<s>
MyISAM	B-Language
was	O
the	O
default	O
storage	B-General_Concept
engine	I-General_Concept
for	O
the	O
MySQL	B-Application
relational	B-Application
database	I-Application
management	I-Application
system	I-Application
versions	O
prior	O
to	O
5.5	O
released	O
in	O
December	O
2009	O
.	O
</s>
<s>
It	O
is	O
based	O
on	O
the	O
older	O
ISAM	B-Application
code	O
,	O
but	O
it	O
has	O
many	O
useful	O
extensions	O
.	O
</s>
<s>
Each	O
MyISAM	B-Language
table	O
is	O
stored	O
on	O
disk	O
in	O
three	O
files	O
(	O
if	O
it	O
is	O
not	O
partitioned	O
)	O
.	O
</s>
<s>
MySQL	B-Application
uses	O
a	O
.frm	B-Language
file	O
to	O
store	O
the	O
definition	O
of	O
the	O
table	O
,	O
but	O
this	O
file	O
is	O
not	O
a	O
part	O
of	O
the	O
MyISAM	B-Language
engine	I-Language
;	O
instead	O
it	O
is	O
a	O
part	O
of	O
the	O
server	O
.	O
</s>
<s>
The	O
data	O
file	O
has	O
a	O
.MYD	B-Language
(	O
MYData	O
)	O
extension	O
.	O
</s>
<s>
The	O
index	O
file	O
has	O
a	O
.MYI	B-Language
(	O
MYIndex	O
)	O
extension	O
.	O
</s>
<s>
COMPRESSED	O
:	O
Compressed	O
tables	O
can	O
be	O
created	O
with	O
a	O
dedicated	O
tool	O
while	O
MySQL	B-Application
is	O
not	O
running	O
,	O
and	O
they	O
are	O
read-only	O
.	O
</s>
<s>
MyISAM	B-Language
files	O
do	O
not	O
depend	O
on	O
the	O
system	O
and	O
,	O
since	O
MyISAM	B-Language
is	O
not	O
transactional	O
,	O
their	O
content	O
does	O
not	O
depend	O
on	O
current	O
server	O
workload	O
.	O
</s>
<s>
MyISAM	B-Language
is	O
optimized	O
for	O
environments	O
with	O
heavy	O
read	O
operations	O
,	O
and	O
few	O
writes	O
,	O
or	O
none	O
at	O
all	O
.	O
</s>
<s>
A	O
typical	O
area	O
in	O
which	O
one	O
could	O
prefer	O
MyISAM	B-Language
is	O
data	O
warehouse	O
because	O
it	O
involves	O
queries	O
on	O
very	O
big	O
tables	O
,	O
and	O
the	O
update	O
of	O
such	O
tables	O
is	O
done	O
when	O
the	O
database	O
is	O
not	O
in	O
use	O
(	O
usually	O
at	O
night	O
)	O
.	O
</s>
<s>
The	O
reason	O
MyISAM	B-Language
allows	O
for	O
fast	O
reads	O
is	O
the	O
structure	O
of	O
its	O
indexes	O
:	O
each	O
entry	O
points	O
to	O
a	O
record	O
in	O
the	O
data	O
file	O
,	O
and	O
the	O
pointer	O
is	O
offset	O
from	O
the	O
beginning	O
of	O
the	O
file	O
.	O
</s>
<s>
Because	O
of	O
this	O
simple	O
mechanism	O
,	O
MyISAM	B-Language
index	O
statistics	O
are	O
usually	O
quite	O
accurate	O
.	O
</s>
<s>
However	O
,	O
the	O
simplicity	O
of	O
MyISAM	B-Language
has	O
several	O
drawbacks	O
.	O
</s>
<s>
The	O
major	O
deficiency	O
of	O
MyISAM	B-Language
is	O
the	O
absence	O
of	O
transactions	O
support	O
.	O
</s>
<s>
Also	O
,	O
foreign	B-Application
keys	I-Application
are	O
not	O
supported	O
.	O
</s>
<s>
In	O
normal	O
use	O
cases	O
,	O
InnoDB	B-Language
seems	O
to	O
be	O
faster	O
than	O
MyISAM	B-Language
.	O
</s>
<s>
Versions	O
of	O
MySQL	O
5.5	O
and	O
greater	O
have	O
switched	O
to	O
the	O
InnoDB	B-Language
engine	O
to	O
ensure	O
referential	B-General_Concept
integrity	I-General_Concept
constraints	O
,	O
and	O
higher	O
concurrency	B-Operating_System
.	O
</s>
<s>
MyISAM	B-Language
supports	O
FULLTEXT	O
indexing	O
and	O
OpenGIS	O
data	O
types	O
.	O
</s>
<s>
MariaDB	B-Application
has	O
a	O
storage	B-General_Concept
engine	I-General_Concept
called	O
Aria	B-Application
,	O
which	O
is	O
described	O
as	O
a	O
"	O
crash-safe	O
alternative	O
to	O
MyISAM	B-Language
"	O
.	O
</s>
<s>
However	O
,	O
the	O
MariaDB	B-Application
developers	O
still	O
work	O
on	O
MyISAM	B-Language
code	O
.	O
</s>
<s>
If	O
it	O
is	O
enabled	O
,	O
MyISAM	B-Language
indices	O
 '	O
cache	O
is	O
divided	O
into	O
segments	O
.	O
</s>
<s>
This	O
improves	O
the	O
concurrency	B-Operating_System
because	O
threads	O
rarely	O
need	O
to	O
lock	O
the	O
entire	O
cache	O
.	O
</s>
<s>
In	O
MariaDB	B-Application
,	O
MyISAM	B-Language
also	O
supports	O
virtual	B-General_Concept
columns	I-General_Concept
.	O
</s>
<s>
Drizzle	B-Application
does	O
not	O
include	O
MyISAM	B-Language
.	O
</s>
