Oracle Database 9.2 Concepts Guide学习笔记1217

31.Oracle drops segments for a transaction-specific temporary table at the end of the

transaction and drops segments for a session-specific temporary table at the end of

the session. If other transactions or sessions share the use of that temporary table,

the segments containing their data remain in the table.

(Oracle在事务结束时删除事务级临时表的段，在会话结束时删除会话级临时表的段。如果事务或会话共享

使用临时表，则段和表数据会保留)

32.Use the V$UNDOSTAT view to monitor and configure your database system to

achieve efficient use of undo space. V$UNDOSTAT shows various undo and

transaction statistics, such as the amount of undo space consumed in the instance.

（通过使用V$UNDOSTAT视图你可以监控和配置你的数据库系统有效使用撤销空间。这个视图显示了撤销的

许多信息和事务的统计，例如用户在历程中使用撤销空间的情况）

33.In manual undo management mode, undo space is managed through rollback segments.

（在撤销表空间的手动管理模式中，撤销空间的管理通过回滚段来管理）

In automatic undo management mode, undo space is managed in undo tablespaces.

To use automatic undo management mode, the database administrator needs only

to create an undo tablespace for each instance and set the UNDO_MANAGEMENT

initialization parameter to AUTO.

（在自动管理模式中，撤销空间的管理在撤销表空间中。使用自动管理管理员需要为每个历程创建一个撤销

表空间并且把初始参数UNDO_MANAGEMENT设置为AUTO）

34.Undo Quota

In automatic undo management mode, the system controls exclusively the

assignment of transactions to undo segments, and controls space allocation for

undo segments. An ill-behaved transaction can potentially consume much of the

undo space, thus paralyzing the entire system. In manual undo management mode,

you can control such possibilities by limiting the size of rollback segments with

small MAXEXTENTS values. However, you then have to explicitly assign long

running transactions to larger rollback segments, using the SET TRANSACTION USE

ROLLBACK SEGMENT statement. This approach has proven to be cumbersome.

（在自动管理模式，系统控制的排它性控制着分配每个事务到撤销段，并且控制着为撤销段分配空间。

一个有问题的事务会潜在的消耗更多的撤销空间，从而导致系统的瘫痪。在手动管理模式，你能够通过限制

回滚段的尺寸给它设置一个小的MAXEXTENTS值来控制这种可能性。但是，你又不得不使用SET TRANSACTION USE

ROLLBACK SEGMENT语句来明确的给一个长时间运行的事务一个较大的回滚段。这种方法已经被证明为是比较麻烦的。）

The Resource Manager directive UNDO_POOL is a more explicit way to control large

transactions. This lets database administrators group users into consumer groups,

with each group assigned a maximum undo space limit. When the total undo space

consumed by a group exceeds the limit, its users cannot make further updates until

undo space is freed up by other member transactions ending.

（资源管理器的UNDO_POOL是一种更直接的方式来管理大事务。这种方式通过DBA把用户分组，每个组都分配一个最大的

撤销空间限制。当总的撤销空间的消耗超过改组的限制时，用户将不能再更新只到其他的成员事务结束释放了使用的撤销

空间。）

The default value of UNDO_POOL is UNLIMITED, where users are allowed to

consume as much undo space as the undo tablespace has. Database administrators

can limit a particular user by using the UNDO_POOL directive.

（UNDO_POOL的默认值是没有限制的，这使得用户可以尽量使用撤销表空间所拥有的空间。DBA可以限制某个用户通过使用

the UNDO_POOL directive.）

Undo Retention Control（Undo保留时的控制）

Long-running queries sometimes fail because undo information required for

consistent read operations is no longer available. This happens when committed

undo blocks are overwritten by active transactions.

（长时间运行的查询有时会因为撤销信息的读一致性操作不再可用而失败。这发生在当提交了的撤销块被活动的事务覆盖的时候）

In general, it is a good idea not to set retention to a value very close to what the

undo tablespace can support, because that may result in excessive movement of

space between undo segments. A 20% buffer of undo space is recommended.

(一般情况下，不要把retention值设置成接近undo tablespace能支持的值，因为那样可能导致在undo segments有过多的空间移动。

推荐设置成undo space的20%)

External Views（外部视图）

Monitor transaction and undo information with V$TRANSACTION and

V$ROLLSTAT. For automatic undo management, the information in V$ROLLSTAT

reflects the behaviors of the automatic undo management undo segments.

The V$UNDOSTAT view displays a histogram of statistical data to show how well

the system is working.

（V$TRANSACTION and V$ROLLSTAT视图监控事务和撤销信息。对于自动管理， V$ROLLSTAT视图反映自动撤销管理撤销段的操作。

V$UNDOSTAT显示系统工作好坏的数据统计）

Tablespaces, Datafiles, and Control Files

You specify operations in terms of database objects rather than filenames.（不知道该如何翻译）

You can enlarge a database in three ways:（通过以下三种方式来扩大数据库）

_1） Add a datafile to a tablespace （给一个表空间增加数据文件）

_ 2）Add a new tablespace（增加新的表空间）

_3） Increase the size of a datafile（增加数据文件的尺寸）

2）

3）

In a database with a locally managed SYSTEM tablespace, dictionary tablespaces

cannot be created. It is possible to plug in a dictionary managed tablespace using

the transportable feature, but it cannot be made writable.

（在一个本地管理系统表空间的数据库里，字典表空间不能被创建。但是它能够使用transportable feature plug in dictionary managed tablespace（这儿不理解是把这个字典管理的表空间plug in到一个dictionary managed tablespace的数据库里还是本地管理系统表空间的数据库里？），但是它不能写数据）

The Data Dictionary

The SYSTEM tablespace always contains the data dictionary tables for the entire

database. The data dictionary tables are stored in datafile 1.

（系统表空间常常包括整个数据库的数据字典表，这些数据字典表存储在数据文件1上）