Database Reference
In-Depth Information
Storage Op-
tion
Explanation
Advantages
Drawbacks
Hybrid
OLAP
(HOLAP
)
This storage op-
tion keeps data
in the relational
database but
creates aggreg-
ations in the
MOLAP format.
If the data re-
quested by a
query is not
found by scan-
ning aggrega-
tions, the rela-
tional database
is queried.
It does not cre-
ate a copy of re-
lational data. If
the requested
data is found in
aggregations,
the response
time is as fast as
it is with MOLAP.
HOLAP is very seldom used be-
cause it requires much tuning to
offer the benefits of either
ROLAP or MOLAP. Aggrega-
tions are only built when using
the
ProcessFull
option, not
with
ProcessIndexes
. So,
each time we run
ProcessUp-
date
on dimensions (effectively
dropping aggregations), we
must also perform the full pro-
cess of all the partitions to re-
build indexes and aggregations.
Since we can specify the storage mode at the partition level, each measure group
could contain both ROLAP and MOLAP partitions. For example, if 90 percent of your
queries check the current data and only 10 percent of them reference the historical
data, you could use MOLAP for the most frequently queried partitions and ROLAP
for historical ones.
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