<s>
In	O
computing	O
,	O
GiST	B-Language
or	O
Generalized	B-Language
Search	I-Language
Tree	I-Language
,	O
is	O
a	O
data	B-General_Concept
structure	I-General_Concept
and	O
API	B-General_Concept
that	O
can	O
be	O
used	O
to	O
build	O
a	O
variety	O
of	O
disk-based	O
search	B-Algorithm
trees	I-Algorithm
.	O
</s>
<s>
GiST	B-Language
is	O
a	O
generalization	O
of	O
the	O
B+	O
tree	O
,	O
providing	O
a	O
concurrent	O
and	O
recoverable	O
height-balanced	O
search	O
tree	O
infrastructure	O
without	O
making	O
any	O
assumptions	O
about	O
the	O
type	O
of	O
data	O
being	O
stored	O
,	O
or	O
the	O
queries	O
being	O
serviced	O
.	O
</s>
<s>
GiST	B-Language
can	O
be	O
used	O
to	O
easily	O
implement	O
a	O
range	O
of	O
well-known	O
indexes	O
,	O
including	O
B+	O
trees	O
,	O
R-trees	B-Library
,	O
hB-trees	O
,	O
RD-trees	O
,	O
and	O
many	O
others	O
;	O
it	O
also	O
allows	O
for	O
easy	O
development	O
of	O
specialized	O
indexes	O
for	O
new	O
data	O
types	O
.	O
</s>
<s>
It	O
cannot	O
be	O
used	O
directly	O
to	O
implement	O
non-height-balanced	O
trees	O
such	O
as	O
quad	B-Data_Structure
trees	I-Data_Structure
or	O
prefix	B-General_Concept
trees	I-General_Concept
(	O
tries	B-General_Concept
)	O
,	O
though	O
like	O
prefix	B-General_Concept
trees	I-General_Concept
it	O
does	O
support	O
compression	O
,	O
including	O
lossy	B-Algorithm
compression	I-Algorithm
.	O
</s>
<s>
GiST	B-Language
can	O
be	O
used	O
for	O
any	O
data	O
type	O
that	O
can	O
be	O
naturally	O
ordered	O
into	O
a	O
hierarchy	O
of	O
supersets	O
.	O
</s>
<s>
Not	O
only	O
is	O
it	O
extensible	B-Architecture
in	O
terms	O
of	O
data	O
type	O
support	O
and	O
tree	O
layout	O
,	O
it	O
allows	O
the	O
extension	O
writer	O
to	O
support	O
any	O
query	O
predicates	O
that	O
they	O
choose	O
.	O
</s>
<s>
GiST	B-Language
is	O
an	O
example	O
of	O
software	O
extensibility	B-Architecture
in	O
the	O
context	O
of	O
database	O
systems	O
:	O
it	O
allows	O
the	O
easy	O
evolution	O
of	O
a	O
database	O
system	O
to	O
support	O
new	O
tree-based	O
indexes	O
.	O
</s>
<s>
It	O
achieves	O
this	O
by	O
factoring	O
out	O
its	O
core	O
system	O
infrastructure	O
from	O
a	O
narrow	O
API	B-General_Concept
that	O
is	O
sufficient	O
to	O
capture	O
the	O
application-specific	O
aspects	O
of	O
a	O
wide	O
variety	O
of	O
index	O
designs	O
.	O
</s>
<s>
The	O
GiST	B-Language
infrastructure	O
code	O
manages	O
the	O
layout	O
of	O
the	O
index	O
pages	O
on	O
disk	O
,	O
the	O
algorithms	O
for	O
searching	O
indexes	O
and	O
deleting	O
from	O
indexes	O
,	O
and	O
complex	O
transactional	O
details	O
such	O
as	O
page-level	O
locking	O
for	O
high	O
concurrency	O
and	O
write-ahead	B-Algorithm
logging	I-Algorithm
for	O
crash	O
recovery	O
.	O
</s>
<s>
Although	O
originally	O
designed	O
for	O
answering	O
Boolean	O
selection	O
queries	O
,	O
GiST	B-Language
can	O
also	O
support	O
nearest-neighbor	B-Algorithm
search	I-Algorithm
,	O
and	O
various	O
forms	O
of	O
statistical	O
approximation	O
over	O
large	O
data	O
sets	O
.	O
</s>
<s>
The	O
most	O
widely	O
used	O
GiST	B-Language
implementation	O
is	O
in	O
the	O
PostgreSQL	B-Application
relational	B-Application
database	I-Application
;	O
it	O
was	O
also	O
implemented	O
in	O
the	O
Informix	B-Application
Universal	O
Server	O
,	O
and	O
as	O
a	O
standalone	O
library	O
,	O
libgist	O
.	O
</s>
<s>
The	O
PostgreSQL	B-Application
GiST	B-Language
implementation	O
includes	O
support	O
for	O
variable	O
length	O
keys	O
,	O
composite	O
keys	O
,	O
concurrency	O
control	O
and	O
recovery	O
;	O
these	O
features	O
are	O
inherited	O
by	O
all	O
GiST	B-Language
extensions	O
.	O
</s>
<s>
There	O
are	O
several	O
contributed	O
modules	O
developed	O
using	O
GiST	B-Language
and	O
distributed	O
with	O
PostgreSQL	B-Application
.	O
</s>
<s>
The	O
PostgreSQL	B-Application
GiST	B-Language
implementation	O
provides	O
the	O
indexing	O
support	O
for	O
the	O
PostGIS	B-Application
(	O
geographic	B-Application
information	I-Application
system	I-Application
)	O
and	O
the	O
BioPostgres	O
bioinformatics	O
system	O
.	O
</s>
