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Transcript
D B 2 PORTAL BLOG
NEWS, VIEWS, AND ITEMS OF INTEREST ON DB2 DATABASE
MANAGEMENT AND MAINFRAME SYSTEMS.
Thursday, February 14, 2013
The Importance of Database Design
- Craig S. Mullins
It is impossible to over-estimate the importance of database design on the
effectiveness and efficiency of application systems. The first step, of course,
is to create a logical data model of the business information that must be
stored in, and accessed through, the database. This is a non-trivial task, but
it is not the focus of today’s blog post, which is implementing optimal and
well-designed database schemata.
Oftentimes the database design does not get the attention it deserves. This
can occur for numerous reasons such as:
•
•
•
•
Insufficient specifications and/or poor logical data modeling
Not enough time in the development schedule
 many changes occurring throughout the development cycle
Database design assigned to, or performed by novices
And even when the database design is being performed by experienced
professionals, with sufficient time and a solid logical model, it is very easy
for design flaws to creep into the database. This is especially the case for
larger and more complex databases required to support mission critical
applications.
Of course, the first step in constructing a physical database should be
transforming the logical design using best practices. The transformation
consists of the following:
•
•
•
•
Transforming entities into tables
Transforming attributes into columns
Transforming domains into data types and constraints
Transforming relationships into primary and foreign keys
But a simple transformation will not result in a complete and correct
physical database design – it is merely the first step. And design flaws can
be introduced even during such a transformation.
The process of normalizing your data should be conducted during the
logical design phase, but sometimes mistakes are made during the logical
modeling process. In simple terms, normalization is the process of
identifying the one best place where each fact belongs. A normalized design
minimizes data redundancy and optimizes data structures by
systematically and properly placing data elements into the appropriate
groupings.
 First normal form (1NF) eliminates repeating groups and non-atomic
data from an entity.
 Second normal form (2NF) ensures that all the attributes of each entity
are dependent on the primary key.
 Third normal form (3NF) ensures that no relationships between
attributes exist within an entity.
Although normalization is a logical process and does not necessarily dictate
physical database design it is good practice to implement normalized
physical databases – especially with today’s powerful hardware and
database systems. A normalized data model will ensure that each entity is
well formed and that each attribute is assigned to the proper entity. And
denormalized data structures should always be documented.
A database that is not normalized can be difficult to modify (because facts
may be strewn about within the database) and prone to data quality issues
(again because one fact may be in more than one place). Failing to achieve
appropriate normalization is a significant design flaw.
Design flaws can make it difficult to access a database in an efficient
manner. Something as simple as creating columns with inconsistent data
type and length across tables can introduce inefficiency. For example,
perhaps JOB_CODE is defined as CHAR(10) in one table and CHAR(15) in
another. If these columns need to be compared or joined, additional work
is required by the DBMS to make the columns comparable. It is easy to see
how a database design issue can make applications inefficient no matter
how adept the developers are. And this is just one type of design flaw.
There are numerous other types of design flaws that can negatively impact
the usability and efficiency of a database implementation (and the
applications that use it). Improperly defined constraints (referential, check,
uniqueness) can cause data quality problems. Improper indexing (to
support constraints and other physical structures) can cause a database to
operate inefficiently or even cease operating altogether.
In DB2 for z/OS, failing to explicitly name a clustering index causes DB2 to
default cluster sequence to the oldest index. Changing index structures,
which sometimes require dropping and recreating the index, can cause the
data to be ordered in a completely different physical sequence, thereby
impacting performance.
Some database design flaws are more subtle. What happens if you create
two incompatible check constraints? For example, consider the following:
CHECK (empno < 100)
CHECK (empno >= 101)
No data can be added to the table because no number is less than 100 and
greater than or equal to 101. Of course, this is an extreme example to
highlight the problem. Another situation can occur if the default value is
not one of the values permitted by the check constraint, for example:
emp_type CHAR(8) DEFAULT 'new'
CHECK (emp_type IN ('temp',
'fulltime',
'contract')), ...
Cycles are another potential problem that can be created in a physical
database schema. A cycle is a referential path that connects a table to itself.
This can occur when multiple tables are related back to each other and it
looks like a loop when diagrammed. DB2 forbids a table from being deleteconnected to itself in a cycle. A table is delete-connected to another table if
it is a dependent of a table specified with a CASCADE delete rule.
Nested triggers can also create problems. A trigger is fired by an INSERT,
UPDATE, or DELETE operation. However, a trigger can also contain
INSERT, UPDATE, or DELETE logic within itself. Therefore, a trigger is
fired by a data modification, but can also cause another data modification,
thereby firing yet another trigger. When a trigger contains INSERT,
UPDATE, and/or DELETE logic (including MERGE statements), the
trigger is said to be a nested trigger. If referential integrity is combined
with triggers, additional cascading updates and/or deletes can occur.
DB2 restricts the number of nested triggers that can be executed within a
single firing event. If this were not done, it could be quite possible to have
triggers firing triggers ad infinitum until all of the data was removed from
an entire database. DB2 limits this cascading effect to 16 levels to prevent
endless looping. If more than 16 levels of nesting occur, the transaction is
aborted. Such nesting may restrict certain types of data modification from
happening at all because the number of nested calls will always be
exceeded. So nesting levels need to be controlled and managed in the
database schema to avoid problems.
And this is by no means a comprehensive list of database design flaws that
can cause real problems for application developers and end users. Getting
the database design correct is imperative if you wish to have any hope of
assuring application performance.
So how can you go about examining the correctness of your database
structures? The best approach is an automated one such as that provided
by Database Examiner, a product offered by DBE Software that
performs a comprehensive validation of a database schema.
The in-depth technology incorporated into Database Examiner allows
you to review your database schema, identify problems, and remediate the
issues to ensure a quality database implementation offering peak
performance.
Simply provide Database Examiner with the ERwin data model, DDL,
or a link to the active database and it will perform a series of
comprehensive diagnostics.
Database Examiner applies the rules of relational technology to detect
flaws, inconsistencies and lack of integrity. The product applies more than
50+ diagnostics that can be organized by category or severity levels. And
you can customize the diagnostics by selecting those to be executed and
assigning each one a severity level.
But Database Examiner doesn’t just highlight the flaws, it also helps to
fix them be providing recommendations and corrections for each issue it
detects, including the generation of SQL DDL scripts to remedy the
flaws. Even better, Database Examiner provides multi-platform DBMS
support, including DB2 for z/OS, DB2 LUW, Oracle, Microsoft SQL Server
and Sybase.
And DBE Software is currently offering folks the ability to download and
use a scaled down “Performance Edition” of Database Examiner, free of
charge for a limited time. Be sure to enter the promo code MULL on the
download request to get the best offer available.
Hopefully this blog entry has convinced you that database design is
important… important enough to take some time to evaluate the quality of
your existing database structures. And to take a look at automating the
process using Database Examiner (using the promo code MULL).
PO ST E D B Y C R A IG S . M ULL IN S AT 1 :1 6 P M