LOC - - - - - - DALLAS 1 row selected.
	34	SELECT loc FROM scott.dept WHERE deptno = 20 FOR UPDATE OF loc; LOC - - - - - - DALLAS 1 row selected.
	35	UPDATE scott.dept SET loc = 'NEW YORK' WHERE deptno = 20; (waits because T1 holds conflicting table lock)
UPDATE scott.dept SET loc = 'NEW YORK' WHERE deptno = 20; (waits because T2 has locked same rows)	36	(deadlock)
Cancel operation ROLLBACK;	37
	38	1 row processed.
LOCK TABLE scott.dept IN EXCLUSIVE MODE;	39
	40	LOCK TABLE scott.dept IN EXCLUSIVE MODE; ORA-00054
	41	LOCK TABLE scott.dept IN ROW EXCLUSIVE MODE NOWAIT; ORA-00054
	42	LOCK TABLE scott.dept IN SHARE MODE; ORA-00054
	43	LOCK TABLE scott.dept IN ROW EXCLUSIVE MODE NOWAIT; ORA-00054
	44	LOCK TABLE scott.dept IN ROW SHARE MODE NOWAIT; ORA-00054
	45	SELECT loc FROM scott.dept WHERE deptno = 20; LOC - - - - - - DALLAS 1 row selected.
	46	SELECT loc FROM scott.dept WHERE deptno = 20 FOR UPDATE OF loc; (waits because T1 has conflicting table lock)
UPDATE scott.dept SET deptno = 30 WHERE deptno = 20; 1 row processed.	47
COMMIT;	48
	49	0 rows selected. (T1 released conflicting lock)
SET TRANSACTION READ ONLY;	50
SELECT loc FROM scott.dept WHERE deptno = 10; LOC - - - - - - BOSTON	51
	52	UPDATE scott.dept SET loc = 'NEW YORK' WHERE deptno = 10; 1 row processed.
SELECT loc FROM scott.dept WHERE deptno = 10; LOC - - - - - - BOSTON (T1 does not see uncommitted data)	53
	54	COMMIT;
SELECT loc FROM scott.dept WHERE deptno = 10; LOC - - - - - - (same results seen even after T2 commits)	55
COMMIT;	56
SELECT loc FROM scott.dept WHERE deptno = 10; LOC - - - - - - NEW YORK (committed data is seen)	57

Oracle Lock Management Services

With Oracle Lock Management services, an application developer can include statements in PL/SQL blocks that:

• Request a lock of a specific type
• Give the lock a unique name recognizable in another procedure in the same or in another instance
• Change the lock type
• Release the lock

Because a reserved user lock is the same as an Oracle lock, it has all the Oracle lock functionality including deadlock detection. User locks never conflict with Oracle locks, because they are identified with the prefix UL.

The Oracle Lock Management services are available through procedures in the DBMS_LOCK package.

See Also: Oracle9i Application Developer's Guide - Fundamentals for more information about Oracle Lock Management services Oracle9i Supplied PL/SQL Packages and Types Reference for information about DBMS_LOCK

Flashback Query

Flashback query lets you view and repair historical data. You can perform queries on the database as of a certain wall clock time or user-specified system commit number (SCN).

Flashback query uses Oracle's multiversion read-consistency capabilities to restore data by applying undo as needed. Administrators can configure undo retention by simply specifying how long undo should be kept in the database. Using flashback query, a user can query the database as it existed this morning, yesterday, or last week. The speed of this operation depends only on the amount of data being queried and the number of changes to the data that need to be backed out.

You set the date and time you want to view. Then, any SQL query you execute operates on data as it existed at that time. If you are an authorized user, then you can correct errors and back out the restored data without needing the intervention of an administrator.

With the AS OF SQL clause, you can choose different snapshots for each table in the query. Associating a snapshot with a table is known as table decoration. If you do not decorate a table with a snapshot, then a default snapshot is used for it. All tables without a specified snapshot get the same default snapshot.

For example, suppose you want to write a query to find all the new customer accounts created in the past hour. You could do set operations on two instances of the same table decorated with different AS OF clauses.

DML and DDL operations can use table decoration to choose snapshots within subqueries. Operations such as INSERT TABLE AS SELECT and CREATE TABLE AS SELECT can be used with table decoration in the subqueries to repair tables from which rows have been mistakenly deleted. Table decoration can be any arbitrary expression: a bind variable, a constant, a string, date operations, and so on. You can open a cursor and dynamically bind a snapshot value (a timestamp or an SCN) to decorate a table with.

See Also: Oracle9i SQL Reference for information on the AS OF clause

Flashback Query Benefits

• Application Transparency

  Packaged applications, like report generation tools that only do queries, can run in flashback query mode by using logon triggers. Applications can run transparently without requiring changes to code. All the constraints that the application needs to be satisfied are guaranteed to hold good, because ther is a consistent version of the database as of the flashback query time.

• Application Performance

  If an application requires recovery actions, it can do so by saving SCNs and flashing back to those SCNs. This is a lot easier and faster than saving data sets and restoring them later, which would be required if the application were to do explicit versioning. Using flashback query, there are no costs for logging that would be incurred by explicit versioning.

• Online Operation

  Flashback query is an online operation. Concurrent DMLs and queries from other sessions are permitted while an object is being queried inside flashback query.The speed of these operations is unaffected. Moreover, different sessions can flash back to different flashback times or SCNs on the same object concurrently. The speed of the flashback query itself depends on the amount of undo that needs to be applied, which is proportional to how far back in time the query goes.

• Easy Manageability

  There is no additional management on the part of the user, except setting the appropriate retention interval, having the right privileges, and so on. No additional logging has to be turned on, because past versions are constructed automatically, as needed.

  Notes: Flashback query does not undo anything. It is only a query mechanism. You can take the output from a flashback query and perform an undo yourself in many circumstances. Flashback query does not tell you what changed. LogMiner does that. Flashback query can be used to undo changes and can be very efficient if you know the rows that need to be moved back in time. You can in theory use it to move a full table back in time but this is very expensive if the table is large since it involves a full table copy. Flashback query does not work through DDL operations that modify columns, or drop or truncate tables. LogMiner is very good for getting change history, but it gives you changes in terms of deltas (insert, update, delete), not in terms of the before and after image of a row. These can be difficult to deal with in some applications.

Some Uses of Flashback Query

Self-Service Repair

Perhaps you accidentally deleted some important rows from a table and wanted to recover the deleted rows. To do the repair, you can move backward in time and see the missing rows and re-insert the deleted row into the current table.

E-Mail or Voice Mail Applications

You might have deleted mail in the past. Using flashback query, you can restore the deleted mail by moving back in time and re-inserting the deleted message into the current message box.

Account Balances

You can view account prior account balances as of a certain day in the month.

Packaged Applications

Packaged applications (like report generation tools) can make use of flashback query without any changes to application logic. Any constraints that the application expects are guaranteed to be satisfied, because users see a consistent version of the Database as of the given time or SCN.

In addition, flashback query could be used after examination of audit information to see the before-image of the data. In DSS enviornments, it could be used for extraction of data as of a consistent point in time from OLTP systems.

See Also: Oracle9i Application Developer's Guide - Fundamentals for more information about using flashback query Oracle9i Supplied PL/SQL Packages and Types Reference for a description of the DBMS_FLASHBACK package Oracle9i Database Administrator's Guide for information about undo tablespaces and setting retention period