<s>
Within	O
computing	O
and	O
computer	B-General_Concept
science	I-General_Concept
,	O
an	O
expression	B-General_Concept
index	I-General_Concept
,	O
also	O
known	O
as	O
a	O
function	O
based	O
index	B-Data_Structure
,	O
is	O
a	O
database	B-Data_Structure
index	I-Data_Structure
that	O
is	O
built	O
on	O
a	O
generic	O
expression	O
,	O
rather	O
than	O
one	O
or	O
more	O
columns	O
.	O
</s>
<s>
A	O
common	O
use	O
for	O
an	O
expression	B-General_Concept
index	I-General_Concept
is	O
to	O
support	O
case-insensitive	O
searching	O
or	O
constraints	O
.	O
</s>
<s>
For	O
example	O
,	O
if	O
a	O
web	O
site	O
wants	O
to	O
make	O
user	O
names	O
case-insensitive	O
,	O
but	O
still	O
preserve	O
the	O
case	O
as	O
originally	O
entered	O
by	O
the	O
user	O
,	O
an	O
index	B-Data_Structure
can	O
be	O
created	O
on	O
the	O
lower-case	O
representation	O
of	O
the	O
user	O
name	O
:	O
</s>
<s>
CREATE	O
INDEX	B-Data_Structure
users__last_name_lower	O
ON	O
users( 	O
lower( last_name )	O
)	O
;	O
</s>
<s>
That	O
will	O
create	O
a	O
unique	O
index	B-Data_Structure
on	O
"	O
lower(last_name )	O
"	O
.	O
</s>
<s>
Any	O
queries	O
that	O
search	O
on	O
"	O
lower(last_name )	O
"	O
could	O
then	O
make	O
use	O
of	O
that	O
index	B-Data_Structure
:	O
</s>
<s>
Major	O
databases	O
which	O
support	O
expression	B-General_Concept
indexes	I-General_Concept
include	O
:	O
IBM	B-Application
Db2	I-Application
(	O
since	O
version	O
10.5	O
)	O
,	O
Oracle	B-General_Concept
Database	I-General_Concept
(	O
since	O
release	O
8i	O
.	O
)	O
</s>
<s>
and	O
PostgreSQL	B-Application
(	O
since	O
at	O
least	O
version	O
7	O
)	O
.	O
</s>
