<s>
Tuple-versioning	B-Application
(	O
also	O
called	O
point-in-time	B-Application
)	O
is	O
a	O
mechanism	O
used	O
in	O
a	O
relational	B-Application
database	I-Application
management	I-Application
system	I-Application
to	O
store	O
past	O
states	O
of	O
a	O
relation	B-Language
.	O
</s>
<s>
Using	O
tuple-versioning	B-Application
techniques	O
,	O
typically	O
two	O
values	O
for	O
time	O
are	O
stored	O
along	O
with	O
each	O
tuple	B-Application
:	O
a	O
start	O
time	O
and	O
an	O
end	O
time	O
.	O
</s>
<s>
These	O
two	O
values	O
indicate	O
the	O
validity	O
of	O
the	O
rest	O
of	O
the	O
values	O
in	O
the	O
tuple	B-Application
.	O
</s>
<s>
Typically	O
when	O
tuple-versioning	B-Application
techniques	O
are	O
used	O
,	O
the	O
current	O
tuple	B-Application
has	O
a	O
valid	O
start	O
time	O
,	O
but	O
a	O
null	O
value	O
for	O
end	O
time	O
.	O
</s>
<s>
Therefore	O
,	O
it	O
is	O
easy	O
and	O
efficient	O
to	O
obtain	O
the	O
current	O
values	O
for	O
all	O
tuples	B-Application
by	O
querying	O
for	O
the	O
null	O
end	O
time	O
.	O
</s>
<s>
A	O
single	O
query	B-Application
that	O
searches	O
for	O
tuples	B-Application
with	O
start	O
time	O
less	O
than	O
,	O
and	O
end	O
time	O
greater	O
than	O
,	O
a	O
given	O
time	O
(	O
where	O
null	O
end	O
time	O
is	O
treated	O
as	O
a	O
value	O
greater	O
than	O
the	O
given	O
time	O
)	O
will	O
give	O
as	O
a	O
result	O
the	O
valid	O
tuples	B-Application
at	O
the	O
given	O
time	O
.	O
</s>
<s>
For	O
example	O
,	O
if	O
a	O
person	O
's	O
job	O
changes	O
from	O
Engineer	O
to	O
Manager	O
,	O
there	O
would	O
be	O
two	O
tuples	B-Application
in	O
an	O
Employee	O
table	B-Application
,	O
one	O
with	O
the	O
value	O
Engineer	O
for	O
job	O
and	O
the	O
other	O
with	O
the	O
value	O
Manager	O
for	O
job	O
.	O
</s>
<s>
The	O
end	O
time	O
for	O
the	O
Engineer	O
tuple	B-Application
would	O
be	O
equal	O
to	O
the	O
start	O
time	O
for	O
the	O
Manager	O
tuple	B-Application
.	O
</s>
<s>
The	O
pattern	O
known	O
as	O
log	B-Application
trigger	I-Application
uses	O
this	O
technique	O
to	O
automatically	O
store	O
historical	O
information	O
of	O
a	O
table	B-Application
in	O
a	O
database	B-Application
.	O
</s>
