Database Reference
In-Depth Information
metadata on V$ views in
DBA_VIEWS
. Conversely, metadata on V_$ views, which are used for
granting access to V$ views, is available through
DBA_VIEWS
.
Some X$ tables are linked to disk storage. For example, the column
DCNAM
of the X$ fixed table
X$KCCDC
holds path names of data file copies created with the RMAN command
COPY
DATAFILE
.
The disk storage for these path names is within the control file. By looking at
V$CONTROLFILE_
RECORD_SECTION
or its foundation
X$KCCRS
, you will be able to identify a section called “DATAFILE
COPY”. This is the control file section represented by
X$KCCDC
(KCCDC is short for Kernel Cache
Control file Data file Copy).
There is an old saying about the ORACLE DBMS which maintains that any well designed
feature has more than a single denomination. The synonymous terms V$ dynamic performance
view and V$ fixed view are an example of this. Another example is automatic undo manage-
ment, which is also known as system managed undo (SMU) or the System Change Number
(SCN), which is occasionally referred to as the System Commit Number.
Layered Architecture
X$ fixed tables are accessed in a layered manner as depicted in Figure 9-1. A public synonym by
the same name exists for each V$ or GV$ fixed view. These synonyms refer to V_$ and GV_$
views owned by SYS. These are true views with metadata in
DBA_VIEWS
. DDL for these views is
in
catalog.sql
. It is of the following form:
CREATE OR REPLACE VIEW {g|v}_$<view_name> AS
SELECT * FROM {g|v$}<fixed_view_name>;
SELECT
on these V_$ and GV_$ views is then granted to
SELECT_CATALOG_ROLE
. All V$ dynamic
performance views hold information pertaining to the current instance, i.e., the instance the
user has connected to. V$ fixed views are based on GV$ fixed views, which are built on top of
one or more X$ fixed tables. On systems running Real Application Clusters, GV$ fixed views
provide access to information about other instances that have mounted the same database.
Array offsets in C start at 0, whereas a minimum index of 1 is preferred in SQL, e.g., the value of
the pseudo-column
ROWNUM
starts at 1. This is why you see that 1 is added to some columns of
X$ tables in the definition of the GV$ view.
Note that
V$OBJECT_USAGE
is the only view—a true view with metadata in
DBA_VIEWS
—that
is not based on a GV$ view. Instead, it retrieves information from data dictionary tables in
tablespace SYSTEM owned by user SYS. Since it violates the aforementioned rules for V$ views,
it doesn't deserve the prefix V$. Apparently no knight armored by a valid Oracle customer
service number dares to dispute the undeserved prefix by opening a service request.
Many X$ table names follow a strict naming convention, where the first few letters repre-
sent a layer or module in the ORACLE kernel. For example KC means Kernel Cache and KT
Kernel Transaction. Table 9-1 has some more abbreviations used by X$ tables and their presumed
meaning.
