Limitations of the relational model
Limitations of the relational model
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Limitations of the relational model
Overview
 application areas for which the relational model
is inadequate - reasons
 drawbacks of relational DBMSs
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Limitations of the relational model
Areas needing advanced database systems
 both in terms of structure of data and of operations to
be performed on data
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computer aided design (CAD)
computer aided manufacturing (CAM)
computer aided software engineering (CASE)
office information systems, multimedia systems and digital
publishing
 geographic information systems
 etc.
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Limitations of the relational model
CAD
 what is CAD?
 do you think CAD needs database systems?
 imagine you were going to use a relational DBMS
(e.g. Postgres) to support a CAD application; what
difficulties do you think you wold encounter?
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Limitations of the relational model
CAD - characteristics | insufficiency of relational
database systems
characteristic
problem
designs - large number of types
(millions, sometimes); small
number of instances
designs - many interdependencies
between subsystems
not suitable for relational model;
why?
design evolves through time implications must be propagated;
some actions cannot be ‘a priori’
foreseen
interdependency  integrity
constraint; the relational model
does not provide sufficient
support for integrity constraints
schema evolution plus integrity
constraints modifications based
on the new schema
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Limitations of the relational model
CAD - characteristics | insufficiency of relational
database systems
characteristic
updates are far reaching; one
change (usually) affects a large
number of objects
design alternatives; versions
co-operative engineering = many
staff work in parallel on multiple
versions of the same product; the
end product must be consistent
and co-ordinated
problem
integrity constraints
no support
no support
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Limitations of the relational model
CAD and relational databases
 could you solve any of the problems mentioned,
basing your solution solely on features of
relational database systems?
 the answer is yes, but some of the features you
are using in your solutions are (probably) not
implicit in the relational model
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Limitations of the relational model
CAM
 similar to CAD but more data processing required
 monitoring
 control
 possibly, both in real time
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Limitations of the relational model
CASE - characteristics that make it
unsuitable for the relational model
 (large) data relating to the stages of the software
development lifecycle
 co-operative engineering
 concurrent sharing of project design, code and
documentation;
 dependencies between components must be tracked
 project management
 scheduling
 cost estimation
 progress monitoring
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Limitations of the relational model
Office Information Systems and Digital
Publishing
characteristic
wide range of data types (e.g.
multimedia types: pictures,
sound, video)
data is shared (retrieve and
update) among many users;
problem
not supported; data cannot be
stored or it cannot be
accessed “inside”
the simple locking of a whole
document - not sufficient;
some versioning system may
be required
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Limitations of the relational model
Geographic information systems
 spatial information - not supported by
relational DBMSs
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Limitations of the relational model
Drawbacks of relational databases
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poor representation of real world entities
semantic overloading
poor support for integrity constraints
homogeneous data structure
limited operations
difficulty in handling recursive queries
impedance mismatch
other problems ...
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Limitations of the relational model
Poor representation of real world objects
 normalisation -> fragmentation
 relations that have no correspondent in the real world
 to infer information -> joins -> very costly
 poor semantics
 everything is a RELATION (see next slide)
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Limitations of the relational model
Semantic overloading
 no distinction between entity and relationship; no
distinction between different types of relations (e.g. “has” “is
the parent of”, “has the address”, “is located”, “earns”)
 result: this semantic cannot be expressed (e.g. be build into the
operators)
 there is very little meaning in relations (i.e. the systems
“knows” very little of the data it contained); what solutions to
increase the semantics exist?
 domains - provided by the relational model, but not fully supported
by relational DBMSs
 keys - provided and supported
 support for enterprise integrity constraints …?
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Limitations of the relational model
Poor support for integrity constraints
 integrity  validity and consistency
 entity integrity + referential integrity + domains
 many systems do not fully support -> build them into applications
-> effort + inconsistencies
 integrity is expressed in terms of constraints (rules that the database
must comply with)
 the constraint checking mechanism is simple
 degree of compliance; categories of integrity constraints ...
 what does it mean must comply with?
 what is the logical model? must be deduced from the DB? the DB must
be consistent with them?must be true of the DB?
 the relational model does not support enterprise integrity constraints
 SQL provides support for constraints as part of the DB def.
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Limitations of the relational model
Homogeneous data structure
 horizontal and vertical homogeneity
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too restrictive
real world objects have a more complex structure
result: unnatural structures + many joins (inefficient)
example: composite parts
on the other hand, this symmetric structure is one of the
strengths of the relational model (why?)
 binary large objects (BLOB)
 are allowed
 typically, they are references to files, therefore some
advantages provided by DBMSs may be lost (e.g. security)
 their inner structure cannot be accessed
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Limitations of the relational model
Limited set of operations
 only a fix set operations
 tuple oriented
 set oriented
 not sufficient for many applications (e.g. geographic
information systems - distance, area, …)
 no new operations cannot be specified
 some DBMSs allow for type extensibility
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Limitations of the relational model
Difficulty in handling recursive queries
Staff_no
Manager_staff_no
Staff_no
Manager_staff_no
S5
S4
S3
S2
S1
S4
S3
S2
S1
NULL
S5
S5
…
S5
S4
…
S3
S3
S2
S1
S4
S3
…
S1
S3
…
S2
S1
S1
NULL
Transitive closure
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Limitations of the relational model
Impedance mismatch
 SQL92 lacks computational completeness
 solution: embedded SQL
 drawback of solution: impedance mismatch
 mixing different paradigms
 SQL: set based operators
 high level programming languages: record based
(or even less) (example C)
 conversion to records needed (inefficient!)
 type mismatch
 solution by Date: build set level facilities in high level
programming languages
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Limitations of the relational model
Other problems
 other problems also exist
 no support for long lived transactions (not inherent to the relational
model)
 schema amendments - difficult
 poor navigational access
 therefore, other approaches might be needed
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Limitations of the relational model
Conclusions
 first generation DBMSs
 1060s-1970s
 two approaches: hierarchical and network
 main problems: complex programs needed to be written to
answer simple queries (navigational databases); minimal data
independence; no widely accepted theoretical foundations
 second generation
 1970 - Codd’s paper
 1970-1980 - many experimental relational DBMSs
 now: over 100 commercial DBMSs (some stretch the definition
of a relational DBMS)
 third generation … (“to be continued”)
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