Building the Data Warehouse

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balance verified = yes

ID used = yes


ATM table

acct = 1234

date = Jan 2

time = 1:31 pm

ID number = Ab00191S

limit exceeded = no

exact timestamp = 1:31:35:05

teller table

acct = 1234

date = Jan 5

time = 3:15 pm

teller ID = JLC

automated verification = no

sequence number = 901

cashbox balance = $112,109.32

Although it is tempting to say that the tables are ready to be cast into the concrete of physical database design, one last design step remains—factoring in the performance characteristics. With the data warehouse, the first step in doing so is deciding on the granularity and partitioning of the data. This is crucial. (Of course, the key structure is changed to add the element of time, to which each unit of data is relevant.)

After granularity and partitioning are factored in, a variety of other physical design activities are embedded into the design, as outlined in Figure 3.20. At the heart of the physical design considerations is the usage of physical I/O (input/output). Physical I/O is the activity that brings data into the computer

Figure 3.19 The corporate DIS is made up of the DIS created as a result of each user view session.

from storage or sends data to storage from the computer. Figure 3.21 shows a simple case of I/O.

Data is transferred to and from the computer to storage in blocks. The I/O event is vital to performance because the transfer of data to and from storage to the computer occurs roughly two to three orders of magnitude slower than the speeds at which the computer runs. The computer runs internally in terms of nanosecond speed. Transfer of data to and from storage occurs in terms of milliseconds. Thus, physical I/O is the main impediment to performance.

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