<s>
In	O
computing	O
,	O
the	O
Halloween	B-General_Concept
Problem	I-General_Concept
refers	O
to	O
a	O
phenomenon	O
in	O
databases	O
in	O
which	O
an	O
update	B-Language
operation	O
causes	O
a	O
change	O
in	O
the	O
physical	O
location	O
of	O
a	O
row	O
,	O
potentially	O
allowing	O
the	O
row	O
to	O
be	O
visited	O
again	O
later	O
in	O
the	O
same	O
update	B-Language
operation	O
.	O
</s>
<s>
This	O
could	O
even	O
cause	O
an	O
infinite	O
loop	O
in	O
some	O
cases	O
where	O
updates	O
continually	O
place	O
the	O
updated	O
record	O
ahead	O
of	O
the	O
scan	O
performing	O
the	O
update	B-Language
operation	O
.	O
</s>
<s>
The	O
potential	O
for	O
this	O
database	O
error	O
was	O
first	O
discovered	O
by	O
Don	O
Chamberlin	O
,	O
Pat	O
Selinger	O
,	O
and	O
Morton	O
Astrahan	O
in	O
1976	O
,	O
on	O
Halloween	B-Application
day	O
while	O
working	O
on	O
a	O
query	O
that	O
was	O
supposed	O
to	O
give	O
a	O
ten	O
percent	O
raise	O
to	O
every	O
employee	O
who	O
earned	O
less	O
than	O
$	O
25,000	O
.	O
</s>
<s>
Pat	O
and	O
Morton	O
discovered	O
this	O
problem	O
on	O
Halloween	B-Application
...	O
</s>
<s>
We	O
have	O
to	O
make	O
sure	O
that	O
when	O
the	O
optimizer	O
is	O
making	O
a	O
plan	O
for	O
processing	O
an	O
update	B-Language
,	O
it	O
does	O
n't	O
use	O
an	O
index	O
that	O
is	O
based	O
on	O
the	O
field	O
that	O
is	O
being	O
updated	O
.	O
</s>
<s>
We	O
'll	O
call	O
it	O
the	O
Halloween	B-General_Concept
Problem	I-General_Concept
and	O
we	O
'll	O
work	O
on	O
it	O
next	O
week.	O
"	O
</s>
