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

    Microstrategy Release Types Platform release Interval:  Annually every twelve (12) months in December Who:  Entire customer base What:  Focus on production level security, stability, and performance defect fixes for all customers. Expectation:  Customer has chosen platform path and wants product stability without new enhancements. Support:  Three (3) years, patches for approved P1 defects, and regular hotfix cadence addresses critical defects. Feature Release Interval:  Quarterly every three (3) months Who:  Customers with specific feature requirements. What:  New functionality developed in close collaboration with customers and customer council. Expectation:  Customer has chosen feature path, will consume further feature releases. Support:  Six (6) months patch support for approved P1 defects and (eighteen) 18 months troubleshooting. Customers upgrade to next feature release for roll-up f...
    1 comment
    Read more

    Microstrategy Document Autotext macros:

    Autotext  code/macros in  Microstrategy Document/dashboard This is a list of the available auto text macros that the Report Services Document engine recognizes. The following auto text codes allow you to add  document variable information to your document. These auto text codes are automatically replaced by information about the document. Auto text codes for MSTR document/dashboard:  AUTOTEXT DESCRIPTION   {&PAGE}  Display the current page.  {&NPAGES}  Display the total number of pages.  {&DATETIME}  Display the current date and time.  {&USER}  Display the user name that is executing the Report Services Document.  {&DOCUMENT}  Display the document name.  {&DOCUMENTID}  Display the document ID.  {&DESCRIPTION}  Display the document description.  {&PROJECT}  Display the project name.  {&EXECUTIONTIME}  Dis...
    2 comments
    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

    RunningSum calculation only on the metric subtotal in MicroStrategy

    RunningSum calculation only on the metric subtotal in MicroStrategy Here are the series of steps to setup report objects in which metrics and subtotals so only the  subtotal field  will contain the  RunningSum  and the  regular metric values  will be  standard sum values . 1) Create Metric 1 which is the sum of the fact that is to be in the columns. 2) Create Metric2 as the RunningSum of Metric1.  NOTE:  The  sortby  parameter for the RunningSum should be set to whichever attribute you want the report sorted by. 3) Create Metric3 as Metric1 + (Metric2 x 0) 4) Create a new subtotal called "Max" which is defined as Max() 5) On the Subtotals/Aggregation tab for Metric 3, set the Total subtotal function to be "Max" and select the check box for "Allow Smart Metric" 6) Create the desired report and place the 3 metrics on the report.  NOTE:  Only Metric3 is required on the gri...
    2 comments
    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 "Error type: Odbc error. Odbc operation attempted

     "Error type: Odbc error. Odbc operation attempted: SQLExecDirect. [HYT00:0: on SQLHANDLE] [MicroStrategy][ODBC Oracle Wire Protocol driver]Timeout expired" is shown when executing reports from Web When users are trying to execute some reports in MicroStrategy web in particular, they may receive the Error “SQL Generation Complete Index out of range” and “Timeout expired” error as shown below: Possible Causes: One possible cause is that the MicroStrategy Intelligence Server using a cached database connection that was already dropped by the RDBMS. To resolve this: Admin should delete the database connection caches and create a new DSNs in case they are sharing DSNs to connect to different databases. In addition, change the settings for the ‘Connection lifetime’ and the ‘Connection idle time out’.  Follow the steps below to perform the mentioned changes and verify the report after each step and some of the settings require i-server r...
    4 comments
    Read more

    Settings for Outer Join between metrics in MicroStrategy

    Settings for Outer Join between metrics in MicroStrategy MicroStrategy adopts multi-pass logic to determine the execution plan for a report. This means that every metric is evaluated in separate SQL passes. Outer Joins come into play when MicroStrategy Engine merges the results from all SQL passes into one report. For a multi-pass report, different Outer Join behaviors can give the user completely different results. In addition, report metrics can be of different types which can, in some cases, influence the result of the outer join. In MicroStrategy, there are two settings that users can access to control Outer Join behavior : Formula Join Type and Metric Join Type . Metric Join Type: VLDB Setting at Database Instance Level Report and Template Levels Report Editor > Data > Report Data Options Metric Level   Metric editor > Tools > Metric Join Type Control Join between Metrics Formula Join Type: Only at Compound/Split...
    1 comment
    Read more

    Fiscal Week, Fiscal Month, Fiscal Quarter and Fiscal Year calculations in Microstrategy

    Fiscal Week, Fiscal Month, Fiscal Quarter and Fiscal Year calculations in Microstrategy FiscalWeek Returns the numeric position of a week within a fiscal year, for a given  input date. This function is useful in financial reporting when the start of the fiscal year is different than the start of the calendar year. Syntax FiscalWeek< firstWeekDay ,  firstMonth >( Date / Time ) Where: • Date / Time  is the input date or timestamp. • firstWeekDay  (default value is 1) is a parameter that determines which day of the week is considered as the first day of the week. You can type an integer value from 1 to 7, with 1 representing Sunday, 2 representing Monday, and so on until 7 representing Saturday. • firstMonth  (default value is 1) is a parameter that determines which month is considered as the start of the fiscal year. You can type an integer value from 1 to 12, with 1 representing January, 2 representing February, and so on until ...
    1 comment
    Read more

    Evaluation Ordering

    Evaluation Ordering Evaluation Ordering is an advanced property that is hidden by default. For information on how to display this property, see  Viewing and changing advanced VLDB properties . An evaluation order is the order in which the MicroStrategy Analytical Engine performs different kinds of calculations during the data population stage. The Evaluation Ordering property determines the order in which calculations are resolved. MicroStrategy objects that are included in the evaluation order include consolidations, compound smart metrics, report limits, subtotals, derived metrics, and derived elements. Some result data can differ depending on the evaluation order of these objects. • 6.x order - Calculate derived metric/smart compound metric before derived elements/consolidation and all subtotals as smart : This option is used primarily to support backward compatibility. It is recommended in most scenarios to update your project to use the 9.x evaluation order described below. • ...
    Post a Comment
    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