Databases Reference
In-Depth Information
systems will have one or more log files, some dedicating different logs to different
components.
Nearly all systems provide some kind of support for error and activity log
maintenance. If the DBMS does not include an automated agent as a separate
component, you might find these functions integrated into the database engine.
Performance monitoring and scheduled activities are a different story, however.
If not supported by the database engine or a separate service, you will need to
either find a way to use operating system utilities or buy (or build) a separate
support application that handles them for you.
Data Dictionary
Another component is the data dictionary, which refers to the storage of struc-
ture definitions—the storage repository of structure definitions. In many rela-
tional databases, the data dictionary comprises a set of system tables like a direc-
tory system. The data dictionary interface software is sometimes considered a
distinct component within a DBMS. However, in other manufacturers' products
the data dictionary is considered an integrated part of the database engine rather
than a separate component. Either way, the data dictionary is part of the data-
base's metadata, and no matter how it is defined, its role is the same. Specifi-
cally, the data dictionary performs the following tasks:
▲
Keeps the definitions of all data structures and relationships.
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Describes data types of all stored data items.
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Specifies field sizes and formats of data items.
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Holds information on space allocations and storage locations for all
data.
Enables the database engine to keep track of the data and store it in the
assigned places.
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Maintains indexes.
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Stores report and screen definitions.
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Query Processor
Another key component is the
query processor,
which may be called the
query optimizer,
the DBMS component responsible for parsing, optimizing,
and compiling queries for execution. Each commercial vendor adopts a
standard query language or develops its own proprietary language for creating
queries.
Structured Query Language
or
SQL
(pronounced see-kwil) is the
standard language for relational DBMSs. Each relational database vendor
enhances the standard SQL command set and adds a few features here and
there, but the essential features of SQL are present in every commercial DBMS.
