how to make barcode in vb.net 2010 The Mini-Dimension in Software

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The Mini-Dimension
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When a dimension table is projected to grow at an inordinate rate or necessitates ETL processing that is overly time-consuming, a mini-dimension can help. A mini-dimension is created by removing a number of the more volatile attributes from the dimension in question and placing them in a new table with its own surrogate key. These attributes share no direct relationship to one another, and there is no natural key. A one-time-only process can populate this table with data by creating a row for each combination of values.
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6 More on Dimension Tables 125
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In the case of the policy dimension, a mini-dimension can be created to isolate the attributes that change more often. These attributes include policy holder demographics, such as marital status and family size, and the policy coverage characteristics, such as the number of covered family members and the deductible amount. In Figure 6-5, these attributes have been removed from the policy table and placed into a mini-dimension called policy_coverage.
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DAY PAYMENT_FACTS INVOICE day_key invoice_key customer_key product_key policy_key policy_coverage_key . . . (facts) . . . POLICY policy_key policy_number policy_holder address . . . Mini-dimension POLICY_COVERAGE policy_coverage_key family_size covered_parties spouse_coverage covered_children deductible_amount . . . Pre-populated with all possible combinations
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POLICY policy_ key 12882 policy_ number 40111 policy_ holder address Coverage changes don t generate new dimension rows...
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Smith, Hal 113 Random Rd.
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POLICY_COVERAGE policy coverage_ marital_ family_ covered_ covered_ key status size parties children 1 2 3 4 5 6 7 8 Single Married Married Married Married Married Married Married 1 2 2 3 3 3 3 4 1 1 2 1 2 2 3 1 0 0 0 0 0 1 1 0
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... and they don t generate new minidimension rows
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Figure 6-5
A mini-dimension relocates volatile attributes
Part III
PRODUCT
PART III
Dimension Design
The policy_coverage table does not have a natural key. Its attributes bear no direct relation to one another, or to any identifier present in the table. In these respects, a mini-dimension is similar to a junk dimension, as described in 3. Like any other dimension table, the mini-dimension is assigned a surrogate key. Rather than have an ETL process update this table on a regular basis, as is the case for other dimensions, it can be populated once, up front, with all possible value combinations. It is important to distinguish the mini-dimension from the arbitrarily split dimension table in the previous section. Unlike the split dimension, the mini-dimension does not share surrogate keys with the original dimension table. There is not a one-to-one relationship between the original dimension and the mini-dimension. Fact tables will carry separate foreign keys which refer to the original dimension table and to the mini-dimension.
Growth Is Controlled
Separation of volatile attributes into a separate table can have an astounding effect, all but eliminating table growth, even as changes occur in the source. This surprising effect can be understood by studying the bottom of Figure 6-5. The policy table contains a single row for Hal Smith, whereas it previously required several. Changes in his demographics and coverage do not impact this table, because those attributes are not present. Since the policy_coverage table has been populated in advance with all possible combinations, changes to Hal s policy don t result in any changes here, either. Hal s coverage can change over and over again, but neither table changes! There is a significant benefit to ETL processing as well. Each time the coverage details of a policy change, it is not necessary to scan through the policy dimension table to determine whether a type 2 change is needed. The ETL process for the main dimension is unaffected. It is necessary for ETL developers to know the current coverage characteristics for each policy. This information is required when loading fact table rows. In Figure 6-5, for example, each payment transaction must be associated with the correct policy_coverage row. ETL developers can keep this process manageable by maintaining a mapping of policy to its current policy_coverage_key. Whenever a fact is loaded, this mapping can be consulted to determine the correct policy_coverage_key. This mapping need only be revisited when coverage changes, which is usually only once per year. It may be maintained in a staging area available only to the ETL process, but as you will see shortly, there may be others who can benefit from this mapping. TIP When tables either (1) grow too quickly or (2) have so many type 2 attributes that change processing becomes a bottleneck, one or more mini-dimensions may help. Move the more volatile attributes into mini-dimension tables and populate them with all possible value combinations. Since a mini-dimension is pre-populated with all possible value combinations, it is necessary to consider the cardinality of each attribute. If the number of possible combinations will result in a mini-dimension that is excessively large, two or more can be created. In the case of the policy example, this might entail building a coverage minidimension and a demographics mini-dimension.
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