Database Reference
In-Depth Information
Example 7:
The following statement lists table comments for tables owned by
schemas beginning with the name
Bruce
:
14.6 Summary and Concluding Remarks
It is now time to summarize what has been discussed in this chapter:
•
The system catalog is the most important database object in a
database system. This is so because it facilitates and supports all
or most of the other database objects. The system catalog typically
contains metadata about the database.
•
The Oracle system catalog contains system tables for various
database objects including (but not confined to) tablespaces,
datafiles, tables, views, table columns, constraints, indexes, users,
roles, privileges, etc. Oracle does not allow direct access of its
catalog tables; rather, it provides views prefixed by
DBA, ALL,
USER
, and
GC$
.
•
Three commonly used Oracle catalog views are
User_Tables
,
User_Tab_Columns
, and
User_Indexes
. In actuality, the Oracle
catalog contains scores of catalog views (review section 14.3).
•
You can query catalog views just as you would any other table. This
often provides useful information to the DBA or software engineer.
•
As a rule, Oracle does not allow direct update of its catalog tables.
However, note that the catalog is automatically updated every
time the physical or logical structure of the database is modified.
The only exception to this rule is the
Comment
statement. This
statement allows the specification of comments for database
tables and columns.
If you consider what has been said about the system catalog in light of Date's Zero
Rule and Codd's twelve rules for relational DBMS suites, you will soon realize the catalog
is an absolute necessity if the DBMS is to stand up to the lofty industry expectations. To
be more direct, if you are evaluating a DBMS suite and discover that it does not host a
comprehensive system catalog, you are pursuing a product that is not going to stand up to
much rigor; you'd be well advised to save your effort for some more meaningful project.
As powerful as useful as SQL is, the language is not without limitations. The next
chapter discusses some of these limitations.
