<s>
The	O
object	B-Architecture
–	I-Architecture
relational	I-Architecture
impedance	I-Architecture
mismatch	I-Architecture
is	O
a	O
set	O
of	O
conceptual	O
and	O
technical	O
difficulties	O
that	O
are	O
often	O
encountered	O
when	O
a	O
relational	B-Application
database	I-Application
management	I-Application
system	I-Application
(	O
RDBMS	O
)	O
is	O
being	O
served	O
by	O
an	O
application	O
program	O
(	O
or	O
multiple	O
application	O
programs	O
)	O
written	O
in	O
an	O
object-oriented	B-Language
programming	I-Language
language	I-Language
or	O
style	O
,	O
particularly	O
because	O
objects	O
or	O
class	O
definitions	O
,	O
must	O
be	O
mapped	B-General_Concept
to	O
database	O
tables	O
defined	O
by	O
a	O
relational	O
schema	B-Application
.	O
</s>
<s>
The	O
term	O
object	B-Architecture
–	I-Architecture
relational	I-Architecture
impedance	I-Architecture
mismatch	I-Architecture
is	O
derived	O
from	O
the	O
electrical	O
engineering	O
term	O
impedance	O
matching	O
.	O
</s>
<s>
Relational	O
schemas	B-Application
are	O
,	O
in	O
contrast	O
,	O
tabular	O
and	O
based	O
on	O
relational	B-Algorithm
algebra	I-Algorithm
,	O
which	O
defines	O
linked	O
heterogeneous	O
tuples	B-Application
(	O
groupings	O
of	O
data	O
fields	O
into	O
a	O
"	O
row	O
"	O
with	O
different	O
types	O
for	O
each	O
field	O
)	O
,	O
where	O
links	O
are	O
always	O
reversible	O
(	O
foreign	O
keys	O
can	O
be	O
followed	O
backwards	O
,	O
as	O
INNER	O
JOIN	O
is	O
symmetric	O
)	O
,	O
which	O
is	O
a	O
characteristic	O
more	O
akin	O
to	O
undirected	O
graphs	O
.	O
</s>
<s>
Object-oriented	B-Language
programs	O
are	O
designed	O
with	O
techniques	O
that	O
result	O
in	O
encapsulated	B-Application
objects	O
whose	O
internal	O
representation	O
can	O
be	O
hidden	O
.	O
</s>
<s>
In	O
an	O
object-oriented	B-Language
framework	O
,	O
the	O
underlying	O
properties	O
of	O
a	O
given	O
object	O
are	O
expected	O
not	O
to	O
be	O
exposed	O
to	O
any	O
interface	O
outside	O
of	O
the	O
one	O
implemented	O
alongside	O
the	O
object	O
.	O
</s>
<s>
However	O
,	O
most	O
object	B-General_Concept
–	I-General_Concept
relational	I-General_Concept
mapping	I-General_Concept
approaches	O
expose	O
the	O
underlying	O
content	O
of	O
an	O
object	O
to	O
interact	O
with	O
an	O
interface	O
that	O
the	O
object	O
implementation	O
cannot	O
specify	O
.	O
</s>
<s>
Hence	O
,	O
this	O
object	B-General_Concept
–	I-General_Concept
relational	I-General_Concept
mapping	I-General_Concept
violates	O
the	O
encapsulation	B-Application
of	O
the	O
object	O
,	O
since	O
many	O
object	O
–	O
relational	O
mappers	O
automatically	O
generate	O
public	O
fields	O
corresponding	O
to	O
database	O
columns	O
.	O
</s>
<s>
A	O
few	O
frameworks	O
use	O
Metaprogramming	O
techniques	O
instead	O
,	O
thus	O
avoiding	O
violating	O
encapsulation	B-Application
.	O
</s>
<s>
In	O
the	O
object-oriented	B-Language
(	O
OO	O
)	O
model	O
it	O
is	O
an	O
absolute	O
characteristic	O
of	O
the	O
data	O
's	O
state	O
.	O
</s>
<s>
Under	O
an	O
object-oriented	B-Language
paradigm	O
,	O
objects	O
have	O
interfaces	B-Application
that	O
together	O
provide	O
the	O
only	O
access	O
to	O
the	O
internals	O
of	O
that	O
object	O
.	O
</s>
<s>
The	O
relational	O
model	O
,	O
on	O
the	O
other	O
hand	O
,	O
utilizes	O
derived	O
relation	O
variables	O
(	O
views	B-Language
)	O
to	O
provide	O
varying	O
perspectives	O
and	O
constraints	O
to	O
ensure	O
integrity	O
.	O
</s>
<s>
Similarly	O
,	O
essential	O
OOP	O
concepts	O
for	O
classes	O
of	O
objects	O
,	O
inheritance	B-Language
and	O
polymorphism	B-Application
,	O
are	O
not	O
supported	O
by	O
relational	B-Application
database	I-Application
systems	O
.	O
</s>
<s>
A	O
proper	O
mapping	O
between	O
relational	O
concepts	O
and	O
object-oriented	B-Language
concepts	O
can	O
be	O
made	O
if	O
relational	B-Application
database	I-Application
tables	O
are	O
linked	O
to	O
associations	B-Application
found	O
in	O
object-oriented	B-Application
analysis	I-Application
.	O
</s>
<s>
Scalar	O
types	O
and	O
their	O
operator	O
semantics	B-Application
can	O
be	O
vastly	O
different	O
between	O
the	O
models	O
,	O
causing	O
problems	O
in	O
mapping	O
.	O
</s>
<s>
For	O
example	O
,	O
most	O
SQL	B-Language
systems	O
support	O
string	O
types	O
with	O
varying	O
collations	O
and	O
constrained	O
maximum	O
lengths	O
(	O
open-ended	O
text	O
types	O
tend	O
to	O
hinder	O
performance	O
)	O
,	O
while	O
most	O
OO	O
languages	O
consider	O
collation	O
only	O
as	O
an	O
argument	O
to	O
sort	B-Algorithm
routines	I-Algorithm
and	O
strings	O
are	O
intrinsically	O
sized	O
to	O
available	O
memory	O
.	O
</s>
<s>
A	O
more	O
subtle	O
,	O
but	O
related	O
example	O
is	O
that	O
SQL	B-Language
systems	O
often	O
ignore	O
trailing	O
white	O
space	O
in	O
a	O
string	O
for	O
the	O
purposes	O
of	O
comparison	O
,	O
whereas	O
OO	O
string	O
libraries	O
do	O
not	O
.	O
</s>
<s>
This	O
may	O
make	O
the	O
mapping	O
to	O
relational	O
schemas	B-Application
less	O
straightforward	O
.	O
</s>
<s>
Relations	O
themselves	O
,	O
being	O
sets	O
of	O
tuples	B-Application
all	O
conforming	O
to	O
the	O
same	O
header	O
(	O
see	O
tuple	B-Application
relational	I-Application
calculus	I-Application
)	O
do	O
not	O
have	O
an	O
ideal	O
counterpart	O
in	O
OO	O
languages	O
.	O
</s>
<s>
Constraints	O
in	O
OO	O
languages	O
are	O
generally	O
not	O
declared	O
as	O
such	O
,	O
but	O
are	O
manifested	O
as	O
exception	O
raising	O
protection	O
logic	O
surrounding	O
code	O
that	O
operates	O
on	O
encapsulated	B-Application
internal	O
data	O
.	O
</s>
<s>
The	O
relational	O
model	O
,	O
on	O
the	O
other	O
hand	O
,	O
calls	O
for	O
declarative	B-Language
constraints	O
on	O
scalar	O
types	O
,	O
attributes	O
,	O
relation	O
variables	O
,	O
and	O
the	O
database	O
as	O
a	O
whole	O
.	O
</s>
<s>
The	O
relational	O
model	O
has	O
an	O
intrinsic	O
,	O
relatively	O
small	O
and	O
well-defined	O
set	O
of	O
primitive	O
operators	O
for	O
usage	O
in	O
the	O
query	O
and	O
manipulation	O
of	O
data	O
,	O
whereas	O
OO	O
languages	O
generally	O
handle	O
query	O
and	O
manipulation	O
through	O
custom-built	O
or	O
lower-level	O
,	O
case	O
-	O
and	O
physical-access-path-specific	O
imperative	B-Application
operations	O
.	O
</s>
<s>
Some	O
OO	O
languages	O
do	O
have	O
support	O
for	O
declarative	B-Language
query	O
sublanguages	B-Application
,	O
but	O
because	O
OO	O
languages	O
typically	O
deal	O
with	O
lists	O
and	O
perhaps	O
hash	B-Algorithm
tables	I-Algorithm
,	O
the	O
manipulative	O
primitives	O
are	O
necessarily	O
distinct	O
from	O
the	O
set-based	O
operations	O
of	O
the	O
relational	O
model	O
.	O
</s>
<s>
In	O
particular	O
,	O
transactions	B-General_Concept
,	O
the	O
smallest	O
unit	B-General_Concept
of	I-General_Concept
work	I-General_Concept
performed	O
by	O
databases	B-Application
,	O
are	O
much	O
larger	O
in	O
relational	B-Application
databases	I-Application
than	O
are	O
any	O
operations	O
performed	O
by	O
classes	O
in	O
OO	O
languages	O
.	O
</s>
<s>
Transactions	B-General_Concept
in	O
relational	B-Application
databases	I-Application
are	O
dynamically	O
bounded	O
sets	O
of	O
arbitrary	O
data	O
manipulations	O
,	O
whereas	O
the	O
granularity	O
of	O
transactions	B-General_Concept
in	O
an	O
OO	O
language	O
is	O
typically	O
on	O
the	O
level	O
of	O
individual	O
assignments	O
to	O
primitive-typed	O
fields	O
.	O
</s>
<s>
Working	O
around	O
the	O
impedance	O
mismatch	O
problem	O
for	O
object-oriented	B-Language
programs	O
starts	O
with	O
recognition	O
of	O
the	O
differences	O
in	O
the	O
specific	O
logic	O
systems	O
being	O
employed	O
.	O
</s>
<s>
The	O
object	B-Architecture
–	I-Architecture
relational	I-Architecture
impedance	I-Architecture
mismatch	I-Architecture
problem	O
is	O
not	O
a	O
universal	O
problem	O
between	O
OO	O
and	O
databases	B-Application
.	O
</s>
<s>
As	O
the	O
name	O
suggests	O
,	O
this	O
impedance	O
problem	O
only	O
occurs	O
with	O
relational	B-Application
databases	I-Application
.	O
</s>
<s>
The	O
most	O
common	O
solution	O
to	O
this	O
problem	O
is	O
to	O
use	O
an	O
alternative	O
database	O
,	O
such	O
as	O
NoSQL	B-General_Concept
or	O
XML	B-Application
database	I-Application
.	O
</s>
<s>
There	O
have	O
been	O
some	O
attempts	O
at	O
building	O
object-oriented	B-Application
database	I-Application
management	I-Application
systems	I-Application
(	O
OODBMS	B-Application
)	O
that	O
would	O
avoid	O
the	O
impedance	O
mismatch	O
problem	O
.	O
</s>
<s>
They	O
have	O
been	O
less	O
successful	O
in	O
practice	O
than	O
relational	B-Application
databases	I-Application
however	O
,	O
partly	O
due	O
to	O
the	O
limitations	O
of	O
OO	O
principles	O
as	O
a	O
basis	O
for	O
a	O
data	O
model	O
.	O
</s>
<s>
There	O
has	O
been	O
research	O
performed	O
in	O
extending	O
the	O
database-like	O
capabilities	O
of	O
OO	O
languages	O
through	O
such	O
notions	O
as	O
transactional	B-Operating_System
memory	I-Operating_System
.	O
</s>
<s>
Frameworks	O
which	O
employ	O
this	O
method	O
will	O
typically	O
have	O
an	O
analogue	O
for	O
a	O
tuple	B-Application
,	O
usually	O
as	O
a	O
"	O
row	O
"	O
in	O
a	O
"	O
dataset	O
"	O
component	O
or	O
as	O
a	O
generic	O
"	O
entity	O
instance	O
"	O
class	O
,	O
as	O
well	O
as	O
an	O
analogue	O
for	O
a	O
relation	O
.	O
</s>
<s>
Ability	O
for	O
the	O
schema	B-Application
to	O
change	O
dynamically	O
.	O
</s>
<s>
Inability	O
to	O
natively	O
utilize	O
uniquely	O
OO	O
aspects	O
,	O
such	O
as	O
polymorphism	B-Application
.	O
</s>
<s>
To	O
address	O
this	O
,	O
reflection	B-Language
and/or	O
code	B-Application
generation	I-Application
are	O
utilized	O
.	O
</s>
<s>
Reflection	B-Language
allows	O
code	O
(	O
classes	O
)	O
to	O
be	O
addressed	O
as	O
data	O
and	O
thus	O
provide	O
automation	O
of	O
the	O
transport	O
,	O
presentation	O
,	O
integrity	O
,	O
etc	O
.	O
</s>
<s>
Generation	O
addresses	O
the	O
problem	O
through	O
addressing	O
the	O
entity	O
structures	O
as	O
data	O
inputs	O
for	O
code	B-Application
generation	I-Application
tools	O
or	O
meta-programming	O
languages	O
,	O
which	O
produce	O
the	O
classes	O
and	O
supporting	O
infrastructure	O
en	O
masse	O
.	O
</s>
<s>
Although	O
object-relational	B-Architecture
impedance	I-Architecture
mismatches	I-Architecture
can	O
occur	O
with	O
object-oriented	B-Language
programming	I-Language
in	O
general	O
,	O
a	O
particular	O
area	O
of	O
difficulty	O
is	O
with	O
object	B-General_Concept
relational	I-General_Concept
mappers	I-General_Concept
(	O
ORMs	O
)	O
.	O
</s>
<s>
Since	O
the	O
ORM	O
is	O
often	O
specified	O
in	O
terms	O
of	O
configuration	O
,	O
annotations	O
,	O
and	O
restricted	O
domain-specific	B-Language
languages	I-Language
,	O
it	O
lacks	O
the	O
flexibility	O
of	O
a	O
full	O
programming	O
language	O
to	O
resolve	O
the	O
impedance	O
mismatch	O
.	O
</s>
<s>
Date	O
and	O
others	O
,	O
that	O
a	O
truly	O
relational	O
DBMS	O
would	O
pose	O
no	O
such	O
problem	O
,	O
as	O
domains	B-Application
and	O
classes	O
are	O
essentially	O
one	O
and	O
the	O
same	O
thing	O
.	O
</s>
<s>
A	O
native	O
mapping	O
between	O
classes	O
and	O
relational	O
schemata	O
is	O
a	O
fundamental	O
design	O
mistake	O
;	O
and	O
that	O
individual	O
tuples	B-Application
within	O
a	O
database	O
table	O
(	O
relation	O
)	O
ought	O
to	O
be	O
viewed	O
as	O
establishing	O
relationships	O
between	O
entities	O
;	O
not	O
as	O
representations	O
for	O
complex	O
entities	O
themselves	O
.	O
</s>
<s>
However	O
,	O
this	O
view	O
tends	O
to	O
diminish	O
the	O
influence	O
and	O
role	O
of	O
object-oriented	B-Language
programming	I-Language
,	O
using	O
it	O
as	O
little	O
more	O
than	O
a	O
field	O
type	O
management	O
system	O
.	O
</s>
<s>
The	O
impedance	O
mismatch	O
is	O
in	O
programming	O
between	O
the	O
domain	O
objects	O
and	O
the	O
user	B-Application
interface	I-Application
.	O
</s>
<s>
Sophisticated	O
user	B-Application
interfaces	I-Application
,	O
to	O
allow	O
operators	O
,	O
managers	O
,	O
and	O
other	O
non-programmers	O
to	O
access	O
and	O
manipulate	O
the	O
records	O
in	O
the	O
database	O
,	O
often	O
require	O
intimate	O
knowledge	O
about	O
the	O
nature	O
of	O
the	O
various	O
database	O
attributes	O
(	O
beyond	O
name	O
and	O
type	O
)	O
.	O
</s>
<s>
In	O
particular	O
,	O
it	O
's	O
considered	O
a	O
good	O
practice	O
(	O
from	O
an	O
end-user	O
productivity	O
point	O
of	O
view	O
)	O
to	O
design	O
user	B-Application
interfaces	I-Application
such	O
that	O
the	O
UI	O
prevents	O
illegal	O
transactions	B-General_Concept
(	O
those	O
which	O
cause	O
a	O
database	O
constraint	O
to	O
be	O
violated	O
)	O
from	O
being	O
entered	O
;	O
to	O
do	O
so	O
requires	O
much	O
of	O
the	O
logic	O
present	O
in	O
the	O
relational	O
schemata	O
to	O
be	O
duplicated	O
in	O
the	O
code	O
.	O
</s>
<s>
Certain	O
code-development	O
frameworks	O
can	O
leverage	O
certain	O
forms	O
of	O
logic	O
that	O
are	O
represented	O
in	O
the	O
database	O
's	O
schema	B-Application
(	O
such	O
as	O
referential	O
integrity	O
constraints	O
)	O
,	O
so	O
that	O
such	O
issues	O
are	O
handled	O
in	O
a	O
generic	O
and	O
standard	O
fashion	O
through	O
library	O
routines	O
rather	O
than	O
ad	O
hoc	O
code	O
written	O
on	O
a	O
case-by-case	O
basis	O
.	O
</s>
<s>
It	O
has	O
been	O
argued	O
that	O
SQL	B-Language
,	O
due	O
to	O
a	O
very	O
limited	O
set	O
of	O
domain	O
types	O
(	O
and	O
other	O
alleged	O
flaws	O
)	O
makes	O
proper	O
object	O
and	O
domain-modelling	O
difficult	O
;	O
and	O
that	O
SQL	B-Language
constitutes	O
a	O
very	O
lossy	O
and	O
inefficient	O
interface	O
between	O
a	O
DBMS	O
and	O
an	O
application	O
program	O
(	O
whether	O
written	O
in	O
an	O
object-oriented	B-Language
style	O
or	O
not	O
)	O
.	O
</s>
<s>
However	O
,	O
SQL	B-Language
is	O
currently	O
the	O
only	O
widely	O
accepted	O
common	O
database	O
language	O
in	O
the	O
marketplace	O
;	O
use	O
of	O
vendor-specific	O
query	O
languages	O
is	O
seen	O
as	O
a	O
bad	O
practice	O
when	O
avoidable	O
.	O
</s>
<s>
Other	O
database	O
languages	O
such	O
as	O
Business	B-Application
System	I-Application
12	I-Application
and	O
Tutorial	O
D	O
have	O
been	O
proposed	O
;	O
but	O
none	O
of	O
these	O
has	O
been	O
widely	O
adopted	O
by	O
DBMS	O
vendors	O
.	O
</s>
<s>
In	O
current	O
versions	O
of	O
mainstream	O
"	O
object	O
–	O
relational	O
"	O
DBMSs	O
like	O
Oracle	O
and	O
Microsoft	O
SQL	B-Language
Server	O
,	O
the	O
above	O
point	O
may	O
be	O
a	O
non-issue	O
.	O
</s>
<s>
With	O
these	O
engines	O
,	O
the	O
functionality	O
of	O
a	O
given	O
database	O
can	O
be	O
arbitrarily	O
extended	O
through	O
stored	O
code	O
(	O
functions	O
and	O
procedures	O
)	O
written	O
in	O
a	O
modern	O
OO	O
language	O
(	O
Java	B-Language
for	O
Oracle	O
,	O
and	O
a	O
Microsoft	O
.NET	O
language	O
for	O
SQL	B-Language
Server	O
)	O
,	O
and	O
these	O
functions	O
can	O
be	O
invoked	O
in-turn	O
in	O
SQL	B-Language
statements	O
in	O
a	O
transparent	O
fashion	O
:	O
that	O
is	O
,	O
the	O
user	O
neither	O
knows	O
nor	O
cares	O
that	O
these	O
functions/procedures	O
were	O
not	O
originally	O
part	O
of	O
the	O
database	O
engine	O
.	O
</s>
<s>
Modern	O
software-development	O
paradigms	O
are	O
fully	O
supported	O
:	O
thus	O
,	O
one	O
can	O
create	O
a	O
set	O
of	O
library	O
routines	O
that	O
can	O
be	O
re-used	O
across	O
multiple	O
database	B-Application
schemas	I-Application
.	O
</s>
<s>
These	O
vendors	O
decided	O
to	O
support	O
OO-language	O
integration	O
at	O
the	O
DBMS	O
back-end	O
because	O
they	O
realized	O
that	O
,	O
despite	O
the	O
attempts	O
of	O
the	O
ISO	O
SQL-99	O
committee	O
to	O
add	O
procedural	O
constructs	O
to	O
SQL	B-Language
,	O
SQL	B-Language
will	O
never	O
have	O
the	O
rich	O
set	O
of	O
libraries	O
and	O
data	O
structures	O
that	O
today	O
's	O
application	O
programmers	O
take	O
for	O
granted	O
,	O
and	O
it	O
is	O
reasonable	O
to	O
leverage	O
these	O
as	O
directly	O
as	O
possible	O
rather	O
than	O
attempting	O
to	O
extend	O
the	O
core	O
SQL	B-Language
language	I-Language
.	O
</s>
<s>
Some	O
,	O
however	O
,	O
would	O
point	O
out	O
that	O
this	O
contention	O
is	O
moot	O
due	O
to	O
the	O
fact	O
that	O
:	O
(	O
1	O
)	O
RDBMSes	O
were	O
never	O
intended	O
to	O
facilitate	O
object	O
modelling	O
,	O
and	O
(	O
2	O
)	O
SQL	B-Language
generally	O
should	O
only	O
be	O
seen	O
as	O
a	O
"	O
lossy	O
"	O
or	O
"	O
inefficient	O
"	O
interface	O
language	O
when	O
one	O
is	O
trying	O
to	O
achieve	O
a	O
solution	O
for	O
which	O
RDBMSes	O
were	O
not	O
designed	O
.	O
</s>
<s>
SQL	B-Language
is	O
very	O
efficient	O
at	O
doing	O
what	O
it	O
was	O
designed	O
to	O
do	O
,	O
namely	O
,	O
to	O
query	O
,	O
sort	O
,	O
filter	O
,	O
and	O
store	O
large	O
sets	O
of	O
data	O
.	O
</s>
<s>
The	O
database	O
model	O
generally	O
assumes	O
that	O
the	O
database	B-Application
management	I-Application
system	I-Application
is	O
the	O
only	O
authoritative	O
repository	O
of	O
state	O
concerning	O
the	O
enterprise	O
;	O
any	O
copies	O
of	O
such	O
state	O
held	O
by	O
an	O
application	O
program	O
are	O
just	O
that	O
temporary	O
copies	O
(	O
which	O
may	O
be	O
out	O
of	O
date	O
,	O
if	O
the	O
underlying	O
database	B-Application
record	I-Application
was	O
subsequently	O
modified	O
by	O
a	O
transaction	O
)	O
.	O
</s>
<s>
Many	O
object-oriented	B-Language
programmers	O
prefer	O
to	O
view	O
the	O
in-memory	O
representations	O
of	O
objects	O
themselves	O
as	O
the	O
canonical	O
data	O
,	O
and	O
view	O
the	O
database	O
as	O
a	O
backing	O
store	O
and	O
persistence	O
mechanism	O
.	O
</s>
<s>
Another	O
point	O
of	O
contention	O
is	O
the	O
proper	O
division	O
of	O
responsibility	O
between	O
application	O
programmers	O
and	O
database	B-General_Concept
administrators	I-General_Concept
(	O
DBA	O
)	O
.	O
</s>
<s>
Due	O
to	O
the	O
need	O
to	O
maintain	O
a	O
production	O
database	B-Application
system	I-Application
24	O
hours	O
a	O
day	O
many	O
DBAs	O
are	O
reluctant	O
to	O
make	O
changes	O
to	O
database	O
schemata	O
that	O
they	O
deem	O
gratuitous	O
or	O
superfluous	O
and	O
in	O
some	O
cases	O
outright	O
refuse	O
to	O
do	O
so	O
.	O
</s>
<s>
Use	O
of	O
developmental	O
databases	B-Application
(	O
apart	O
from	O
production	O
systems	O
)	O
can	O
help	O
somewhat	O
;	O
but	O
when	O
the	O
newly	O
developed	O
application	O
"	O
goes	O
live	O
"	O
the	O
DBA	O
will	O
need	O
to	O
approve	O
any	O
changes	O
.	O
</s>
<s>
In	O
organizations	O
with	O
a	O
non-dysfunctional	O
relationship	O
between	O
DBAs	O
and	O
developers	O
,	O
though	O
,	O
the	O
above	O
issue	O
should	O
not	O
present	O
itself	O
,	O
as	O
the	O
decision	O
to	O
change	O
a	O
database	B-Application
schema	I-Application
or	O
not	O
would	O
only	O
be	O
driven	O
by	O
business	O
needs	O
:	O
a	O
new	O
requirement	O
to	O
persist	O
additional	O
data	O
or	O
a	O
performance	O
boost	O
of	O
a	O
critical	O
application	O
would	O
both	O
trigger	O
a	O
schema	B-Application
modification	O
,	O
for	O
example	O
.	O
</s>
<s>
Declarative	B-Language
vs.	O
imperative	B-Application
interfaces	B-Application
Relational	O
thinking	O
tends	O
to	O
use	O
data	O
as	O
interfaces	B-Application
,	O
not	O
behavior	O
as	O
interfaces	B-Application
.	O
</s>
<s>
It	O
thus	O
has	O
a	O
declarative	B-Language
tilt	O
in	O
design	O
philosophy	O
in	O
contrast	O
to	O
OO	O
's	O
behavioral	O
tilt	O
.	O
</s>
<s>
Schema	B-Application
bound	O
Objects	O
do	O
not	O
have	O
to	O
follow	O
a	O
"	O
parent	O
schema	B-Application
"	O
for	O
which	O
attributes	O
or	O
accessors	O
an	O
object	O
has	O
,	O
while	O
table	B-Application
rows	I-Application
must	O
follow	O
the	O
entity	O
's	O
schema	B-Application
.	O
</s>
<s>
The	O
closest	O
thing	O
in	O
OO	O
is	O
inheritance	B-Language
,	O
but	O
it	O
is	O
generally	O
tree-shaped	O
and	O
optional	O
.	O
</s>
<s>
Dynamic	O
database	B-Application
systems	I-Application
that	O
allow	O
ad	O
hoc	O
columns	O
may	O
relax	O
schema	B-Application
bound-ness	O
,	O
but	O
such	O
systems	O
are	O
either	O
currently	O
rare	O
,	O
or	O
their	O
classification	O
as	O
"	O
relational	O
"	O
is	O
in	O
question	O
.	O
</s>
<s>
Access	O
rules	O
In	O
relational	B-Application
databases	I-Application
,	O
attributes	O
are	O
accessed	O
and	O
altered	O
through	O
predefined	O
relational	O
operators	O
,	O
while	O
OO	O
allows	O
each	O
class	O
to	O
create	O
its	O
own	O
state	O
alteration	O
interface	O
and	O
practices	O
.	O
</s>
<s>
The	O
resulting	O
tightly	O
bound	O
entity	O
containing	O
both	O
nouns	O
and	O
the	O
verbs	O
is	O
usually	O
called	O
a	O
class	O
,	O
or	O
in	O
OO	O
analysis	O
,	O
a	O
concept	B-Application
.	O
</s>
<s>
Relational	O
designs	O
generally	O
do	O
not	O
assume	O
there	O
is	O
anything	O
natural	O
or	O
logical	O
about	O
such	O
tight	O
associations	B-Application
(	O
outside	O
of	O
relational	O
operators	O
)	O
.	O
</s>
<s>
Relations	O
,	O
on	O
the	O
other	O
hand	O
,	O
have	O
no	O
inherent	O
concept	B-Application
of	O
this	O
kind	O
of	O
identity	O
.	O
</s>
<s>
That	O
said	O
,	O
it	O
is	O
a	O
common	O
practice	O
to	O
fabricate	O
"	O
identity	O
"	O
for	O
records	O
in	O
a	O
database	O
through	O
use	O
of	O
globally	O
unique	O
candidate	B-Application
keys	I-Application
;	O
though	O
many	O
consider	O
this	O
a	O
poor	O
practice	O
for	O
any	O
database	B-Application
record	I-Application
which	O
does	O
not	O
have	O
a	O
one-to-one	O
correspondence	O
with	O
a	O
real	O
world	O
entity	O
.	O
</s>
<s>
Normalization	B-Application
Relational	B-Application
normalization	I-Application
practices	O
are	O
often	O
ignored	O
by	O
OO	O
designs	O
.	O
</s>
<s>
An	O
alternate	O
view	O
is	O
that	O
a	O
collection	O
of	O
objects	O
,	O
interlinked	O
via	O
pointers	O
of	O
some	O
sort	O
,	O
is	O
equivalent	O
to	O
a	O
network	B-General_Concept
database	I-General_Concept
;	O
which	O
in	O
turn	O
can	O
be	O
viewed	O
as	O
an	O
extremely	O
denormalized	O
relational	B-Application
database	I-Application
.	O
</s>
<s>
Schema	B-Application
inheritance	B-Language
Most	O
relational	B-Application
databases	I-Application
do	O
not	O
support	O
schema	B-Application
inheritance	B-Language
.	O
</s>
<s>
OO	O
advocates	O
point	O
out	O
that	O
inheritance/subtyping	O
models	O
need	O
not	O
be	O
limited	O
to	O
trees	O
(	O
though	O
this	O
is	O
a	O
limitation	O
in	O
many	O
popular	O
OO	O
languages	O
such	O
as	O
Java	B-Language
)	O
,	O
but	O
non-tree	O
OO	O
solutions	O
are	O
seen	O
as	O
more	O
difficult	O
to	O
formulate	O
than	O
set-based	O
variation-on-a-theme	O
management	O
techniques	O
preferred	O
by	O
relational	O
.	O
</s>
<s>
At	O
the	O
least	O
,	O
they	O
differ	O
from	O
techniques	O
commonly	O
used	O
in	O
relational	B-Algorithm
algebra	I-Algorithm
.	O
</s>
<s>
Object-oriented	B-Language
methods	O
generally	O
assume	O
that	O
the	O
primary	O
user	O
of	O
the	O
object-oriented	B-Language
code	I-Language
and	O
its	O
interfaces	B-Application
are	O
the	O
application	O
developers	O
.	O
</s>
<s>
However	O
,	O
relational	B-Application
queries	I-Application
and	O
"	O
views	B-Language
"	O
are	O
common	O
techniques	O
to	O
present	O
information	O
in	O
application	O
-	O
or	O
task-specific	O
configurations	O
.	O
</s>
<s>
As	O
a	O
result	O
of	O
the	O
object	B-Architecture
–	I-Architecture
relational	I-Architecture
impedance	I-Architecture
mismatch	I-Architecture
,	O
it	O
is	O
often	O
argued	O
by	O
partisans	O
on	O
both	O
sides	O
of	O
the	O
debate	O
that	O
the	O
other	O
technology	O
ought	O
to	O
be	O
abandoned	O
or	O
reduced	O
in	O
scope	O
.	O
</s>
<s>
Conversely	O
,	O
some	O
OO	O
advocates	O
argue	O
that	O
more	O
OO-friendly	O
persistence	O
mechanisms	O
,	O
such	O
as	O
OODBMS	B-Application
,	O
ought	O
to	O
be	O
developed	O
and	O
used	O
,	O
and	O
that	O
relational	O
technology	O
ought	O
to	O
be	O
phased	O
out	O
.	O
</s>
<s>
Many	O
(	O
if	O
not	O
most	O
)	O
programmers	O
and	O
DBAs	O
hold	O
neither	O
of	O
these	O
viewpoints	O
;	O
and	O
view	O
the	O
object	B-Architecture
–	I-Architecture
relational	I-Architecture
impedance	I-Architecture
mismatch	I-Architecture
as	O
a	O
mere	O
fact	O
of	O
life	O
that	O
information	B-General_Concept
technology	I-General_Concept
has	O
to	O
deal	O
with	O
.	O
</s>
<s>
It	O
is	O
also	O
argued	O
that	O
the	O
O/R	B-General_Concept
mapping	I-General_Concept
is	O
paying	O
off	O
in	O
some	O
situations	O
,	O
but	O
is	O
probably	O
oversold	O
:	O
it	O
has	O
advantages	O
besides	O
drawbacks	O
.	O
</s>
