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dml locks(dml_locks)

原创 Linux操作系统 作者:tengrid 时间:2009-05-18 18:55:06 0 删除 编辑
Parameter type Integer
Default value Derived: 4 * TRANSACTIONS
Modifiable No
Range of values 20 to unlimited; a setting of 0 disables enqueues
Basic No
Real Application
You must set this parameter for every instance, and all instances must
have positive values or all must be 0.

A DML lock is a lock obtained on a table that is undergoing a DML operation (insert,
update, delete). DML_LOCKS specifies the maximum number of DML locks—one for
each table modified in a transaction. The value should equal the grand total of locks on
tables currently referenced by all users. For example, if three users are modifying data
in one table, then three entries would be required. If three users are modifying data in
two tables, then six entries would be required.
The default value assumes an average of four tables referenced for each transaction.
For some systems, this value may not be enough.
Enqueues are shared memory structures that serialize access to database resources. If
you set the value of DML_LOCKS to 0, enqueues are disabled and performance is
slightly increased. However, you cannot use DROP TABLE, CREATE INDEX, or explicit
lock statements such as LOCK TABLE IN EXCLUSIVE MODE.
Oracle holds more locks during parallel DML than during serial execution. Therefore,
if your database supports a lot of parallel DML, you may need to increase the value of
this parameter. - 64bit (缺省值)
NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
dml_locks                            integer     2440
transactions                         integer     610

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