Database Reference
In-Depth Information
nevertheless, a chapter entitled how ASo Works and how to Design for Performance
warrants a few remarks about aggregations:
7.5.1 Aggregation and the Bitmap
By now, you might be able to guess at how aggregation and the bitmap interact. repeating
some of what was said above in Section 7.4.3.2: The bitmap algorithm selects the hierar-
chy with the longest required bitmap for any dimension. remember that within the set
number of bits, the actual encoding on the record on disk and in memory can reflect only
one hierarchy at a time
. In the input-level data, aka the level-0 view, the longest bitmap
(primary) hierarchy will be what is seen for each dimension on each record. A member,
as represented by the hierarchy on the record can reflect only one hierarchy at a time,
even if it has one or more aliases in other hierarchies, cannot simultaneously show (refer-
ring to the product dimension as seen in Figure 7.6) [hDtv] as a descendant of both [All
merchandise] and [high End merchandise]. Similarly, a store cannot be represented in
the bitmap as both [Brick & mortar] and [10000] square feet.
okay, let me back up a second and mention that an Aggregated view is not one
“answer” or one record, but a set of records. An Aggregated view of ASosamp “at level-1
of the [Age] dimension” would have a set of records for each of the level-1 members of
the members [Age] or a set of records for [teens], [Adult], and [Senior] (Figure 7.9). The
set of records for [teens] would have records with
all of the level-0 nonformula
combi-
nations
of the other dimensions
.
Therefore, to keep the example and notation simple,
lets ignore all dimensions other than [measures], [year], [Age] members. now let us
assume that the level-0 view has fact data existing for the following [measures], [year],
[Age] members combinations
only
:
• ([original Price], [Curr year], [1 to 13 years])
• ([original Price], [Curr year], [14 to 19 years])
Figure 7.9
Selected portions of the ASOsamp outline. (From Oracle Essbase Administration Services. With permission.)
