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What is Denormalization
Denormalization refers to a refinement to the relational schema
such that the degree of
normalization for a modified relation is less than the degree of
at least one of the original
relations. Denormalization can also be referred to a process in
which we combine two relations
into one new relation, and the new relation is still normalized
but contains more nulls than theoriginal relations.
Normalization
Normalization is a logical database design that is
structurally consistent and has minimalredundancy. Normalization
forces us to understand completely each attribute that has to
be
represented in the database. This may be the most important
factor that contributes to the overall
success of the system.
In addition, the following factors have to be considered:
denormalization maes implementationmore comple!" denormalization
often sacrifices fle!ibility" denormalization may speed up
retrievals but it slows down updates.
Then why to denormalize relations
It is sometimes argued that a normalized database design does
not provide ma!imum processing
efficiency. There may be circumstances where it may be necessary
to accept the loss of some ofthe benefits of a fully normalized
design in favor of performance.
Benefts o Normalization
Normalization produces smaller tables with smaller
rows:
#ore rows per page $less logical I%&'
#ore rows per I%& $more efficient'
#ore rows fit in cache $less physical I%&'
The benefits of normalization include:
(earching, sorting, and creating inde!es are faster, since
tables are narrower, and more
rows fit on a data page.
)ou usually wind up with more tables. )ou can have more
clustered inde!es $you get
only one per table' so you get more fle!ibility in tuning
*ueries.
Inde! searching is often faster, since inde!es tend to be
narrower and shorter.
#ore tables allow better use of segments to control physical
placement of data.
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)ou usually wind up with fewer inde!es per table, so data
modification commands are
faster.
)ou wind up with fewer null values and less redundant data,
maing your database more
compact.
Triggers e!ecute more *uicly if you are not maintaining
redundant data.
Data modification anomalies are reduced.
Normalization is conceptually cleaner and easier to
maintain and change as your needs
change.
+ood reasons for denormalizing are:
ll or nearly all of the most fre*uent *ueries re*uire access to
the full set of -oined data
ma-ority of applications perform table scans when -oining
tables
omputational comple!ity of derived columns re*uires temporary
tables or e!cessively
comple! *ueries
Disadvantages o Denormalization
Denormalization has these disadvantages:
It usually speeds retrieval but can slow data modification.
It is always application/specific and needs to be re/evaluated
if the application changes.
It can increase the size of tables.
In some instances, it simplifies coding" in others, it maes
coding more comple!.
Perormance Advantages o Denormalization
Denormalization can improve performance by:
#inimizing the need for -oins
0educing the number of foreign eys on tables
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0educing the number of inde!es, saving storage space and
reducing data modification
time
1recomputing aggregate values, that is, computing them at data
modification time rather
than at select time
0educing the number of tables $in some cases'
