<s>
In	O
databases	B-Application
,	O
a	O
partial	B-Language
index	I-Language
,	O
also	O
known	O
as	O
filtered	B-Language
index	I-Language
is	O
an	O
index	B-Data_Structure
which	O
has	O
some	O
condition	O
applied	O
to	O
it	O
so	O
that	O
it	O
includes	O
a	O
subset	O
of	O
rows	O
in	O
the	O
table	B-Application
.	O
</s>
<s>
This	O
allows	O
the	O
index	B-Data_Structure
to	O
remain	O
small	O
,	O
even	O
though	O
the	O
table	B-Application
may	O
be	O
rather	O
large	O
,	O
and	O
have	O
extreme	O
selectivity	O
.	O
</s>
<s>
Suppose	O
you	O
have	O
a	O
transaction	O
table	B-Application
where	O
entries	O
start	O
out	O
with	O
STATUS	O
=	O
'	O
A	O
 '	O
(	O
active	O
)	O
,	O
and	O
then	O
may	O
pass	O
through	O
other	O
statuses	O
( 	O
 '	O
P	O
 '	O
for	O
pending	O
,	O
'	O
W	O
 '	O
for	O
"	O
being	O
worked	O
on	O
"	O
)	O
before	O
reaching	O
a	O
final	O
status	O
,	O
'	O
F	O
 '	O
,	O
at	O
which	O
point	O
it	O
is	O
no	O
longer	O
likely	O
to	O
be	O
processed	O
again	O
.	O
</s>
<s>
In	O
PostgreSQL	B-Application
,	O
a	O
useful	O
partial	B-Language
index	I-Language
might	O
be	O
defined	O
as	O
:	O
</s>
<s>
This	O
index	B-Data_Structure
would	O
not	O
bother	O
storing	O
any	O
of	O
the	O
millions	O
of	O
rows	O
that	O
have	O
reached	O
"	O
final	O
"	O
status	O
,	O
'	O
F	O
 '	O
,	O
and	O
would	O
allow	O
queries	O
looking	O
for	O
transactions	O
that	O
still	O
"	O
need	O
work	O
"	O
to	O
efficiently	O
search	O
via	O
this	O
index	B-Data_Structure
.	O
</s>
<s>
Similarly	O
,	O
a	O
partial	B-Language
index	I-Language
can	O
be	O
used	O
to	O
index	B-Data_Structure
only	O
those	O
rows	O
where	O
a	O
column	O
is	O
not	O
null	O
,	O
which	O
will	O
be	O
of	O
benefit	O
when	O
the	O
column	O
usually	O
is	O
null	O
.	O
</s>
<s>
This	O
index	B-Data_Structure
would	O
allow	O
the	O
following	O
query	O
to	O
read	O
only	O
the	O
updated	O
tuples	B-Application
:	O
</s>
<s>
It	O
is	O
not	O
necessary	O
that	O
the	O
condition	O
be	O
the	O
same	O
as	O
the	O
index	B-Data_Structure
criterion	O
;	O
Stonebraker	O
's	O
paper	O
below	O
presents	O
a	O
number	O
of	O
examples	O
with	O
indexes	O
similar	O
to	O
the	O
following	O
:	O
</s>
<s>
In	O
SQL	B-Application
Server	I-Application
,	O
this	O
type	O
of	O
index	B-Data_Structure
is	O
called	O
a	O
filtered	B-Language
index	I-Language
.	O
</s>
<s>
Partial	B-Language
indexes	I-Language
have	O
been	O
supported	O
in	O
PostgreSQL	B-Application
since	O
version	O
7.2	O
,	O
released	O
in	O
February	O
2002	O
.	O
</s>
<s>
SQLite	B-Language
supports	O
partial	B-Language
indexes	I-Language
since	O
version	O
3.8.0	O
.	O
</s>
<s>
MongoDB	B-Application
supports	O
partial	B-Language
indexes	I-Language
since	O
version	O
3.2	O
.	O
</s>
