<s>
The	O
recursive	B-Application
join	I-Application
is	O
an	O
operation	O
used	O
in	O
relational	B-Application
databases	I-Application
,	O
also	O
sometimes	O
called	O
a	O
"	O
fixed-point	O
join	B-Language
"	O
.	O
</s>
<s>
It	O
is	O
a	O
compound	O
operation	O
that	O
involves	O
repeating	O
the	O
join	B-Language
operation	O
,	O
typically	O
accumulating	O
more	O
records	O
each	O
time	O
,	O
until	O
a	O
repetition	O
makes	O
no	O
change	O
to	O
the	O
results	O
(	O
as	O
compared	O
to	O
the	O
results	O
of	O
the	O
previous	O
iteration	O
)	O
.	O
</s>
<s>
For	O
example	O
,	O
if	O
a	O
database	O
of	O
family	O
relationships	O
is	O
to	O
be	O
searched	O
,	O
and	O
the	O
record	O
for	O
each	O
person	O
has	O
"	O
mother	O
"	O
and	O
"	O
father	O
"	O
fields	O
,	O
a	O
recursive	B-Application
join	I-Application
would	O
be	O
one	O
way	O
to	O
retrieve	O
all	O
of	O
a	O
person	O
's	O
known	O
ancestors	O
:	O
first	O
the	O
person	O
's	O
direct	O
parents	O
 '	O
records	O
would	O
be	O
retrieved	O
,	O
then	O
the	O
parents	O
 '	O
information	O
would	O
be	O
used	O
to	O
retrieve	O
the	O
grandparents	O
 '	O
records	O
,	O
and	O
so	O
on	O
until	O
no	O
new	O
records	O
are	O
being	O
found	O
.	O
</s>
<s>
In	O
this	O
example	O
,	O
as	O
in	O
many	O
real	O
cases	O
,	O
the	O
repetition	O
involves	O
only	O
a	O
single	O
database	O
table	O
,	O
and	O
so	O
is	O
more	O
specifically	O
a	O
"	O
recursive	O
self-join	O
"	O
.	O
</s>
<s>
Recursive	B-Application
joins	I-Application
can	O
be	O
very	O
time-consuming	O
unless	O
optimized	O
through	O
indexing	O
,	O
the	O
addition	O
of	O
extra	O
key	O
fields	O
,	O
or	O
other	O
techniques	O
.	O
</s>
<s>
Graph	B-Algorithm
traversals	I-Algorithm
come	O
at	O
a	O
lower	O
cost	O
than	O
the	O
method	O
of	O
recursive	B-Application
joins	I-Application
.	O
</s>
<s>
Recursive	B-Application
joins	I-Application
are	O
highly	O
characteristic	O
of	O
hierarchical	O
data	O
,	O
and	O
therefore	O
become	O
a	O
serious	O
issue	O
with	O
XML	B-Protocol
data	O
.	O
</s>
<s>
In	O
XML	B-Protocol
,	O
operations	O
such	O
as	O
determining	O
whether	O
one	O
element	O
contains	O
another	O
are	O
extremely	O
common	O
,	O
and	O
the	O
recursive	B-Application
join	I-Application
is	O
perhaps	O
the	O
most	O
obvious	O
way	O
to	O
implement	O
them	O
when	O
the	O
XML	B-Protocol
data	O
is	O
stored	O
in	O
a	O
relational	B-Application
database	I-Application
.	O
</s>
<s>
The	O
standard	O
way	O
to	O
define	O
recursive	B-Application
joins	I-Application
in	O
the	O
SQL:1999	B-Language
standard	O
is	O
by	O
way	O
of	O
recursive	O
common	O
table	O
expressions	O
.	O
</s>
<s>
Database	O
management	O
systems	O
that	O
support	O
recursive	O
CTEs	O
include	O
Microsoft	B-Application
SQL	I-Application
Server	I-Application
,	O
Oracle	B-General_Concept
,	O
PostgreSQL	B-Application
and	O
others	O
.	O
</s>
