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18/6/2014 Kimball Group Desig n Tip #167 Complementary Fact Table Types Kimball Group Print

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Design Tip #167 Complementary Fact Table TypesBob Becker on June 17, 2014

There are three fundamental types of fact tables in the data warehouse presentation area:

transaction fact tables[1], periodic snapshot fact tables[2], and accumulating snapshot fact

tables[3]. Most DW/BI design teams are very familiar with transaction fact tables. They arethe most common fact table type and are often the primary workhorse schema for manyorganizations. Many teams have also incorporated periodic snapshot fact tables in theirpresentation areas. Fewer organizations leverage the accumulating snapshot fact table.Design teams often dont appreciate how an accumulating snapshot fact table cancomplement transaction and/or periodic snapshot fact tables.

Each of the fact table types is a design response to the w ide variety of requirements posedby the business community. Often the best design response is a combination of two, or evenall three, fact table types. The multiple fact tables complement one another, each supportinga unique view of the business processes that would be difficult to achieve with only one facttable type.

A logistics supply chain is an excellent scenario to illustrate all three fact table types workingtogether to support a rich set of business requirements. Well use a simplified view of thefinished goods logistics pipeline of a large auto manufacturer to help understand thestrengths and appropriate use of each fact table type.

Our auto manufacturer has plants where vehicles are assembled. The finished vehicleseventually find their way to dealers where they will be sold to the ultimate owners. Ourfictitious auto manufacturer maintains finished goods inventory in a large parking lot really

a warehouse located just outs ide the assembly plant doors. Vehicles (inventory) areshipped from the finished goods warehouse via freight train to one of several regionalparking lots. From these regional warehouses, the inventory is shipped via carrier truck todea ler locations. Once the vehicle arrives at the dea ler, it is prepped and put on the dealerlot (store inventory) for final sale.

The logistics business users need to understand the number of vehicles flowing out of finalassembly, in and out of each warehouse, and the final customer demand for various vehicletypes, colors, models, and so on. The company also needs to understand and analyze theinventory levels at each stage of the logistics chain. Logistics management wants tounderstand the time it takes for a vehicle to move from the assembly plant to the finalcustomer, depending on vehicle type, warehouses, and carriers. Moving vehicles more quicklyand efficiently through the logistics pipeline helps the company minimize inventory levels andreduce carrying costs.

A robust des ign to support our auto manufacturers finished goods logistics pipelineillustrates all three fact table types.

Transaction Fact Table

A key component of the logistics pipeline is the flow of inventory from one location toanother. The flow of vehicles is captured in a series of inventory movement transactions. Anassembly plant releases a vehicle into the finished goods inventory via an inventorymovement transaction. The vehicle is then shipped via rail to the regional warehouse whereit is received into its inventory; later it is removed from inventory and shipped via truck to thedea ler where it is received into the dea ler inventory. For each of these inventory moves, aninventory movement or shipping/receiving transaction is generated. The inventory flow is agreat opportunity for a transaction fact table. The grain of this fact table is one row for each
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inventory movement transaction for each vehicle. Similarly the final sale of the vehicle shouldbe captured in a sales transaction fact table with one row for each vehicle so ld.

Transaction fact tables are an appropriate design response to business requirements thatlook for an understanding of the intensity or quantity of a business process. Transaction facttables he lp answer the how many? question. For example, how many white sports utilityvehicles (SUVs) were sold last week? What were the dollar sales? How many all-wheel drivevehicles were released by assembly into salable inventory? How many vehicles did we shipwith a given carrier? How many vehicles were received by our dealers this month? Comparedto last month, last quarter, or last year? Theres a reason transaction fact tables are theworkhorse fact table type: they support critically important business requirements. On theother hand, transaction fact tables are less effective in answering questions regarding thestate of our inventory levels or the speed/efficiency of the logistics pipeline. To support thesebusiness requirements, we look to other fact table types to complement transaction facttables.

Periodic Snapshot Fact Table

The second requirement is understanding the total amount of inventory at any point in thepipeline. Supporting analysis of inventory levels is a task well suited for a periodic snapshotfact table. At any point in time, each vehicle is in a single physical location such as finishedgoods inventory at the plant, in a regional distribution center, on a dealer lot, or in-transit ona railcar or truck. To support inventory analysis, the periodic snapshot fact table has a grainof one row per vehicle each day. A location dimension will support the analysis of inventory ineach point of the pipeline.

The periodic snapshot fact table does an excellent job of helping understand the volume ofvehicles in our pipeline. It answers the how much? question. How much total inventory dowe have? How much inventory is available in white vehicles? SUVs? Four doors? Sportmodels? In California? On dealer lots? Compared to prior months, quarters, or years? Theperiodic snapshot supports trending of inventory levels over time. The inventory movementtransaction fact table and inventory periodic snapshot together support a wide range of thebusiness requirements surrounding the logistics pipeline. However, even with both of thesefact tables, it will be difficult to address the pipeline efficiency requirements. To complete thepicture, an accumulating snapshot fact table will complement the transaction and periodicsnapshot fact tables.

Accumulating Snapshot Fact Table

The third set of requirements for the logistics pipeline is supporting analysis of the speed atwhich vehicles travel through the pipeline (no pun intended). Each vehicle w ill pass through aseries of milestones during its travels to the final owner. To support the analyticrequirements for measuring and understanding the efficiencies of our logistics pipeline, anaccumulating snapshot fact table will be populated w ith one row per vehicle. As each vehiclemoves through the pipeline, the accumulating snapshot will be updated w ith the date ofeach movement and current location of the vehicle. The accumulating snapshot w ill havenumerous date keys, such as date released by assembly, date shipped to distributioncenter, date received at distribution center, and so on until date of final sale. Fact tablemetrics w ill include a series of date lags that measure the time it took for a vehicle to movebetween pipeline steps .

The accumulating snapshot fact table supports the key efficiency measures o f velocity.How

quickly do vehicles move through the pipeline? Whats the average length of time fromassembly release to final customer sale? Is it different for autos than SUVs? Hybrids versusnon-hybrids? White versus red vehicles? Which carriers/railroads are most effective? Theaccumulating snapshot will be updated da ily for vehicles currently in the logistics pipeline.Thus, the accumulating snapshot can also be used to look at the current state of the pipelineto identify stuck vehicles, such as find all the vehicles that have been at a regionaldistribution center or on a dealer lot for more than n days. How many vehicles have beenin transit via rail or truck for more than n days? Where are a ll the SUVs? This fact table canhelp the logistics team identify and move the vehicles in highest demand, identify efficiencyimprovement opportunities, and identify preferred shipping partners.

In the case of our auto manufacturer, it is clear that the three fact table types complementone another. Implementing all three fact table types is an appropriate response to the richset of business requirements. Implementing only one or even only two of the fact table types


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would have made it very difficult, if not imposs ible, to support all the requirements.

Article printed from Kimball Group: http://www.kimballgroup.com

URL to article: http://www.kimballgroup.com/2014/06/17/design-tip-167-complementary-fact-table-types/

URLs in this post:

[1] transaction fact tables: http://www.kimballgroup.com/data-warehouse-business-intelligence-resources/kimball-techniques/dimensional-modeling-techniques/transaction-fact-table/

[2] periodic snapshot fact tables: http://www.kimballgroup.com/data-warehouse-business-intelligence-resources/kimball-techniques/dimensional-modeling-techniques/periodic-snapshot-fact-table/

[3] accumulating snapshot fact tables: http://www.kimballgroup.com/data-warehouse-business-intelligence-resources/kimball-techniques/dimensional-modeling-techniques/accumulating-snapshot-fact-table/

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