barcode vb.net 2010 Density vs. Sparsity in Software

Printer QR Code ISO/IEC18004 in Software Density vs. Sparsity

Density vs. Sparsity
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You have probably noticed another key difference between transaction and snapshot. While transaction fact tables are sparse, snapshots are dense. In a transaction fact table, a row is only recorded for a combination of dimension values if an activity links them. If there is no transaction on a particular day, no row is recorded. In a snapshot, however, rows are recorded regardless of activity. Failure to do so would render it useless. The account snapshot in Figure 11-2 records a row for each account at the end of each snapshot period. Since the period is a day, each account will receive exactly one row per day in the fact table. Figure 11-3 shows how this differs from what is stored in the transaction fact table. The table on the left shows transactions for a particular account during a span of two weeks. Each of these transactions generates a single row in the account_facts table. The table on the right illustrates the balance of the account at the end of each day. Each of these periodic balances is represented by a single row in account_snapshot_facts. (Ignore the shaded regions. These will be discussed shortly.) As you can see, the snapshot is dense; each account is represented for each day. Not so in the transaction fact table. If you examine the data closely, you will see there were a total of nine transactions in this account during these two weeks. In the snapshot, however, 14 rows are recorded for the account during the two-week span one for each snapshot period. TIP Snapshot fact tables are dense. Information is recorded each period in accordance with the grain statement, regardless of whether any activity took place. This density is a crucial feature of the snapshot table. If a row is not recorded for each snapshot period, it becomes difficult to determine status. For example, no transactions
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11 Transactions, Snapshots, and Accumulating Snapshots 267
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Account: Period: Granular transaction data stored in account_facts: Transaction Type Initial Deposit Withdrawal Check Check Check Deposit Check Check Withdrawal Transaction Amount 2000.00 (20.00) (35.50) (17.02) (75.00) 75.00 (800.00) (68.29) (100.00)
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7922-3002 2/1/2009 2/14/2009 Periodic status data stored in account_snapshot_facts: Day 2/1/2009 2/2/2009 2/3/2009 2/4/2009 2/5/2009 2/6/2009 2/7/2009 2/8/2009 2/9/2009 2/10/2009 2/11/2009 2/12/2009 2/13/2009 2/14/2009 Balance 2000.00 1980.00 1927.48 1927.48 1927.48 1927.48 1127.48 1127.48 1127.48 1059.19 1059.19 1059.19 1059.19 959.19 Two transactions occurred on this day No transactions occurred on these days Offsetting transactions occurred on this day
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Day 2/1/2009 2/2/2009 2/3/2009 2/3/2009 2/6/2009 2/6/2009 2/7/2009 2/10/2009 2/14/2009
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Figure 11-3
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Account transactions vs. status for a two-week period
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occurred on February 13, but a row will be recorded in the snapshot. Without this row, determining the balance for this date would require the following steps: 1. Check the date in question (February 13). 2. If no balance is found, check the previous date. 3. Repeat step 2 until a balance is found. In this case, it would be necessary to go back to February 10 to find a balance. This is unacceptably onerous, particularly if a large number of accounts are being studied. Note that this density does not necessarily imply that the snapshot will have more rows than the transaction fact table. Relative sizes will be determined by the snapshot s grain and transaction volume. If accounts average more than one transaction per day, the snapshot may actually be smaller. In some cases, the density might not be quite so high. A star schema that tracks product inventory in stores would be likely to record rows only for products that are in stock. This avoids the pesky appearance of zero-valued rows making their way into inventory reports.
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Part IV
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PART IV
Fact Table Design
While a bank will likely want accounts with zero balance to show up on reports, the inventory managers may not want products with no inventory to show up. Products may be discontinued or stocked only at specific locations. The grain statement of the star should make it clear whether rows are to be recorded for zero-valued levels. For the inventory star, the grain can be stated as end-of-day snapshots of in-stock products by location.
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