<s>
Database	B-Application
management	I-Application
systems	I-Application
provide	O
multiple	O
types	O
of	O
indexes	O
to	O
improve	O
performance	O
and	O
data	O
integrity	O
across	O
diverse	O
applications	O
.	O
</s>
<s>
Index	B-Data_Structure
types	O
include	O
b-trees	B-Architecture
,	O
bitmaps	B-Data_Structure
,	O
and	O
r-trees	B-Library
.	O
</s>
<s>
In	O
database	B-Application
management	I-Application
systems	I-Application
,	O
a	O
reverse	O
key	B-Application
index	B-Data_Structure
strategy	O
reverses	O
the	O
key	B-Application
value	O
before	O
entering	O
it	O
in	O
the	O
index	B-Data_Structure
.	O
</s>
<s>
E.g.	O
,	O
the	O
value	O
24538	O
becomes	O
83542	O
in	O
the	O
index	B-Data_Structure
.	O
</s>
<s>
Reversing	O
the	O
key	B-Application
value	O
is	O
particularly	O
useful	O
for	O
indexing	O
data	O
such	O
as	O
sequence	O
numbers	O
,	O
where	O
each	O
new	O
key	B-Application
value	O
is	O
greater	O
than	O
the	O
prior	O
value	O
,	O
i.e.	O
,	O
values	O
monotonically	O
increase	O
.	O
</s>
<s>
Reverse	O
key	B-Application
indexes	O
have	O
become	O
particularly	O
important	O
in	O
high	O
volume	O
transaction	B-Application
processing	I-Application
systems	I-Application
because	O
they	O
reduce	O
contention	B-General_Concept
for	O
index	B-Data_Structure
blocks	B-General_Concept
.	O
</s>
<s>
Reversed	O
key	B-Application
indexes	O
use	O
b-tree	B-Architecture
structures	O
,	O
but	O
preprocess	O
key	B-Application
values	O
before	O
inserting	O
them	O
.	O
</s>
<s>
Simplifying	O
,	O
b-trees	B-Architecture
place	O
similar	O
values	O
on	O
a	O
single	O
index	B-Data_Structure
block	O
,	O
e.g.	O
,	O
storing	O
24538	O
on	O
the	O
same	O
block	O
as	O
24539	O
.	O
</s>
<s>
However	O
,	O
if	O
the	O
application	O
inserts	O
values	O
in	O
sequence	O
,	O
each	O
insert	O
must	O
have	O
access	O
to	O
the	O
newest	O
block	O
in	O
the	O
index	B-Data_Structure
in	O
order	O
to	O
add	O
the	O
new	O
value	O
.	O
</s>
<s>
This	O
is	O
particularly	O
a	O
problem	O
in	O
clustered	B-Architecture
databases	I-Architecture
,	O
which	O
may	O
require	O
the	O
block	O
to	O
be	O
copied	O
from	O
one	O
computer	O
's	O
memory	O
to	O
another	O
's	O
to	O
allow	O
the	O
next	O
user	O
to	O
perform	O
their	O
insert	O
.	O
</s>
<s>
Reversing	O
the	O
key	B-Application
spreads	O
similar	O
new	O
values	O
across	O
the	O
entire	O
index	B-Data_Structure
instead	O
of	O
concentrating	O
them	O
in	O
any	O
one	O
leaf	O
block	O
.	O
</s>
<s>
This	O
means	O
that	O
24538	O
appears	O
on	O
the	O
same	O
block	O
as	O
14538	O
while	O
24539	O
goes	O
to	O
a	O
different	O
block	O
,	O
eliminating	O
this	O
cause	O
of	O
contention	B-General_Concept
.	O
</s>
<s>
Reverse	B-Data_Structure
indexes	I-Data_Structure
are	O
just	O
as	O
efficient	O
as	O
unreversed	O
indexes	O
for	O
finding	O
specific	O
values	O
,	O
although	O
they	O
are	O
n't	O
helpful	O
for	O
range	O
queries	O
.	O
</s>
<s>
When	O
searching	O
the	O
index	B-Data_Structure
,	O
the	O
query	O
processor	O
simply	O
reverses	O
the	O
search	O
target	O
before	O
looking	O
it	O
up	O
.	O
</s>
<s>
As	O
time	O
passes	O
,	O
in	O
standard	O
b-trees	B-Architecture
,	O
index	B-Data_Structure
blocks	B-General_Concept
for	O
lower	O
values	O
end	O
up	O
containing	O
few	O
values	O
,	O
with	O
a	O
commensurate	O
increase	O
in	O
unused	O
space	O
,	O
referred	O
to	O
as	O
"	O
rot	O
"	O
.	O
</s>
<s>
Rot	O
not	O
only	O
wastes	O
space	O
,	O
but	O
slows	O
query	O
speeds	O
,	O
because	O
a	O
smaller	O
fraction	O
of	O
a	O
rotten	O
index	B-Data_Structure
's	O
blocks	B-General_Concept
fit	O
in	O
memory	O
at	O
any	O
one	O
time	O
.	O
</s>
<s>
In	O
a	O
b-tree	B-Architecture
,	O
if	O
14538	O
gets	O
deleted	O
,	O
its	O
index	B-Data_Structure
space	O
remains	O
empty	O
.	O
</s>
