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 "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

    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

    Developer not starting with "invalid picture" message

    Developer not starting with "invalid picture" message. This error could be due to the fact that t here is insufficient disk space on the drive used for the Windows Temp directory or the Optional Work Drive. https://support.microsoft.com/en-us/help/964421/error-481-invalid-picture Users may see the "Invalid Picture" error thrown when attempting to launch MicroStrategy Developer in Windows: CAUSE: This error is caused by a lack of available space on the user's hard disk to properly launch the platform. ACTION: To remedy the issue, simply clear up some active space on the hard drive to allow MicroStrategy to properly launch.
    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

    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

    Types of prompts in Microstrategy

    Types of prompts in Microstrategy The different types of prompts allow you to create a  prompt  for nearly every part of a report. Prompts can be used in many objects including reports, filters, metrics, and custom groups, but all prompts require user interaction when the report is executed. The correct prompt type to create depends on what report objects you want users to be able to base a filter on to filter data, as described in the list below. Filter definition prompts   allow users to determine how the report's data is filtered, based on one of the following objects: Attributes in a hierarchy : Users can select prompt answers from one or more attribute elements from one or more attributes. The attribute elements that they select are used to filter data displayed on the report. This prompt lets you give users the largest number of attribute elements to choose from when they answer the prompt to define their filtering criteria. For example, on a repor...
    1 comment
    Read more

    Types of filters in Microstrategy

    Types of filters in Microstrategy Below are the types of filters: 1. Attribute qualification filter These types of qualifications restrict data related to attributes on the report. a) Attribute form qualification Filters data related to a business attribute’s form(s), such as ID or description. •  For example, the attribute Customer has the forms ID, First Name, Last Name, Address, and Birth Date. An attribute form qualification might filter on the form Last Name, the operator Begins With, and the letter H. The results show a list of customers whose last names start with the letter H. b) Attribute element list qualification Filters data related to a business attribute’s elements, such as New York, Washington, and San Francisco, which are elements of the attribute City. • For example, the attribute Customer has the elements John Smith, Jane Doe, William Hill, and so on. An attribute element list qualification can filter data to display only those customer...
    6 comments
    Read more

    Allow a Visualization to Update the Data in Another Visualization in Dossier

    Allow a Visualization to Update the Data in Another Visualization After adding multiple visualizations to a dossier, you can select values in one visualization (that is, the source) to automatically update data in another visualization (that is, the target). This is done by creating a filter on a visualization that targets other visualizations. To Add a Target Visualization to Your Dossier: Open the dossier with the visualization. Click  Insert Visualization   . A blank visualization appears in the dossier. From the Visualizations panel, select  Grid   . Drag an attribute from the Datasets panel to the  Rows  area of the Editor panel to add attributes to the rows. Drag an attribute from the Datasets panel to the  Columns  area of the Editor panel to add attributes to the columns. Drag a metric from the Datasets panel to the  Metrics  area of the Editor panel, to add a metric to the grid. The Metric Names attribute automatically appears i...
    Post a Comment
    Read more

    Create a Visualization Filter in a Dossier

    Create a Visualization Filter You can select attribute values to filter data that appears in a visualization by creating an attribute filter. Create a visualization filter by dragging attributes and metrics from the Datasets panel. To create an attribute filter Open the dossier  you want to modify. Select the chapter that contains the visualization to filter. Click  Filter  . An empty filter appears in the dossier. Drag the attributes and metrics for filtering from the Datasets panel to the empty filter. Use the Ctrl key to select multiple attributes and metrics. In the filter, click  Select Target . Click on target visualizations or visualization filters to select them. You can select targets across chapters and pages, as long as they are all on the same page. Click  Apply . Click  More   in the filter and select options for modifying the filter. The available options are based on the objects you select.
    Post a Comment
    Read more

    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