<s>
Similarity	B-Algorithm
search	I-Algorithm
is	O
the	O
most	O
general	O
term	O
used	O
for	O
a	O
range	O
of	O
mechanisms	O
which	O
share	O
the	O
principle	O
of	O
searching	O
(	O
typically	O
,	O
very	O
large	O
)	O
spaces	O
of	O
objects	O
where	O
the	O
only	O
available	O
comparator	O
is	O
the	O
similarity	O
between	O
any	O
pair	O
of	O
objects	O
.	O
</s>
<s>
Nearest	B-Algorithm
neighbor	I-Algorithm
search	I-Algorithm
and	O
range	B-General_Concept
queries	I-General_Concept
are	O
important	O
subclasses	O
of	O
similarity	B-Algorithm
search	I-Algorithm
,	O
and	O
a	O
number	O
of	O
solutions	O
exist	O
.	O
</s>
<s>
Research	O
in	O
similarity	B-Algorithm
search	I-Algorithm
is	O
dominated	O
by	O
the	O
inherent	O
problems	O
of	O
searching	O
over	O
complex	O
objects	O
.	O
</s>
<s>
Such	O
objects	O
cause	O
most	O
known	O
techniques	O
to	O
lose	O
traction	O
over	O
large	O
collections	O
,	O
due	O
to	O
a	O
manifestation	O
of	O
the	O
so-called	O
curse	B-Algorithm
of	I-Algorithm
dimensionality	I-Algorithm
,	O
and	O
there	O
are	O
still	O
many	O
unsolved	O
problems	O
.	O
</s>
<s>
Unfortunately	O
,	O
in	O
many	O
cases	O
where	O
similarity	B-Algorithm
search	I-Algorithm
is	O
necessary	O
,	O
the	O
objects	O
are	O
inherently	O
complex	O
.	O
</s>
<s>
The	O
most	O
general	O
approach	O
to	O
similarity	B-Algorithm
search	I-Algorithm
relies	O
upon	O
the	O
mathematical	O
notion	O
of	O
metric	O
space	O
,	O
which	O
allows	O
the	O
construction	O
of	O
efficient	O
index	O
structures	O
in	O
order	O
to	O
achieve	O
scalability	O
in	O
the	O
search	O
domain	O
.	O
</s>
<s>
Similarity	B-Algorithm
search	I-Algorithm
evolved	O
independently	O
in	O
a	O
number	O
different	O
scientific	O
and	O
computing	O
contexts	O
,	O
according	O
to	O
various	O
needs	O
.	O
</s>
<s>
Metric	O
search	O
is	O
similarity	B-Algorithm
search	I-Algorithm
which	O
takes	O
place	O
within	O
metric	O
spaces	O
.	O
</s>
<s>
The	O
research	O
domain	O
of	O
metric	O
search	O
can	O
thus	O
be	O
characterised	O
as	O
the	O
study	O
of	O
pre-processing	O
algorithms	O
over	O
large	O
and	O
relatively	O
static	O
collections	O
of	O
data	O
which	O
,	O
using	O
the	O
properties	O
of	O
metric	O
spaces	O
,	O
allow	O
efficient	O
similarity	B-Algorithm
search	I-Algorithm
to	O
be	O
performed	O
.	O
</s>
<s>
A	O
popular	O
approach	O
for	O
similarity	B-Algorithm
search	I-Algorithm
is	O
locality	B-Algorithm
sensitive	I-Algorithm
hashing	I-Algorithm
(	O
LSH	O
)	O
.	O
</s>
<s>
It	O
hashes	B-Error_Name
input	O
items	O
so	O
that	O
similar	O
items	O
map	O
to	O
the	O
same	O
"	O
buckets	O
"	O
in	O
memory	O
with	O
high	O
probability	O
(	O
the	O
number	O
of	O
buckets	O
being	O
much	O
smaller	O
than	O
the	O
universe	O
of	O
possible	O
input	O
items	O
)	O
.	O
</s>
<s>
It	O
is	O
often	O
applied	O
in	O
nearest	B-Algorithm
neighbor	I-Algorithm
search	I-Algorithm
on	O
large	O
scale	O
high-dimensional	O
data	O
,	O
e.g.	O
,	O
image	O
databases	O
,	O
document	O
collections	O
,	O
time-series	O
databases	O
,	O
and	O
genome	O
databases	O
.	O
</s>
