<s>
In	O
database	B-Application
systems	I-Application
,	O
isolation	O
determines	O
how	O
transaction	B-General_Concept
integrity	O
is	O
visible	O
to	O
other	O
users	O
and	O
systems	O
.	O
</s>
<s>
A	O
lower	O
isolation	B-General_Concept
level	I-General_Concept
increases	O
the	O
ability	O
of	O
many	O
users	O
to	O
access	O
the	O
same	O
data	O
at	O
the	O
same	O
time	O
,	O
but	O
increases	O
the	O
number	O
of	O
concurrency	B-Operating_System
effects	O
(	O
such	O
as	O
dirty	O
reads	O
or	O
lost	O
updates	O
)	O
users	O
might	O
encounter	O
.	O
</s>
<s>
Conversely	O
,	O
a	O
higher	O
isolation	B-General_Concept
level	I-General_Concept
reduces	O
the	O
types	O
of	O
concurrency	B-Operating_System
effects	O
that	O
users	O
may	O
encounter	O
,	O
but	O
requires	O
more	O
system	O
resources	O
and	O
increases	O
the	O
chances	O
that	O
one	O
transaction	B-General_Concept
will	O
block	O
another	O
.	O
</s>
<s>
In	O
two-tier	O
systems	O
,	O
a	O
transaction	B-General_Concept
processing	O
(	O
TP	O
)	O
manager	O
is	O
required	O
to	O
maintain	O
isolation	O
.	O
</s>
<s>
In	O
n-tier	O
systems	O
(	O
such	O
as	O
multiple	O
websites	O
attempting	O
to	O
book	O
the	O
last	O
seat	O
on	O
a	O
flight	O
)	O
,	O
a	O
combination	O
of	O
stored	O
procedures	O
and	O
transaction	B-General_Concept
management	O
is	O
required	O
to	O
commit	O
the	O
booking	O
and	O
send	O
confirmation	O
to	O
the	O
customer	O
.	O
</s>
<s>
Isolation	O
is	O
one	O
of	O
the	O
four	O
ACID	O
properties	O
,	O
along	O
with	O
atomicity	B-General_Concept
,	O
consistency	B-General_Concept
and	O
durability	B-General_Concept
.	O
</s>
<s>
Concurrency	B-Operating_System
control	I-Operating_System
comprises	O
the	O
underlying	O
mechanisms	O
in	O
a	O
DBMS	B-Application
which	O
handle	O
isolation	O
and	O
guarantee	O
related	O
correctness	O
.	O
</s>
<s>
It	O
is	O
heavily	O
used	O
by	O
the	O
database	O
and	O
storage	O
engines	O
both	O
to	O
guarantee	O
the	O
correct	O
execution	O
of	O
concurrent	O
transactions	O
,	O
and	O
(	O
via	O
different	O
mechanisms	O
)	O
the	O
correctness	O
of	O
other	O
DBMS	B-Application
processes	O
.	O
</s>
<s>
The	O
transaction-related	O
mechanisms	O
typically	O
constrain	O
the	O
database	O
data	O
access	O
operations	O
 '	O
timing	O
(	O
transaction	B-General_Concept
schedules	I-General_Concept
)	O
to	O
certain	O
orders	O
characterized	O
as	O
the	O
serializability	B-Operating_System
and	O
recoverability	O
schedule	B-General_Concept
properties	O
.	O
</s>
<s>
Constraining	O
database	O
access	O
operation	O
execution	O
typically	O
means	O
reduced	O
performance	O
(	O
measured	O
by	O
rates	O
of	O
execution	O
)	O
,	O
and	O
thus	O
concurrency	B-Operating_System
control	I-Operating_System
mechanisms	O
are	O
typically	O
designed	O
to	O
provide	O
the	O
best	O
performance	O
possible	O
under	O
the	O
constraints	O
.	O
</s>
<s>
Often	O
,	O
when	O
possible	O
without	O
harming	O
correctness	O
,	O
the	O
serializability	B-Operating_System
property	O
is	O
compromised	O
for	O
better	O
performance	O
.	O
</s>
<s>
Two-phase	B-Operating_System
locking	I-Operating_System
is	O
the	O
most	O
common	O
transaction	B-General_Concept
concurrency	B-Operating_System
control	I-Operating_System
method	O
in	O
DBMSs	O
,	O
used	O
to	O
provide	O
both	O
serializability	B-Operating_System
and	O
recoverability	O
for	O
correctness	O
.	O
</s>
<s>
In	O
order	O
to	O
access	O
a	O
database	O
object	O
a	O
transaction	B-General_Concept
first	O
needs	O
to	O
acquire	O
a	O
lock	B-Operating_System
for	O
this	O
object	O
.	O
</s>
<s>
Depending	O
on	O
the	O
access	O
operation	O
type	O
(	O
e.g.	O
,	O
reading	O
or	O
writing	O
an	O
object	O
)	O
and	O
on	O
the	O
lock	B-Operating_System
type	O
,	O
acquiring	O
the	O
lock	B-Operating_System
may	O
be	O
blocked	O
and	O
postponed	O
,	O
if	O
another	O
transaction	B-General_Concept
is	O
holding	O
a	O
lock	B-Operating_System
for	O
that	O
object	O
.	O
</s>
<s>
The	O
ANSI/ISO	O
standard	O
SQL	B-Language
92	O
refers	O
to	O
three	O
different	O
read	O
phenomena	O
when	O
a	O
transaction	B-General_Concept
retrieves	O
data	O
that	O
another	O
transaction	B-General_Concept
might	O
have	O
updated	O
.	O
</s>
<s>
In	O
transaction	B-General_Concept
1	O
,	O
a	O
query	O
is	O
performed	O
,	O
then	O
in	O
transaction	B-General_Concept
2	O
,	O
an	O
update	O
is	O
performed	O
,	O
and	O
finally	O
in	O
transaction	B-General_Concept
1	O
,	O
the	O
same	O
query	O
is	O
performed	O
again	O
.	O
</s>
<s>
A	O
dirty	O
read	O
(	O
aka	O
uncommitted	O
dependency	O
)	O
occurs	O
when	O
a	O
transaction	B-General_Concept
retrieves	O
a	O
row	O
that	O
has	O
been	O
updated	O
by	O
another	O
transaction	B-General_Concept
that	O
is	O
not	O
yet	O
committed	O
.	O
</s>
<s>
In	O
this	O
example	O
,	O
transaction	B-General_Concept
1	O
retrieves	O
the	O
row	O
with	O
id	O
1	O
,	O
then	O
transaction	B-General_Concept
2	O
updates	O
the	O
row	O
with	O
id	O
1	O
,	O
and	O
finally	O
transaction	B-General_Concept
1	O
retrieves	O
the	O
row	O
with	O
id	O
1	O
again	O
.	O
</s>
<s>
Now	O
if	O
transaction	B-General_Concept
2	O
rolls	O
back	O
its	O
update	O
(	O
already	O
retrieved	O
by	O
transaction	B-General_Concept
1	O
)	O
or	O
performs	O
other	O
updates	O
,	O
then	O
the	O
view	O
of	O
the	O
row	O
may	O
be	O
wrong	O
in	O
transaction	B-General_Concept
1	O
.	O
</s>
<s>
At	O
the	O
READ	O
UNCOMMITTED	O
isolation	B-General_Concept
level	I-General_Concept
,	O
the	O
second	O
SELECT	B-Language
in	O
transaction	B-General_Concept
1	O
retrieves	O
the	O
updated	O
row	O
:	O
this	O
is	O
a	O
dirty	O
read	O
.	O
</s>
<s>
At	O
the	O
READ	O
COMMITTED	O
,	O
REPEATABLE	O
READ	O
,	O
and	O
SERIALIZABLE	B-Operating_System
isolation	B-General_Concept
levels	I-General_Concept
,	O
the	O
second	O
SELECT	B-Language
in	O
transaction	B-General_Concept
1	O
retrieves	O
the	O
initial	O
row	O
.	O
</s>
<s>
Transaction	B-General_Concept
1	O
Transaction	B-General_Concept
2BEGIN	O
;	O
</s>
<s>
SELECT	B-Language
age	O
FROM	O
users	O
WHERE	O
id	O
=	O
1	O
;	O
</s>
<s>
A	O
non-repeatable	O
read	O
occurs	O
when	O
a	O
transaction	B-General_Concept
retrieves	O
a	O
row	O
twice	O
and	O
that	O
row	O
is	O
updated	O
by	O
another	O
transaction	B-General_Concept
that	O
is	O
committed	O
in	O
between	O
.	O
</s>
<s>
In	O
this	O
example	O
,	O
transaction	B-General_Concept
1	O
retrieves	O
the	O
row	O
with	O
id	O
1	O
,	O
then	O
transaction	B-General_Concept
2	O
updates	O
the	O
row	O
with	O
id	O
1	O
and	O
is	O
committed	O
,	O
and	O
finally	O
transaction	B-General_Concept
1	O
retrieves	O
the	O
row	O
with	O
id	O
1	O
again	O
.	O
</s>
<s>
At	O
the	O
READ	O
UNCOMMITTED	O
and	O
READ	O
COMMITTED	O
isolation	B-General_Concept
levels	I-General_Concept
,	O
the	O
second	O
SELECT	B-Language
in	O
transaction	B-General_Concept
1	O
retrieves	O
the	O
updated	O
row	O
:	O
this	O
is	O
a	O
non-repeatable	O
read	O
.	O
</s>
<s>
At	O
the	O
REPEATABLE	O
READ	O
and	O
SERIALIZABLE	B-Operating_System
isolation	B-General_Concept
levels	I-General_Concept
,	O
the	O
second	O
SELECT	B-Language
in	O
transaction	B-General_Concept
1	O
retrieves	O
the	O
initial	O
row	O
.	O
</s>
<s>
Transaction	B-General_Concept
1	O
Transaction	B-General_Concept
2BEGIN	O
;	O
</s>
<s>
SELECT	B-Language
age	O
FROM	O
users	O
WHERE	O
id	O
=	O
1	O
;	O
</s>
<s>
COMMIT	O
;	O
SELECT	B-Language
age	O
FROM	O
users	O
WHERE	O
id	O
=	O
1	O
;	O
</s>
<s>
A	O
phantom	O
read	O
occurs	O
when	O
a	O
transaction	B-General_Concept
retrieves	O
a	O
set	O
of	O
rows	O
twice	O
and	O
new	O
rows	O
are	O
inserted	O
into	O
or	O
removed	O
from	O
that	O
set	O
by	O
another	O
transaction	B-General_Concept
that	O
is	O
committed	O
in	O
between	O
.	O
</s>
<s>
In	O
this	O
example	O
,	O
transaction	B-General_Concept
1	O
retrieves	O
the	O
set	O
of	O
rows	O
with	O
age	O
greater	O
than	O
17	O
,	O
then	O
transaction	B-General_Concept
2	O
inserts	O
a	O
row	O
with	O
age	O
26	O
and	O
is	O
committed	O
,	O
and	O
finally	O
transaction	B-General_Concept
1	O
retrieves	O
the	O
set	O
of	O
rows	O
with	O
age	O
greater	O
than	O
17	O
again	O
.	O
</s>
<s>
At	O
the	O
READ	O
UNCOMMITTED	O
,	O
READ	O
COMMITTED	O
,	O
and	O
REPEATABLE	O
READ	O
isolation	B-General_Concept
levels	I-General_Concept
,	O
the	O
second	O
SELECT	B-Language
in	O
transaction	B-General_Concept
1	O
retrieves	O
the	O
new	O
set	O
of	O
rows	O
that	O
includes	O
the	O
inserted	O
row	O
:	O
this	O
is	O
a	O
phantom	O
read	O
.	O
</s>
<s>
At	O
the	O
SERIALIZABLE	B-Operating_System
isolation	B-General_Concept
level	I-General_Concept
,	O
the	O
second	O
SELECT	B-Language
in	O
transaction	B-General_Concept
1	O
retrieves	O
the	O
initial	O
set	O
of	O
rows	O
.	O
</s>
<s>
Transaction	B-General_Concept
1	O
Transaction	B-General_Concept
2BEGIN	O
;	O
</s>
<s>
SELECT	B-Language
name	O
FROM	O
users	O
WHERE	O
age	O
>	O
17	O
;	O
</s>
<s>
COMMIT	O
;	O
SELECT	B-Language
name	O
FROM	O
users	O
WHERE	O
age	O
>	O
17	O
;	O
</s>
<s>
In	O
the	O
first	O
strategy	O
,	O
lock-based	O
concurrency	B-Operating_System
control	I-Operating_System
,	O
transaction	B-General_Concept
2	O
is	O
committed	O
after	O
transaction	B-General_Concept
1	O
is	O
committed	O
or	O
rolled	O
back	O
.	O
</s>
<s>
It	O
produces	O
the	O
serial	O
schedule	B-General_Concept
T1	O
,	O
T2	O
.	O
</s>
<s>
In	O
the	O
other	O
strategy	O
,	O
multiversion	B-Operating_System
concurrency	I-Operating_System
control	I-Operating_System
,	O
transaction	B-General_Concept
2	O
is	O
committed	O
immediately	O
while	O
transaction	B-General_Concept
1	O
,	O
which	O
started	O
before	O
transaction	B-General_Concept
2	O
,	O
continues	O
to	O
operate	O
on	O
an	O
old	O
snapshot	O
of	O
the	O
database	O
taken	O
at	O
the	O
start	O
of	O
transaction	B-General_Concept
1	O
,	O
and	O
when	O
transaction	B-General_Concept
1	O
eventually	O
tries	O
to	O
commit	O
,	O
if	O
the	O
result	O
of	O
committing	O
would	O
be	O
equivalent	O
to	O
the	O
serial	O
schedule	B-General_Concept
T1	O
,	O
T2	O
,	O
then	O
transaction	B-General_Concept
1	O
is	O
committed	O
;	O
otherwise	O
,	O
there	O
is	O
a	O
commit	O
conflict	O
and	O
transaction	B-General_Concept
1	O
is	O
rolled	O
back	O
with	O
a	O
serialization	O
failure	O
.	O
</s>
<s>
Under	O
lock-based	O
concurrency	B-Operating_System
control	I-Operating_System
,	O
non-repeatable	O
reads	O
and	O
phantom	O
reads	O
may	O
occur	O
when	O
read	B-Operating_System
locks	I-Operating_System
are	O
not	O
acquired	O
when	O
performing	O
a	O
SELECT	B-Language
,	O
or	O
when	O
the	O
acquired	O
locks	B-Operating_System
on	O
affected	O
rows	O
are	O
released	O
as	O
soon	O
as	O
the	O
SELECT	B-Language
is	O
performed	O
.	O
</s>
<s>
Under	O
multiversion	B-Operating_System
concurrency	I-Operating_System
control	I-Operating_System
,	O
non-repeatable	O
reads	O
and	O
phantom	O
reads	O
may	O
occur	O
when	O
the	O
requirement	O
that	O
a	O
transaction	B-General_Concept
affected	O
by	O
a	O
commit	O
conflict	O
must	O
be	O
rolled	O
back	O
is	O
relaxed	O
.	O
</s>
<s>
Of	O
the	O
four	O
ACID	O
properties	O
in	O
a	O
DBMS	B-Application
(	O
Database	B-Application
Management	I-Application
System	I-Application
)	O
,	O
the	O
isolation	O
property	O
is	O
the	O
one	O
most	O
often	O
relaxed	O
.	O
</s>
<s>
When	O
attempting	O
to	O
maintain	O
the	O
highest	O
level	O
of	O
isolation	O
,	O
a	O
DBMS	B-Application
usually	O
acquires	O
locks	B-Operating_System
on	O
data	O
which	O
may	O
result	O
in	O
a	O
loss	O
of	O
concurrency	B-Operating_System
,	O
or	O
implements	O
multiversion	B-Operating_System
concurrency	I-Operating_System
control	I-Operating_System
.	O
</s>
<s>
This	O
requires	O
adding	O
logic	O
for	O
the	O
application	B-Application
to	O
function	O
correctly	O
.	O
</s>
<s>
Most	O
DBMSs	O
offer	O
a	O
number	O
of	O
transaction	B-General_Concept
isolation	I-General_Concept
levels	I-General_Concept
,	O
which	O
control	O
the	O
degree	O
of	O
locking	O
that	O
occurs	O
when	O
selecting	O
data	O
.	O
</s>
<s>
For	O
many	O
database	O
applications	O
,	O
the	O
majority	O
of	O
database	B-General_Concept
transactions	I-General_Concept
can	O
be	O
constructed	O
to	O
avoid	O
requiring	O
high	O
isolation	B-General_Concept
levels	I-General_Concept
(	O
e.g.	O
</s>
<s>
SERIALIZABLE	B-Operating_System
level	O
)	O
,	O
thus	O
reducing	O
the	O
locking	O
overhead	O
for	O
the	O
system	O
.	O
</s>
<s>
Conversely	O
,	O
if	O
higher	O
isolation	B-General_Concept
levels	I-General_Concept
are	O
used	O
,	O
the	O
possibility	O
of	O
deadlock	B-Operating_System
is	O
increased	O
,	O
which	O
also	O
requires	O
careful	O
analysis	O
and	O
programming	O
techniques	O
to	O
avoid	O
.	O
</s>
<s>
Since	O
each	O
isolation	B-General_Concept
level	I-General_Concept
is	O
stronger	O
than	O
those	O
below	O
,	O
in	O
that	O
no	O
higher	O
isolation	B-General_Concept
level	I-General_Concept
allows	O
an	O
action	O
forbidden	O
by	O
a	O
lower	O
one	O
,	O
the	O
standard	O
permits	O
a	O
DBMS	B-Application
to	O
run	O
a	O
transaction	B-General_Concept
at	O
an	O
isolation	B-General_Concept
level	I-General_Concept
stronger	O
than	O
that	O
requested	O
(	O
e.g.	O
,	O
a	O
"	O
Read	O
committed	O
"	O
transaction	B-General_Concept
may	O
actually	O
be	O
performed	O
at	O
a	O
"	O
Repeatable	O
read	O
"	O
isolation	B-General_Concept
level	I-General_Concept
)	O
.	O
</s>
<s>
The	O
isolation	B-General_Concept
levels	I-General_Concept
defined	O
by	O
the	O
ANSI/ISO	O
SQL	B-Language
standard	O
are	O
listed	O
as	O
follows	O
.	O
</s>
<s>
This	O
is	O
the	O
highest	O
isolation	B-General_Concept
level	I-General_Concept
.	O
</s>
<s>
With	O
a	O
lock-based	O
concurrency	B-Operating_System
control	I-Operating_System
DBMS	B-Application
implementation	O
,	O
serializability	B-Operating_System
requires	O
read	O
and	O
write	B-Operating_System
locks	I-Operating_System
(	O
acquired	O
on	O
selected	O
data	O
)	O
to	O
be	O
released	O
at	O
the	O
end	O
of	O
the	O
transaction	B-General_Concept
.	O
</s>
<s>
Also	O
range-locks	O
must	O
be	O
acquired	O
when	O
a	O
SELECT	B-Language
query	O
uses	O
a	O
ranged	O
WHERE	O
clause	O
,	O
especially	O
to	O
avoid	O
the	O
phantom	O
reads	O
phenomenon	O
.	O
</s>
<s>
When	O
using	O
non-lock	O
based	O
concurrency	B-Operating_System
control	I-Operating_System
,	O
no	O
locks	B-Operating_System
are	O
acquired	O
;	O
however	O
,	O
if	O
the	O
system	O
detects	O
a	O
write	O
collision	O
among	O
several	O
concurrent	O
transactions	O
,	O
only	O
one	O
of	O
them	O
is	O
allowed	O
to	O
commit	O
.	O
</s>
<s>
See	O
snapshot	B-Operating_System
isolation	I-Operating_System
for	O
more	O
details	O
on	O
this	O
topic	O
.	O
</s>
<s>
From	O
:	O
(	O
Second	O
Informal	O
Review	O
Draft	O
)	O
ISO/IEC	O
9075:1992	O
,	O
Database	O
Language	O
SQL	B-Language
-	O
July	O
30	O
,	O
1992	O
:	O
</s>
<s>
The	O
execution	O
of	O
concurrent	O
SQL-transactions	O
at	O
isolation	B-General_Concept
level	I-General_Concept
SERIALIZABLE	B-Operating_System
is	O
guaranteed	O
to	O
be	O
serializable	B-Operating_System
.	O
</s>
<s>
A	O
serializable	B-Operating_System
execution	O
is	O
defined	O
to	O
be	O
an	O
execution	O
of	O
the	O
operations	O
of	O
concurrently	O
executing	O
SQL-transactions	O
that	O
produces	O
the	O
same	O
effect	O
as	O
some	O
serial	O
execution	O
of	O
those	O
same	O
SQL-transactions	O
.	O
</s>
<s>
A	O
serial	O
execution	O
is	O
one	O
in	O
which	O
each	O
SQL-transaction	O
executes	O
to	O
completion	O
before	O
the	O
next	O
SQL-transaction	O
begins	O
.	O
</s>
<s>
In	O
this	O
isolation	B-General_Concept
level	I-General_Concept
,	O
a	O
lock-based	O
concurrency	B-Operating_System
control	I-Operating_System
DBMS	B-Application
implementation	O
keeps	O
read	O
and	O
write	B-Operating_System
locks	I-Operating_System
(	O
acquired	O
on	O
selected	O
data	O
)	O
until	O
the	O
end	O
of	O
the	O
transaction	B-General_Concept
.	O
</s>
<s>
However	O
,	O
range-locks	O
are	O
not	O
managed	O
,	O
so	O
phantom	O
reads	O
can	O
occur	O
.	O
</s>
<s>
Write	B-Operating_System
skew	I-Operating_System
is	O
possible	O
at	O
this	O
isolation	B-General_Concept
level	I-General_Concept
in	O
some	O
systems	O
.	O
</s>
<s>
Write	B-Operating_System
skew	I-Operating_System
is	O
a	O
phenomenon	O
where	O
two	O
writes	O
are	O
allowed	O
to	O
the	O
same	O
column(s )	O
in	O
a	O
table	O
by	O
two	O
different	O
writers	O
(	O
who	O
have	O
previously	O
read	O
the	O
columns	O
they	O
are	O
updating	O
)	O
,	O
resulting	O
in	O
the	O
column	O
having	O
data	O
that	O
is	O
a	O
mix	O
of	O
the	O
two	O
transactions	O
.	O
</s>
<s>
In	O
this	O
isolation	B-General_Concept
level	I-General_Concept
,	O
a	O
lock-based	O
concurrency	B-Operating_System
control	I-Operating_System
DBMS	B-Application
implementation	O
keeps	O
write	B-Operating_System
locks	I-Operating_System
(	O
acquired	O
on	O
selected	O
data	O
)	O
until	O
the	O
end	O
of	O
the	O
transaction	B-General_Concept
,	O
but	O
read	B-Operating_System
locks	I-Operating_System
are	O
released	O
as	O
soon	O
as	O
the	O
SELECT	B-Language
operation	O
is	O
performed	O
(	O
so	O
the	O
non-repeatable	O
reads	O
phenomenon	O
can	O
occur	O
in	O
this	O
isolation	B-General_Concept
level	I-General_Concept
)	O
.	O
</s>
<s>
As	O
in	O
the	O
previous	O
level	O
,	O
range-locks	O
are	O
not	O
managed	O
.	O
</s>
<s>
Putting	O
it	O
in	O
simpler	O
words	O
,	O
read	O
committed	O
is	O
an	O
isolation	B-General_Concept
level	I-General_Concept
that	O
guarantees	O
that	O
any	O
data	O
read	O
is	O
committed	O
at	O
the	O
moment	O
it	O
is	O
read	O
.	O
</s>
<s>
It	O
makes	O
no	O
promise	O
whatsoever	O
that	O
if	O
the	O
transaction	B-General_Concept
re-issues	O
the	O
read	O
,	O
it	O
will	O
find	O
the	O
same	O
data	O
;	O
data	O
is	O
free	O
to	O
change	O
after	O
it	O
is	O
read	O
.	O
</s>
<s>
This	O
is	O
the	O
lowest	O
isolation	B-General_Concept
level	I-General_Concept
.	O
</s>
<s>
In	O
this	O
level	O
,	O
dirty	O
reads	O
are	O
allowed	O
,	O
so	O
one	O
transaction	B-General_Concept
may	O
see	O
not-yet-committed	O
changes	O
made	O
by	O
other	O
transactions	O
.	O
</s>
<s>
The	O
default	O
isolation	B-General_Concept
level	I-General_Concept
of	O
different	O
DBMS	B-Application
's	O
varies	O
quite	O
widely	O
.	O
</s>
<s>
Most	O
databases	B-Application
that	O
feature	O
transactions	O
allow	O
the	O
user	O
to	O
set	O
any	O
isolation	B-General_Concept
level	I-General_Concept
.	O
</s>
<s>
Some	O
DBMS	B-Application
's	O
also	O
require	O
additional	O
syntax	O
when	O
performing	O
a	O
SELECT	B-Language
statement	O
to	O
acquire	O
locks	B-Operating_System
(	O
e.g.	O
</s>
<s>
SELECT	B-Language
…	O
FOR	O
UPDATE	O
to	O
acquire	O
exclusive	O
write	B-Operating_System
locks	I-Operating_System
on	O
accessed	O
rows	O
)	O
.	O
</s>
<s>
However	O
,	O
the	O
definitions	O
above	O
have	O
been	O
criticized	O
as	O
being	O
ambiguous	O
,	O
and	O
as	O
not	O
accurately	O
reflecting	O
the	O
isolation	O
provided	O
by	O
many	O
databases	B-Application
:	O
</s>
<s>
This	O
paper	O
shows	O
a	O
number	O
of	O
weaknesses	O
in	O
the	O
anomaly	O
approach	O
to	O
defining	O
isolation	B-General_Concept
levels	I-General_Concept
.	O
</s>
<s>
In	O
particular	O
,	O
lock-based	O
isolation	B-General_Concept
levels	I-General_Concept
have	O
different	O
characteristics	O
than	O
their	O
ANSI	O
equivalents	O
.	O
</s>
<s>
This	O
is	O
disconcerting	O
because	O
commercial	O
database	B-Application
systems	I-Application
typically	O
use	O
locking	O
implementations	O
.	O
</s>
<s>
Additionally	O
,	O
the	O
ANSI	O
phenomena	O
do	O
not	O
distinguish	O
between	O
a	O
number	O
of	O
types	O
of	O
isolation	B-General_Concept
level	I-General_Concept
behavior	O
that	O
are	O
popular	O
in	O
commercial	O
systems	O
.	O
</s>
<s>
There	O
are	O
also	O
other	O
criticisms	O
concerning	O
ANSI	O
SQL	B-Language
's	O
isolation	O
definition	O
,	O
in	O
that	O
it	O
encourages	O
implementors	O
to	O
do	O
"	O
bad	O
things	O
"	O
:	O
</s>
<s>
...	O
it	O
relies	O
in	O
subtle	O
ways	O
on	O
an	O
assumption	O
that	O
a	O
locking	O
schema	O
is	O
used	O
for	O
concurrency	B-Operating_System
control	I-Operating_System
,	O
as	O
opposed	O
to	O
an	O
optimistic	O
or	O
multi-version	O
concurrency	B-Operating_System
scheme	O
.	O
</s>
<s>
Anomaly	O
serializable	B-Operating_System
is	O
not	O
the	O
same	O
as	O
serializable	B-Operating_System
.	O
</s>
<s>
That	O
is	O
,	O
it	O
is	O
necessary	O
,	O
but	O
not	O
sufficient	O
that	O
a	O
serializable	B-Operating_System
schedule	B-General_Concept
should	O
be	O
free	O
of	O
all	O
three	O
phenomena	O
types	O
.	O
</s>
