<s>
Operational	B-General_Concept
historian	I-General_Concept
refers	O
to	O
a	O
complementary	O
set	O
of	O
time-series	O
database	O
applications	O
that	O
are	O
developed	O
for	O
operational	O
process	O
data	O
.	O
</s>
<s>
Historian	O
software	O
is	O
often	O
embedded	O
or	O
used	O
in	O
conjunction	O
with	O
standard	O
DCS	O
and	O
PLC	B-Architecture
control	O
systems	O
to	O
provide	O
enhanced	O
data	O
capture	O
,	O
validation	O
,	O
compression	O
,	O
and	O
aggregation	O
capabilities	O
.	O
</s>
<s>
Raw	O
data	O
may	O
be	O
accessed	O
via	O
OPC	O
HDA	O
,	O
SQL	B-Language
,	O
or	O
REST	B-Protocol
API	I-Protocol
interfaces	O
.	O
</s>
<s>
Operational	B-General_Concept
historians	I-General_Concept
are	O
typically	O
used	O
within	O
the	O
manufacturing	O
facility	O
by	O
engineers	O
and	O
operators	O
for	O
supervisory	O
functions	O
and	O
analysis	O
.	O
</s>
<s>
An	O
operational	B-General_Concept
historian	I-General_Concept
will	O
typically	O
capture	O
all	O
instrumentation	O
and	O
control	O
data	O
,	O
whereas	O
an	O
enterprise	B-General_Concept
historians	I-General_Concept
that	O
is	O
deployed	O
to	O
support	O
business	O
functions	O
will	O
take	O
a	O
subset	O
of	O
the	O
plant	O
data	O
.	O
</s>
<s>
Usual	O
challenges	O
the	O
operational	B-General_Concept
historians	I-General_Concept
must	O
address	O
are	O
as	O
follows	O
:	O
</s>
<s>
As	O
opposed	O
to	O
enterprise	B-General_Concept
historians	I-General_Concept
,	O
the	O
data	O
access	O
layer	O
in	O
the	O
operational	B-General_Concept
historian	I-General_Concept
is	O
designed	O
to	O
offer	O
sophisticated	O
data	O
fetching	O
modes	O
without	O
complex	O
information	O
analysis	O
facilities	O
.	O
</s>
<s>
Even	O
though	O
the	O
operational	B-General_Concept
historians	I-General_Concept
are	O
rarely	O
relational	B-Application
database	I-Application
management	I-Application
systems	I-Application
,	O
they	O
often	O
offer	O
SQL-based	O
interfaces	O
to	O
query	O
the	O
database	O
.	O
</s>
<s>
In	O
most	O
of	O
such	O
implementations	O
,	O
the	O
dialect	O
does	O
not	O
follow	O
the	O
SQL	B-Language
standard	O
in	O
order	O
to	O
provide	O
syntax	O
for	O
specifying	O
data	O
access	O
operations	O
parameters	O
.	O
</s>
