LOCK

Function

LOCK TABLE obtains a table-level lock.

MogDB always tries to select the lock mode with minimum constraints when automatically requesting a lock for a statement referenced by a table. Use LOCK if users need a more strict lock mode. For example, suppose an application runs a transaction at the Read Committed isolation level and needs to ensure that data in a table remains stable in the duration of the transaction. To achieve this, you could obtain SHARE lock mode over the table before the query. This will prevent concurrent data changes and ensure subsequent reads of the table see a stable view of committed data. It is because the SHARE lock mode conflicts with the ROW EXCLUSIVE lock acquired by writers, and your LOCK TABLE name IN SHARE MODE statement will wait until any concurrent holders of ROW EXCLUSIVE mode locks commit or roll back. Therefore, once you obtain the lock, there are no uncommitted writes outstanding; furthermore none can begin until you release the lock.

Precautions

• LOCK TABLE is useless outside a transaction block: the lock would remain held only to the completion of the statement. If LOCK TABLE is out of any transaction block, an error is reported.
• If no lock mode is specified, then ACCESS EXCLUSIVE, the most restrictive mode, is used.
• LOCK TABLE … IN ACCESS SHARE MODE requires the SELECT permission on the target table. All other forms of LOCK require table-level UPDATE and/or the DELETE permission.
• There is no UNLOCK TABLE statement. Locks are always released at transaction end.
• LOCK TABLE only deals with table-level locks, and so the mode names involving ROW are all misnomers. These mode names should generally be read as indicating the intention of the user to acquire row-level locks within the locked table. Also, ROW EXCLUSIVE mode is a shareable table lock. Note that all the lock modes have identical semantics so far as LOCK TABLE is concerned, differing only in the rules about which modes conflict with which. For details about the rules, see Table 1.
• If the xc_maintenance_mode parameter is not enabled, an error is reported when an ACCESS EXCLUSIVE lock is applied for a system catalog.

Syntax

Lock ::= LOCK [ TABLE ] {[ ONLY ] name [, ...]| {name [ * ]} [, ...]}
    [ IN {ACCESS SHARE | ROW SHARE | ROW EXCLUSIVE | SHARE UPDATE EXCLUSIVE | SHARE | SHARE ROW EXCLUSIVE | EXCLUSIVE | ACCESS EXCLUSIVE} MODE ]
    [ NOWAIT ];

Parameter Description

Table 1 Lock mode conflicts

LOCK parameters are as follows:

• name

  Specifies the name (optionally schema-qualified) of an existing table to lock.

  Tables are locked one-by-one in the order specified in the LOCK TABLE statement.

  Value range: an existing table name

• ONLY

  If ONLY is specified, only that table is locked. If ONLY is not specified, the table and all its sub-tables are locked.

• ACCESS SHARE

  Conflicts with the ACCESS EXCLUSIVE lock mode only.

  The SELECT statement acquires a lock of this mode on referenced tables. Typically, any command that reads a table without modifying it acquires this lock mode.

• ROW SHARE

  It conflicts with the EXCLUSIVE and ACCESS EXCLUSIVE lock modes.

  SELECT FOR UPDATE and SELECT FOR SHARE automatically acquire the ROW SHARE lock on the target table and add the ACCESS SHARE lock to other referenced tables except FOR SHARE and FOR UPDATE.

• ROW EXCLUSIVE

  Allows concurrent read of a table but does not allow modification of data in the table like ROW SHARE. UPDATE, DELETE, and INSERT automatically acquire the ROW SHARE lock on the target table and add the ACCESS SHARE lock to other referenced tables. Generally, all statements that modify table data acquire the ROW EXCLUSIVE lock for tables.

• SHARE UPDATE EXCLUSIVE

  Protects a table against concurrent schema changes and VACUUM runs.

  Acquired by VACUUM (without FULL), ANALYZE and CREATE INDEX CONCURRENTLY statements, and some forms of ALTER TABLE.

• SHARE

  Allows concurrent queries of a table but does not allow modification of the table.

  Acquired by CREATE INDEX (without CONCURRENTLY).

• SHARE ROW EXCLUSIVE

  Protects a table against concurrent data changes, and is self-exclusive so that only one session can hold it at a time.

  No SQL statements automatically acquire this lock mode.

• EXCLUSIVE

  Allows concurrent queries of the target table but does not allow any other operations.

  This mode allows only concurrent ACCESS SHARE locks; that is, only reads from the table can proceed in parallel with a transaction holding this lock mode.

  No SQL statements automatically acquire this lock mode on user tables. However, it will be acquired on some system catalogs in case of some operations.

• ACCESS EXCLUSIVE

  Guarantees that the holder is the only transaction accessing the table in any way.

  Acquired by the ALTER TABLE, DROP TABLE, TRUNCATE, and REINDEX statements.

  This is also the default lock mode for LOCK TABLE statements that do not specify a mode explicitly.

• NOWAIT

  Specifies that LOCK TABLE should not wait for any conflicting locks to be released: if the specified lock(s) cannot be acquired immediately without waiting, the transaction is aborted.

  If NOWAIT is not specified, LOCK TABLE obtains a table-level lock, waiting if necessary for any conflicting locks to be released.

Example

-- Obtain a SHARE ROW EXCLUSIVE lock on a primary key table when going to perform a delete operation.
mogdb=# CREATE TABLE tpcds.reason_t1 AS TABLE tpcds.reason;

mogdb=# START TRANSACTION;

mogdb=# LOCK TABLE tpcds.reason_t1 IN SHARE ROW EXCLUSIVE MODE;

mogdb=# DELETE FROM tpcds.reason_t1 WHERE r_reason_desc IN(SELECT r_reason_desc FROM tpcds.reason_t1 WHERE r_reason_sk < 6 );

mogdb=# DELETE FROM tpcds.reason_t1 WHERE r_reason_sk = 7;

mogdb=# COMMIT;

-- Delete the tpcds.reason_t1 table.
mogdb=# DROP TABLE tpcds.reason_t1;