Skip to main content

Star schemas and aggregate (or summary) fact tables


Aggregate tables can further improve query performance by reducing the number of rows over which higher-level metrics must be aggregated. 
However, the use of aggregate tables with dimension tables is not a valid physical modeling strategy. Whenever aggregation is performed over fact data, it is a general requirement that tables joined to the fact table must be at the same attribute level or at a higher level. If the auxiliary table is at a lower level, fact rows will be replicated prior to aggregation and this will result in inflated metric values (also known as "multiple counting").

With the above Time dimension table, a fact table at the level of Day functions correctly because there is exactly one row in DIM_TIME for each day. To aggregate the facts to the level of Quarter, it is valid to join the fact table to the dimension table and group by the quarter ID from the dimension table.

Sql
select DT.QUARTER_ID,
   max(DT.QUARTER_DESC) Quarter_Desc
   sum(FT.REVENUE) Revenue
from DAY_FACT_TABLE FT
   join DIM_TIME DT
      on (FT.DAY_ID = DT.DAY_ID)
group by DT.QUARTER_ID

If, however, there is an aggregate fact table already at the level of Quarter, the results will not be correct. This is because the query must join on Quarter ID, but the quarter ID is not a unique key of the dimension table. Because any given quarter of a year contains 90, 91 or 92 days, the dimension table will contain that many rows with the same quarter ID. Thus fact rows will be replicated prior to taking the sum, and the sum will be too high.

Sql
select FT.QUARTER_ID,
   max(DT.QUARTER_DESC) Quarter_Desc
   sum(FT.REVENUE) Revenue
from QTR_FACT_TABLE FT
   join DIM_TIME DT
      on (FT.QUARTER_ID = DT.QUARTER_ID)
group by FT.QUARTER_ID

This is a generally recognized problem with star schemas, and is not strictly a MicroStrategy limitation.

Star schemas will function correctly with MicroStrategy SQL Generation Engine 8.x as long as they obey the general data warehousing principle that fact tables should not be at a higher level than the dimension tables to which they are joined.

If aggregate tables are required, it is necessary to provide higher-level lookup tables with unique rows corresponding to each aggregate table's key. Logical views are a way to do this without adding tables or views to the warehouse. For example, LWV_LU_QUARTER may be defined using the following SQL statement:

Sql
select distinct QUARTER_ID, QUARTER_DESC, YEAR_ID
from DIM_TIME

 

With this logical view, it becomes possible for MicroStrategy SQL Generation Engine 8.x to query the quarter-level fact table as follows. Since the logical view has distinct rows per quarter, multiple counting will not occur in this query.

Sql
select FT.QUARTER_ID,
   max(LQ.QUARTER_DESC) Quarter_Desc
   sum(FT.REVENUE) Revenue
from QTR_FACT_TABLE FT
   join (select distinct QUARTER_ID, QUARTER_DESC, YEAR_ID
         from DIM_TIME) LQ
      on (FT.QUARTER_ID = LQ.QUARTER_ID)
group by FT.QUARTER_ID

For more information on the use of logical views in MicroStrategy SQL Generation Engine 8.1.x and 9.x, consult the MicroStrategy Project Design Guide manual, Appendix B: Logical Tables, "Creating logical tables."

Aggregate tables store pre-summarized totals at a higher level of aggregation than the most granular fact table. They allow reports to be generated from small, rather than large, tables; therefore, performance is enhanced. A successful aggregation strategy seeks to choose aggregate tables that will have the most impact while taking the least amount of space.

Aggregation decisions are driven by the following factors:

  • Usage patterns: Build aggregate tables that are likely to be used the most.
  • Compression ratios: The compression ratio between two tables is defined as the size of the aggregate compared to the size of the smallest table from which the aggregate can be derived.
  • Volatility: Changes in hierarchies over time impact the accuracy of aggregate tables. Sometimes aggregate tables must be rebuilt as a result of changes in dimensions.
A good candidate for aggregation should have at most 10-15 percent of the size of the smallest table from which it is derived.

EXAMPLE:
The MicroStrategy Tutorial project uses aggregate tables by default. A simple metric sum (Revenue) will go to different aggregate tables depending on the attributes on the template.

  1. Create a report with Year on the rows and Revenue on the columns.
  2. Execute the report and view the SQL:

  3. Drill from Year to Item and view the SQL:

The query will go from using ORDER_FACT to ORDER_DETAIL. When Year is on the template, the engine selects the smaller table and the fact is calculated as:

sum(a11.ORDER_AMT)
    instead of:

    sum((a11.QTY_SOLD * (a11.UNIT_PRICE - a11.DISCOUNT)))
    Labels:

    Comments

    1. hinaFebruary 5, 2021 at 12:19 PM

      Is there any solution now to use aggregate tables with star schema without creating logical tables?

      ReplyDelete

    Post a Comment

    Popular posts from this blog

    Derived metric based on attribute values

    Derived metric based on attribute values Here is how could create and display data correctly on using below simple steps.  Create a report with Category, Subcategory and Revenue. Create New Metric in a report or VI.  Case((Category@ID = 1), Revenue, 0) Booksand Name it as Revenue for  where 2 is Category ID for "Books"  Report will display result as below.  Result for new metric is blank. Now to fix this create a new Derived metric on Category attribute first with formula as  Max(Category) {~ }  and calling Books Now Edit the "Revenue for Books metric and Replace Category@ID with this new Books metric formula would looks like this  Case((Books = 1), Revenue, 0).  Report result would now display as expected as shown below
    2 comments
    Read more

    Create an alert-based subscription in MicroStrategy Distribution Services

    Create an alert-based subscription in MicroStrategy Distribution Services on Web Subscription to a report or Report Services document which will be executed when a certain conditional threshold is met based on another executing report. For example, a scheduled report executes which shows the Revenue by day for the past week. If the Revenue on any one day falls below a certain value, a subscription to another report or Report Services document can be triggered and delivered to a recipient. An alert based subscription can only be created directly on a report; however, another report or Report Services document can be delivered when the alert based subscription is triggered. Note: you need a grid report to create an alert and you cannot create if you want to create on a document with text boxes. The following example will walk through the basic steps on how to setup a subscription based on an alert like this: Follow the brief  steps bel...
    3 comments
    Read more

    Documents with derived attributes cannot be edited within MicroStrategy Developer 10.x

    Documents with derived attributes cannot be edited within MicroStrategy Developer 10.x When trying to edit a document created in MicroStrategy Web using MicroStrategy Developer 10.x, the 'Edit' button is found greyed out and inaccessible as per the screenshot below. When navigating to the same document in MicroStrategy Web 10.x, it is possible to edit the document. STEPS TO REPRODUCE: Create a document in MicroStrategy Developer or Web 10.x with any number of attributes. See below: Notice in Developer, you can currently right click on this document and select the 'Edit' option. Open up the same document in MicroStrategy Web 10.x and add a derived attribute, right clicking an attribute and selecting Insert new attribute. Save the report in MicroStrategy Web 10.x with the same name. Open MicroStrategy Developer 10.x, and right click the document, you will notice that the Edit option is greyed out as seen below: Navigate to the same document wi...
    1 comment
    Read more

    Microstrategy Caches explained

    Microstrategy Caches Improving Response Time: Caching A  cache is a result set that is stored on a system to improve response time in future requests.  With caching, users can retrieve results from Intelligence Server rather than re-executing queries against a database. To delete all object caches for a project 1 In Developer, log into a project. You must log in with a user account that has administrative privileges. 2 From the  Administration  menu, point to  Projects , and then select  Project Configuration . The Project Configuration Editor opens. 3 Expand  Caching , expand  Auxiliary Caches , then select  Objects . To delete all configuration object caches for a server 1 Log in to the project source. 2 From the  Administration  menu in Developer, point to  Server , and then select  Purge Server Object Caches . 4 Click  Purge Now . To purge web cache follow the steps in the link ...
    3 comments
    Read more

    Metric values are repeated across rows when a report is executed in MicroStrategy

    Metric values are repeated across rows when a report is executed in MicroStrategy When comparing report results between DB Query Tool and MicroStrategy, some reports show repeated metric values in MicroStrategy where there were none in DB Query Tool. To illustrate the issue, a fact table CAT_ITEM_SLS has been added into the MicroStrategy Tutorial project and populated with a small set of three rows. CAT_ID ITEM_ID REVENUE  1 1  10   1 2  20  2  2  30  Report results in DB Query Tool: Report results in MicroStrategy: In MicroStrategy, the row for "Art As Experience" in the Spring 2007 catalog repeats the $20 value from the Winter 2007 catalog, where DB Query Tool shows the $30 value from the fact table. CAUSE The discrepancy occurs because the attribute elements for Catalog and Item are in a many-to-many relationship, but the attribute relationship in the MicroStrategy schema is defined incorrectly w...
    1 comment
    Read more

    Microstrategy Custom number formatting symbols

    Custom number formatting symbols If none of the built-in number formats meet your needs, you can create your own custom format in the Number tab of the Format Cells dialog box. Select  Custom  as the Category and create the format using the number format symbols listed in the table below. Each custom format can have up to four optional sections, one each for: Positive numbers Negative numbers Zeros Text Each section is optional. Separate the sections by semicolons, as shown in the example below: #,###;(#,###);0;"Error: Entry must be numeric" For more examples, see  Custom number formatting examples . To jump to a section of the formatting symbol table, click one of the following: Numeric symbols Character/text symbols Date and time symbols Text color symbols Currency symbols Conditional symbols Numeric symbols For details on how numeric symbols apply to the Big Decimal data type, refer to the  Project Design Guide . ...
    3 comments
    Read more

    Relationship with Report Filter options for Levels metrics

    Relationship with Report Filter options for Levels metrics You can define how the report filter affects the metric calculation. From the  Relationship with Report Filter  drop-down list, select one of the following: • To include only data that meets the conditions in the report filter in the metric calculation, select  Standard filtering . • To raise the level of the report filter to the level of the target, if possible, then apply the report filter to the metric calculation, select  Absolute filtering . For example, the report filter contains the Washington, DC, Boston, and New York call centers, but the Revenue metric is calculated at the Region level. Because Call Center is a child attribute of Region, the report filter's level is raised to the Region level, and the report filter is treated as if it includes the regions that contain Washington, DC, Boston, and New York (in this case, Mid-Atlantic and Northeast). Data from all call centers in the Mid-At...
    Post a Comment
    Read more

    Case functions Microstrategy

    Ca se functions Microstrategy Case functions return specified data in a SQL query based on the evaluation of user-defined conditions. In general, a user specifies a list of conditions and corresponding return values. Case This function evaluates multiple expressions until a condition is determined to be true, then returns a corresponding value. If all conditions are false, a default value is returned.  Case  can be used for categorizing data based on multiple conditions. This is a single-value function. Syntax Case ( Condition1 ,  ReturnValue1 ,  Condition2 , ReturnValue2 ,...,  DefaultValue ) Example Case(([Total Revenue] < 300000), 0, ([Total Revenue] < 600000), 1, 2) sum(Case (Day@DESC in (“Sat”,”Sun”), Sales, 0) {~+} Sum(Case(Category@DESC In("Books","Electronics"),Revenue,0)){~+} CaseV (case vector) CaseV  evaluates a single metric and returns different values according to the results. It can be used to perfo...
    3 comments
    Read more

    Microstrategy Dossiers explained

    Microstrategy  Dossiers With the release of MicroStrategy 10.9, we’ve taken a leap forward in our dashboarding capabilities by simplifying the user experience, adding storytelling, and collaboration.MSTR has  evolved dashboards to the point that they are more than dashboards - they are  interactive, collaborative analytic stories . Ultimately, it was time to go beyond dashboards, both in concept and in name, and so  the've  renamed VI dashboards to  ‘ dossiers ’.  Dossiers can be created by using the new Desktop product or Workstation or simply from the Web interface which replaces Visual Insights. All the existing visual Insights dashboards will be converted to Dossiers   With MicroStrategy 10.9, there was an active focus on making it easier to build dashboards for the widest audience of end users. To achieve this, some key new capabilities were added that make it easier to author, read, interact and collaborate on dashboards ...
    7 comments
    Read more

    Personalizing file locations, email and file subscriptions using macros in Microstrategy

    Personalizing file locations MSTr allows to dynamically specify the  File Location  and  Backup File Location  in a file device using macros.  For example, if you specify the  File Location  as  C:\Reports\{&RecipientName}\ ,  all subscriptions using that file device are delivered to subfolders of  C:\Reports\ . Subscribed reports or documents for each recipient are delivered to a subfolder with that recipient’s name, such as  C:\Reports\Jane Smith\  or  C:\Reports\Hiro Protagonist\ . The table below lists the macros that can be used in the  File Location  and  Backup File Location  fields in a file device: Description Macro Date on which the subscription is sent {&Date} Time at which the subscription is sent {&Time} Name of the recipient {&RecipientName} User ID (32-character GUID) of the recipient {&RecipientID} Distribution Services add...
    Post a Comment
    Read more