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 URL API Parameters

    MicroStrategy URL Structure The following table summarizes the root URL structure used for every request to MicroStrategy Web. Environment Main Application URL Administration URL J2EE http://webserver/MicroStrategy/servlet/mstrWeb http://webserver/MicroStrategy/servlet/mstrWebAdmin .NET http://webserver/MicroStrategy/asp/Main.aspx http://webserver/MicroStrategy/asp/Admin.aspx Every request sent to MicroStrategy Web calls a central controller. Parameters are appended to  Main.aspx  or  mstrWeb  (in a .NET and J2EE environment, respectively) to indicate to the controller how the request should be internally forwarded and handled. The following examples show a URL for accessing a MicroStrategy folder when the user does not have an existing session. The URL contains not only the parameters needed to connect to MicroStrategy Web, but also the parameters needed to log on and create a session. J2EE environment: <a href="http:...
    2 comments
    Read more

    Microstrategy Report Execution Process

    The Report Execution Process  Report execution process at a high level: The report execution process is a three-step process:  1. Query Stage : Retrieve data from the warehouse  2. Populate and Evaluate : Fill report data required for display  3. Cross-tab : Pivot and display sorting and page-by Each of the engines plays an important part in the report execution process.  As you can imagine, the SQL Engine performs its role during the Query stage while the Query Engine and Analytical Engine can be involved in all three stages.
    3 comments
    Read more

    Client Rendering Optimizations for Dashboard Performance Optimizations

      The amount of data retrieved and objects being used in a Report Services Dashboard have a direct impact in the size of the final Dashboard. The bigger the Dashboard size the longer it will take to be prepared, be sent to the client, and render.   Client Rendering Once the data reaches the end user's browser window the data has to be formatted according to the definition of the Dashboard as specified in the formatting set by the architect. To do so the browser will have to either build the HTML page in DHTML mode or initialize the flash container and parse the XML.   Client rendering greatly varies depending on the hardware used. More powerful machines will render dashboard faster for a list of recommended client hardware specifications please refer to the Readme File for the specific version of MicroStrategy.   Optimization Techniques common to DHTML and Flash Client rendering time greatly relies in the amount of XML that needs to be parsed. In order to ensure that...
    Post a Comment
    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

    Evaluation order of calculations

    Evaluation order of calculations Evaluation order is the order in which objects are calculated by MicroStrategy’s Analytical Engine. Changing the order in which data is calculated can change report results. You change the evaluation order of a report’s data calculation by changing the order in which compound smart metrics, consolidations, derived metrics, derived elements, report limits, and subtotals on the report are calculated. The  default order of calculation is as follows: Compound smart metrics (which are compound metrics with smart totals enabled) Consolidations, which are evaluated by their relative position on the report template: Rows, from left to right Columns, from top to bottom Report limits Subtotals Compound metrics that are not the direct aggregations of other metrics can be used in the evaluation order by setting the  Allow Smart Metrics  option of the Metric Editor to  Yes .
    Post a 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

    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 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 . ...
    4 comments
    Read more