<s>
In	O
computer	B-General_Concept
science	I-General_Concept
,	O
a	O
grid	B-Data_Structure
file	I-Data_Structure
or	O
bucket	O
grid	O
is	O
a	O
point	O
access	O
method	O
which	O
splits	O
a	O
space	O
into	O
a	O
non-periodic	O
grid	O
where	O
one	O
or	O
more	O
cells	O
of	O
the	O
grid	O
refer	O
to	O
a	O
small	O
set	O
of	O
points	O
.	O
</s>
<s>
Grid	B-Data_Structure
files	I-Data_Structure
(	O
a	O
symmetric	B-Algorithm
data	B-General_Concept
structure	I-General_Concept
)	O
provide	O
an	O
efficient	O
method	O
of	O
storing	O
these	O
indexes	O
on	O
disk	O
to	O
perform	O
complex	O
data	O
lookups	O
.	O
</s>
<s>
Grid	B-Data_Structure
files	I-Data_Structure
do	O
not	O
contain	O
any	O
data	O
themselves	O
but	O
instead	O
contain	O
references	O
to	O
the	O
correct	O
bucket	O
.	O
</s>
<s>
A	O
grid	B-Data_Structure
file	I-Data_Structure
is	O
usually	O
used	O
in	O
cases	O
where	O
a	O
single	O
value	O
can	O
be	O
referenced	O
by	O
multiple	O
keys	O
.	O
</s>
<s>
A	O
grid	B-Data_Structure
file	I-Data_Structure
began	O
being	O
used	O
because	O
"	O
traditional	O
file	O
structures	O
that	O
provide	O
multikey	O
access	O
to	O
records	O
,	O
for	O
example	O
,	O
inverted	O
files	O
,	O
are	O
extensions	O
of	O
file	O
structures	O
originally	O
designed	O
for	O
single-key	O
access	O
.	O
</s>
<s>
In	O
a	O
traditional	O
single	O
dimensional	O
data	B-General_Concept
structure	I-General_Concept
(	O
e.g.	O
</s>
<s>
hash	B-Application
)	O
,	O
a	O
search	O
on	O
a	O
single	O
criterion	O
is	O
usually	O
very	O
simple	O
but	O
searching	O
for	O
a	O
second	O
criterion	O
can	O
be	O
much	O
more	O
complex	O
.	O
</s>
<s>
Grid	B-Data_Structure
files	I-Data_Structure
represent	O
a	O
special	O
kind	O
of	O
hashing	O
,	O
where	O
the	O
traditional	O
hash	B-Application
is	O
replaced	O
by	O
a	O
grid	O
directory	O
.	O
</s>
<s>
A	O
single	O
record	B-Application
represents	O
a	O
single	O
household	O
,	O
and	O
all	O
records	O
are	O
grouped	O
into	O
buckets	O
.	O
</s>
<s>
A	O
grid	B-Data_Structure
file	I-Data_Structure
can	O
be	O
used	O
to	O
provide	O
an	O
efficient	O
index	O
for	O
this	O
structure	O
,	O
where	O
records	O
come	O
in	O
groupings	O
of	O
26	O
,	O
each	O
of	O
them	O
relating	O
to	O
street	O
names	O
in	O
a	O
city	O
starting	O
with	O
one	O
of	O
the	O
letters	O
of	O
the	O
alphabet	O
.	O
</s>
<s>
This	O
structure	O
can	O
be	O
thought	O
of	O
as	O
an	O
array	B-Data_Structure
,	O
table	B-Application
,	O
or	O
grid	O
with	O
two	O
dimensions	O
which	O
we	O
will	O
call	O
the	O
x	O
and	O
y	O
axes	O
.	O
</s>
<s>
Each	O
record	B-Application
in	O
this	O
structure	O
is	O
known	O
as	O
a	O
cell	O
.	O
</s>
<s>
An	O
extra	O
cell	O
,	O
or	O
record	B-Application
header	O
,	O
may	O
be	O
required	O
to	O
store	O
the	O
name	O
of	O
the	O
city	O
.	O
</s>
<s>
The	O
cells	O
in	O
the	O
grid	B-Data_Structure
file	I-Data_Structure
would	O
then	O
consist	O
of	O
a	O
city	O
header	O
,	O
and	O
six	O
(	O
one	O
for	O
each	O
continent	O
,	O
not	O
including	O
Antarctica	O
)	O
groupings	O
of	O
26	O
cells	O
relating	O
to	O
the	O
streets	O
with	O
the	O
same	O
starting	O
letter	O
,	O
in	O
the	O
same	O
city	O
,	O
on	O
the	O
same	O
continent	O
and	O
could	O
now	O
be	O
thought	O
of	O
as	O
a	O
three-dimensional	O
array	B-Data_Structure
.	O
</s>
<s>
Since	O
a	O
single	O
entry	O
in	O
the	O
grid	B-Data_Structure
file	I-Data_Structure
contains	O
pointers	O
to	O
all	O
records	O
indexed	O
by	O
the	O
specified	O
keys	O
:	O
</s>
<s>
However	O
,	O
because	O
of	O
the	O
nature	O
of	O
the	O
grid	B-Data_Structure
file	I-Data_Structure
,	O
which	O
gives	O
it	O
its	O
advantages	O
,	O
there	O
are	O
also	O
some	O
disadvantages	O
:	O
</s>
