<s>
In	O
databases	O
an	O
index	B-Data_Structure
is	O
a	O
data	O
structure	O
,	O
part	O
of	O
the	O
database	O
,	O
used	O
by	O
a	O
database	O
system	O
to	O
efficiently	O
navigate	O
access	O
to	O
user	O
data	O
.	O
</s>
<s>
Index	B-Data_Structure
data	O
are	O
system	O
data	O
distinct	O
from	O
user	O
data	O
,	O
and	O
consist	O
primarily	O
of	O
pointers	O
.	O
</s>
<s>
Index	B-Operating_System
locking	I-Operating_System
is	O
a	O
technique	O
used	O
to	O
maintain	O
index	B-Data_Structure
integrity	O
.	O
</s>
<s>
A	O
portion	O
of	O
an	O
index	B-Data_Structure
is	O
locked	O
during	O
a	O
database	O
transaction	O
when	O
this	O
portion	O
is	O
being	O
accessed	O
by	O
the	O
transaction	O
as	O
a	O
result	O
of	O
attempt	O
to	O
access	O
related	O
user	O
data	O
.	O
</s>
<s>
Additionally	O
,	O
special	O
database	O
system	O
transactions	O
(	O
not	O
user-invoked	O
transactions	O
)	O
may	O
be	O
invoked	O
to	O
maintain	O
and	O
modify	O
an	O
index	B-Data_Structure
,	O
as	O
part	O
of	O
a	O
system	O
's	O
self-maintenance	O
activities	O
.	O
</s>
<s>
When	O
a	O
portion	O
of	O
an	O
index	B-Data_Structure
is	O
locked	O
by	O
a	O
transaction	O
,	O
other	O
transactions	O
may	O
be	O
blocked	O
from	O
accessing	O
this	O
index	B-Data_Structure
portion	O
(	O
blocked	O
from	O
modifying	O
,	O
and	O
even	O
from	O
reading	O
it	O
,	O
depending	O
on	O
lock	O
type	O
and	O
needed	O
operation	O
)	O
.	O
</s>
<s>
Index	B-Operating_System
Locking	I-Operating_System
Protocol	O
guarantees	O
that	O
phantom	O
read	O
phenomenon	O
wo	O
n't	O
occur	O
.	O
</s>
<s>
Index	B-Operating_System
locking	I-Operating_System
protocol	O
states	O
:	O
</s>
<s>
Every	O
relation	O
must	O
have	O
at	O
least	O
one	O
index	B-Data_Structure
.	O
</s>
<s>
A	O
transaction	O
Ti	O
that	O
performs	O
a	O
lookup	O
must	O
lock	O
all	O
the	O
index	B-Data_Structure
leaf	O
nodes	O
that	O
it	O
accesses	O
,	O
in	O
S-mode	O
,	O
even	O
if	O
the	O
leaf	O
node	O
does	O
not	O
contain	O
any	O
tuple	O
satisfying	O
the	O
index	B-Data_Structure
lookup	O
(	O
e.g.	O
</s>
<s>
The	O
rules	O
of	O
the	O
two-phase	B-Operating_System
locking	I-Operating_System
protocol	O
must	O
be	O
observed	O
.	O
</s>
<s>
Specialized	O
concurrency	B-Operating_System
control	I-Operating_System
techniques	O
exist	O
for	O
accessing	O
indexes	O
.	O
</s>
<s>
These	O
techniques	O
depend	O
on	O
the	O
index	B-Data_Structure
type	O
,	O
and	O
take	O
advantage	O
of	O
its	O
structure	O
.	O
</s>
<s>
They	O
are	O
typically	O
much	O
more	O
effective	O
than	O
applying	O
to	O
indexes	O
common	O
concurrency	B-Operating_System
control	I-Operating_System
methods	O
applied	O
to	O
user	O
data	O
.	O
</s>
<s>
Notable	O
and	O
widely	O
researched	O
are	O
specialized	O
techniques	O
for	O
B-trees	B-Architecture
(	O
B-Tree	B-Architecture
concurrency	B-Operating_System
control	I-Operating_System
)	O
which	O
are	O
regularly	O
used	O
as	O
database	B-Data_Structure
indexes	I-Data_Structure
.	O
</s>
<s>
Index	B-Operating_System
locks	I-Operating_System
are	O
used	O
to	O
coordinate	O
threads	B-Operating_System
accessing	O
indexes	O
concurrently	O
,	O
and	O
typically	O
shorter-lived	O
than	O
the	O
common	O
transaction	O
locks	O
on	O
user	O
data	O
.	O
</s>
