Database Reference
In-Depth Information
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i'll say this from time to time throughout the topic: please bear in mind that commands that set parameters,
modify the database, or make fundamental changes should not be done lightly, and definitely should be tested prior to
performing them on your “real” system. the preceding command will, in fact, cause the database to do more work. it will
consume resources.
Caution
To turn off and remove the block change-tracking file, you'd use the
ALTER DATABASE
command once again:
SYS@ORA12CR1> alter database disable block change tracking;
Database altered.
Note that this command will erase the block change-tracking file. It does not just disable the feature—it removes
the file as well.
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On certain operating systems, such as Windows, you might find that if you run my example—creating a block
change-tracking file and then disabling it—the file appears to still exist. this is an Os-specific issue—it does not happen
on many operating systems. it will happen only if you
CREATE
and
DISABLE
the change-tracking file from a single session.
the session that created the block change-tracking file will leave that file open and some operating systems will not
permit you to erase a file that has been opened by a previous process (for example, the session process that created the
file). this is harmless; you just need to remove the file yourself later.
Note
You can enable this new block change-tracking feature in either
ARCHIVELOG
or
NOARCHIVELOG
mode. But
remember, a database in
NOARCHIVELOG
mode, where the redo log generated daily is not retained, can't recover all
changes in the event of a media (disk or device) failure! A
NOARCHIVELOG
mode database will lose data some day.
We will cover these two database modes in more detail in Chapter 9.
Flashback Logs
Flashback logs were introduced in Oracle 10
g
in support of the
FLASHBACK DATABASE
command, a new feature of the
Enterprise Edition of the database in that release. Flashback logs contain “before images” of modified database blocks
that can be used to return the database to the way it was at some prior point in time.
Flashback Database
The
FLASHBACK DATABASE
command was introduced to speed up the otherwise slow process of point-in-time database
recovery. It can be used in place of a full database restore and a rolling forward using archive logs, and it is primarily
designed to speed up the recovery from an “accident.” For example, let's take a look at what a DBA might do to recover
from an accidentally dropped schema, in which the right schema was dropped, just in the wrong database (it was
meant to be dropped in the test environment). The DBA immediately recognizes the mistake he has made and shuts
down the database right away. Now what?
