<s>
A	O
database	B-General_Concept
engine	I-General_Concept
(	O
or	O
storage	B-General_Concept
engine	I-General_Concept
)	O
is	O
the	O
underlying	O
software	O
component	O
that	O
a	O
database	B-Application
management	I-Application
system	I-Application
(	O
DBMS	B-General_Concept
)	O
uses	O
to	O
create	B-General_Concept
,	I-General_Concept
read	I-General_Concept
,	I-General_Concept
update	I-General_Concept
and	I-General_Concept
delete	I-General_Concept
(	O
CRUD	B-General_Concept
)	O
data	O
from	O
a	O
database	O
.	O
</s>
<s>
Most	O
database	B-Application
management	I-Application
systems	I-Application
include	O
their	O
own	O
application	B-Application
programming	I-Application
interface	I-Application
(	O
API	B-Application
)	O
that	O
allows	O
the	O
user	O
to	O
interact	O
with	O
their	O
underlying	O
engine	O
without	O
going	O
through	O
the	O
user	O
interface	O
of	O
the	O
DBMS	B-General_Concept
.	O
</s>
<s>
The	O
term	O
"	O
database	B-General_Concept
engine	I-General_Concept
"	O
is	O
frequently	O
used	O
interchangeably	O
with	O
"	O
database	B-Application
server	I-Application
"	O
or	O
"	O
database	B-Application
management	I-Application
system	I-Application
"	O
.	O
</s>
<s>
A	O
"	O
database	B-Application
instance	I-Application
"	O
refers	O
to	O
the	O
processes	O
and	O
memory	B-Language
structures	O
of	O
the	O
running	O
database	B-General_Concept
engine	I-General_Concept
.	O
</s>
<s>
Many	O
of	O
the	O
modern	O
DBMS	B-General_Concept
support	O
multiple	O
storage	B-General_Concept
engines	I-General_Concept
within	O
the	O
same	O
database	O
.	O
</s>
<s>
For	O
example	O
,	O
MySQL	B-Application
supports	O
InnoDB	B-Language
as	O
well	O
as	O
MyISAM	B-Language
.	O
</s>
<s>
Some	O
storage	B-General_Concept
engines	I-General_Concept
are	O
transactional	B-General_Concept
.	O
</s>
<s>
Information	O
in	O
a	O
database	O
is	O
stored	O
as	O
bits	O
laid	O
out	O
as	O
data	B-General_Concept
structures	I-General_Concept
in	O
storage	O
that	O
can	O
be	O
efficiently	O
read	O
from	O
and	O
written	O
to	O
given	O
the	O
properties	O
of	O
hardware	O
.	O
</s>
<s>
Typically	O
the	O
storage	O
itself	O
is	O
designed	O
to	O
meet	O
requirements	O
of	O
various	O
areas	O
that	O
extensively	O
utilize	O
storage	O
,	O
including	O
databases	B-Application
.	O
</s>
<s>
A	O
DBMS	B-General_Concept
in	O
operation	O
always	O
simultaneously	O
utilizes	O
several	O
storage	O
types	O
(	O
e.g.	O
,	O
memory	B-Language
,	O
and	O
external	O
storage	O
)	O
,	O
with	O
respective	O
layout	O
methods	O
.	O
</s>
<s>
In	O
principle	O
the	O
database	O
storage	O
can	O
be	O
viewed	O
as	O
a	O
linear	B-General_Concept
address	I-General_Concept
space	I-General_Concept
,	O
where	O
every	O
bit	O
of	O
data	O
has	O
its	O
unique	O
address	O
in	O
this	O
address	O
space	O
.	O
</s>
<s>
In	O
practice	O
,	O
only	O
a	O
very	O
small	O
percentage	O
of	O
addresses	O
are	O
kept	O
as	O
initial	O
reference	O
points	O
(	O
which	O
also	O
requires	O
storage	O
)	O
;	O
most	O
data	O
is	O
accessed	O
by	O
indirection	O
using	O
displacement	O
calculations	O
(	O
distance	O
in	O
bits	O
from	O
the	O
reference	O
points	O
)	O
and	O
data	B-General_Concept
structures	I-General_Concept
which	O
define	O
access	O
paths	O
(	O
using	O
pointers	O
)	O
to	O
all	O
needed	O
data	O
in	O
an	O
effective	O
manner	O
,	O
optimized	O
for	O
the	O
needed	O
data	O
access	O
operations	O
.	O
</s>
<s>
A	O
database	O
,	O
while	O
in	O
operation	O
,	O
resides	O
simultaneously	O
in	O
several	O
types	O
of	O
storage	O
,	O
forming	O
a	O
storage	B-General_Concept
hierarchy	I-General_Concept
.	O
</s>
<s>
By	O
the	O
nature	O
of	O
contemporary	O
computers	O
most	O
of	O
the	O
database	O
part	O
inside	O
a	O
computer	O
that	O
hosts	O
the	O
DBMS	B-General_Concept
resides	O
(	O
partially	O
replicated	O
)	O
in	O
volatile	O
storage	O
.	O
</s>
<s>
Data	O
(	O
pieces	O
of	O
the	O
database	O
)	O
that	O
are	O
being	O
processed/manipulated	O
reside	O
inside	O
a	O
processor	O
,	O
possibly	O
in	O
processor	B-General_Concept
's	I-General_Concept
caches	I-General_Concept
.	O
</s>
<s>
These	O
data	O
are	O
being	O
read	O
from/written	O
to	O
memory	B-Language
,	O
typically	O
through	O
a	O
computer	B-General_Concept
bus	I-General_Concept
(	O
so	O
far	O
typically	O
volatile	O
storage	O
components	O
)	O
.	O
</s>
<s>
Computer	O
memory	B-Language
is	O
communicating	O
data	O
(	O
transferred	O
to/from	O
)	O
external	O
storage	O
,	O
typically	O
through	O
standard	O
storage	O
interfaces	O
or	O
networks	O
(	O
e.g.	O
,	O
fibre	B-Architecture
channel	I-Architecture
,	O
iSCSI	B-Protocol
)	O
.	O
</s>
<s>
A	O
storage	B-General_Concept
array	I-General_Concept
,	O
a	O
common	O
external	O
storage	O
unit	O
,	O
typically	O
has	O
storage	B-General_Concept
hierarchy	I-General_Concept
of	O
its	O
own	O
,	O
from	O
a	O
fast	O
cache	O
,	O
typically	O
consisting	O
of	O
(	O
volatile	O
and	O
fast	O
)	O
DRAM	O
,	O
which	O
is	O
connected	O
(	O
again	O
via	O
standard	O
interfaces	O
)	O
to	O
drives	O
,	O
possibly	O
with	O
different	O
speeds	O
,	O
like	O
flash	O
drives	O
and	O
magnetic	O
disk	B-Device
drives	I-Device
(	O
non-volatile	O
)	O
.	O
</s>
<s>
A	O
data	B-General_Concept
structure	I-General_Concept
is	O
an	O
abstract	O
construct	O
that	O
embeds	O
data	O
in	O
a	O
well	O
defined	O
manner	O
.	O
</s>
<s>
An	O
efficient	O
data	B-General_Concept
structure	I-General_Concept
allows	O
manipulation	O
of	O
the	O
data	O
in	O
efficient	O
ways	O
.	O
</s>
<s>
A	O
certain	O
data	B-General_Concept
structure	I-General_Concept
type	O
may	O
be	O
very	O
effective	O
in	O
certain	O
operations	O
,	O
and	O
very	O
ineffective	O
in	O
others	O
.	O
</s>
<s>
A	O
data	B-General_Concept
structure	I-General_Concept
type	O
is	O
selected	O
upon	O
DBMS	B-General_Concept
development	O
to	O
best	O
meet	O
the	O
operations	O
needed	O
for	O
the	O
types	O
of	O
data	O
it	O
contains	O
.	O
</s>
<s>
Type	O
of	O
data	B-General_Concept
structure	I-General_Concept
selected	O
for	O
a	O
certain	O
task	O
typically	O
also	O
takes	O
into	O
consideration	O
the	O
type	O
of	O
storage	O
it	O
resides	O
in	O
(	O
e.g.	O
,	O
speed	O
of	O
access	O
,	O
minimal	O
size	O
of	O
storage	O
chunk	O
accessed	O
,	O
etc	O
.	O
)	O
.	O
</s>
<s>
In	O
some	O
DBMSs	O
database	O
administrators	O
have	O
the	O
flexibility	O
to	O
select	O
among	O
options	O
of	O
data	B-General_Concept
structures	I-General_Concept
to	O
contain	O
user	O
data	O
for	O
performance	O
reasons	O
.	O
</s>
<s>
Sometimes	O
the	O
data	B-General_Concept
structures	I-General_Concept
have	O
selectable	O
parameters	O
to	O
tune	O
the	O
database	O
performance	O
.	O
</s>
<s>
Databases	B-Application
may	O
store	O
data	O
in	O
many	O
data	B-General_Concept
structure	I-General_Concept
types	O
.	O
</s>
<s>
In	O
contrast	O
to	O
conventional	O
row-orientation	O
,	O
relational	O
databases	B-Application
can	O
also	O
be	O
column-oriented	B-General_Concept
or	O
correlational	O
in	O
the	O
way	O
they	O
store	O
data	O
in	O
any	O
particular	O
structure	O
.	O
</s>
<s>
Even	O
for	O
in-memory	B-General_Concept
databases	I-General_Concept
clustering	O
provides	O
performance	O
advantage	O
due	O
to	O
common	O
utilization	O
of	O
large	O
caches	O
for	O
input-output	O
operations	O
in	O
memory	B-Language
,	O
with	O
similar	O
resulting	O
behavior	O
.	O
</s>
<s>
Indexing	O
is	O
a	O
technique	O
some	O
storage	B-General_Concept
engines	I-General_Concept
use	O
for	O
improving	O
database	O
performance	O
.	O
</s>
<s>
In	O
large	O
databases	B-Application
,	O
this	O
can	O
reduce	O
query	O
time/cost	O
by	O
orders	O
of	O
magnitude	O
.	O
</s>
