<s>
In	O
computer	B-General_Concept
science	I-General_Concept
,	O
an	O
inverted	B-Algorithm
index	I-Algorithm
(	O
also	O
referred	O
to	O
as	O
a	O
postings	O
list	O
,	O
postings	B-Algorithm
file	I-Algorithm
,	O
or	O
inverted	B-Algorithm
file	I-Algorithm
)	O
is	O
a	O
database	B-Data_Structure
index	I-Data_Structure
storing	O
a	O
mapping	O
from	O
content	O
,	O
such	O
as	O
words	O
or	O
numbers	O
,	O
to	O
its	O
locations	O
in	O
a	O
table	B-Application
,	O
or	O
in	O
a	O
document	O
or	O
a	O
set	O
of	O
documents	O
(	O
named	O
in	O
contrast	O
to	O
a	O
forward	O
index	O
,	O
which	O
maps	O
from	O
documents	O
to	O
content	O
)	O
.	O
</s>
<s>
The	O
purpose	O
of	O
an	O
inverted	B-Algorithm
index	I-Algorithm
is	O
to	O
allow	O
fast	O
full-text	B-Application
searches	I-Application
,	O
at	O
a	O
cost	O
of	O
increased	O
processing	O
when	O
a	O
document	O
is	O
added	O
to	O
the	O
database	O
.	O
</s>
<s>
The	O
inverted	B-Algorithm
file	I-Algorithm
may	O
be	O
the	O
database	O
file	O
itself	O
,	O
rather	O
than	O
its	O
index	O
.	O
</s>
<s>
It	O
is	O
the	O
most	O
popular	O
data	B-General_Concept
structure	I-General_Concept
used	O
in	O
document	B-Algorithm
retrieval	I-Algorithm
systems	I-Algorithm
,	O
used	O
on	O
a	O
large	O
scale	O
for	O
example	O
in	O
search	B-Application
engines	I-Application
.	O
</s>
<s>
Additionally	O
,	O
several	O
significant	O
general-purpose	O
mainframe-based	O
database	B-Application
management	I-Application
systems	I-Application
have	O
used	O
inverted	B-Algorithm
list	I-Algorithm
architectures	O
,	O
including	O
ADABAS	B-Application
,	O
DATACOM/DB	B-Application
,	O
and	O
Model	B-Application
204	I-Application
.	O
</s>
<s>
There	O
are	O
two	O
main	O
variants	O
of	O
inverted	B-Algorithm
indexes	I-Algorithm
:	O
A	O
record-level	O
inverted	B-Algorithm
index	I-Algorithm
(	O
or	O
inverted	B-Algorithm
file	I-Algorithm
index	I-Algorithm
or	O
just	O
inverted	B-Algorithm
file	I-Algorithm
)	O
contains	O
a	O
list	O
of	O
references	O
to	O
documents	O
for	O
each	O
word	O
.	O
</s>
<s>
A	O
word-level	O
inverted	B-Algorithm
index	I-Algorithm
(	O
or	O
full	B-Algorithm
inverted	I-Algorithm
index	I-Algorithm
or	O
inverted	B-Algorithm
list	I-Algorithm
)	O
additionally	O
contains	O
the	O
positions	O
of	O
each	O
word	O
within	O
a	O
document	O
.	O
</s>
<s>
The	O
latter	O
form	O
offers	O
more	O
functionality	O
(	O
like	O
phrase	B-Algorithm
searches	I-Algorithm
)	O
,	O
but	O
needs	O
more	O
processing	O
power	O
and	O
space	O
to	O
be	O
created	O
.	O
</s>
<s>
The	O
inverted	B-Algorithm
index	I-Algorithm
data	B-General_Concept
structure	I-General_Concept
is	O
a	O
central	O
component	O
of	O
a	O
typical	O
search	B-Application
engine	I-Application
indexing	I-Application
algorithm	I-Application
.	O
</s>
<s>
A	O
goal	O
of	O
a	O
search	B-Application
engine	I-Application
implementation	O
is	O
to	O
optimize	O
the	O
speed	O
of	O
the	O
query	O
:	O
find	O
the	O
documents	O
where	O
word	O
X	O
occurs	O
.	O
</s>
<s>
Once	O
a	O
forward	O
index	O
is	O
developed	O
,	O
which	O
stores	O
lists	O
of	O
words	O
per	O
document	O
,	O
it	O
is	O
next	O
inverted	O
to	O
develop	O
an	O
inverted	B-Algorithm
index	I-Algorithm
.	O
</s>
<s>
Instead	O
of	O
listing	O
the	O
words	O
per	O
document	O
in	O
the	O
forward	O
index	O
,	O
the	O
inverted	B-Algorithm
index	I-Algorithm
data	B-General_Concept
structure	I-General_Concept
is	O
developed	O
which	O
lists	O
the	O
documents	O
per	O
word	O
.	O
</s>
<s>
With	O
the	O
inverted	B-Algorithm
index	I-Algorithm
created	O
,	O
the	O
query	O
can	O
now	O
be	O
resolved	O
by	O
jumping	O
to	O
the	O
word	O
ID	O
(	O
via	O
random	B-General_Concept
access	I-General_Concept
)	O
in	O
the	O
inverted	B-Algorithm
index	I-Algorithm
.	O
</s>
<s>
These	O
were	O
effectively	O
inverted	B-Algorithm
indexes	I-Algorithm
with	O
a	O
small	O
amount	O
of	O
accompanying	O
commentary	O
that	O
required	O
a	O
tremendous	O
amount	O
of	O
effort	O
to	O
produce	O
.	O
</s>
<s>
In	O
bioinformatics	O
,	O
inverted	B-Algorithm
indexes	I-Algorithm
are	O
very	O
important	O
in	O
the	O
sequence	O
assembly	O
of	O
short	O
fragments	O
of	O
sequenced	O
DNA	O
.	O
</s>
<s>
The	O
matching	O
requires	O
constructing	O
an	O
inverted	B-Algorithm
index	I-Algorithm
of	O
all	O
substrings	O
of	O
a	O
certain	O
length	O
from	O
the	O
reference	O
DNA	O
sequence	O
.	O
</s>
<s>
Since	O
the	O
human	O
DNA	O
contains	O
more	O
than	O
3	O
billion	O
base	O
pairs	O
,	O
and	O
we	O
need	O
to	O
store	O
a	O
DNA	O
substring	O
for	O
every	O
index	O
and	O
a	O
32-bit	O
integer	O
for	O
index	O
itself	O
,	O
the	O
storage	O
requirement	O
for	O
such	O
an	O
inverted	B-Algorithm
index	I-Algorithm
would	O
probably	O
be	O
in	O
the	O
tens	O
of	O
gigabytes	O
.	O
</s>
<s>
For	O
historical	O
reasons	O
,	O
inverted	B-Algorithm
list	I-Algorithm
compression	O
and	O
bitmap	O
compression	O
were	O
developed	O
as	O
separate	O
lines	O
of	O
research	O
,	O
and	O
only	O
later	O
were	O
recognized	O
as	O
solving	O
essentially	O
the	O
same	O
problem	O
.	O
</s>
