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

    Scheduling a report or document to be sent to an FTP in MSTR

    Scheduling a report or document to be sent to an FTP server You can have a report or document automatically delivered to a location on your FTP server on a specific schedule. To do so, you must subscribe to the report or document, as described in the steps below. You can customize your subscription by typing macros in the  File Name ,  Sub-folder , or  Zip File Name  fields. These macros are automatically replaced with the appropriate text when the report or document is delivered. For example, you create a subscription to a document. If you type  {&Project}  in the  File Name field, the name of the project in which the document is saved is displayed in the name of the document when it is delivered. • This procedure assumes that an administrator has already added your FTP server as a new device in Developer. Steps to do so are included in the  System Administrator Help . To send a report or document to an FTP server on a schedule ...
    Post a Comment
    Read more

    Logical Views to specify an outer join between two attribute lookup tables when only attributes are on a report

    Logical Views to specify an outer join between two attribute lookup tables when only attributes are on a report Apart from using the VLDB properties to create the left outer join,  article describes how to use the Logical View to specify an outer join between two attribute lookup tables when only attributes are on a report. This method exists as attribute only outer joins will not be generated on their own by the MicroStrategy SQL engine. This is because they are only necessary with r agged/unbalanced hierarchies which are not supported as null attribute IDs are not supported (parent elements with no child elements or child elements with no parents).  Brief instructions are provided using the example below. Consider, two attributes: Parent01 and Child01 have a parent-child relationship. Their Lookup tables are defined, as follows Parent01 Child01 Note that although there are 4 ID values for the attribute Parent01, there is no defined relationship ...
    Post a 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

    Microstrategy Dossier training videos

    Microstrategy Dossier training videos Adding data to a Dossier: Creating a visualization filter in Dossier: Sending Dossier to User libraries: How to format Dossiers: Adding a designer filter to a Dossier: Use Bookmarks in Dossiers: Dynamic Links in Dossier: Recreating documents from Dossiers: Exploring and searching in Dossiers: Exploring sample dossiers in MSTR library: Using Page to Page Targets in Dossiers:
    2 comments
    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

    Create a transaction services photo uploader

    Create a transaction services photo uploader   1.  Create a new table "photo_upload" in Tutorial warehouse database (the default location: C:\Program Files\MicroStrategy\Tutorial Reporting\TUTORIAL_DATA_7200.mdb), as shown below:    2. The 'photo_upload' table has to be pre-populated with *exactly* 10 rows of data, the values for the 'ID' column should be 1-10 and the values for the 'uploaded' column should all be 0 3.  In MicroStrategy Desktop, create a freeform report "R1" based on the new table "photo_upload" in Tutorial data created at step 1, as shown below:   SELECT Location, Description, ID, uploaded, numbers FROM PHOTO_UPLOAD 4.  Create another table for transaction insert SQL. Make sure to create an 'autonumber' type ID as primary key for this table, or auto_increment ID for different DBs.                     5. Create...
    Post a Comment
    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...
    2 comments
    Read more

    Predictive modelling in Data Science using Microstrategy

    Creating a predictive modelling in MicroStrategy MicroStrategy Data Mining Services has been evolving to include more data mining algorithms and functionality. One key feature is MicroStrategy Developer’s Training Metric Wizard. The Training Metric Wizard can be used to create several different types of predictive models including linear and exponential regression, logistic regression, decision tree, cluster, time series, and association rules. Linear and exponential regression The linear regression data mining technique should be familiar to you if you have ever tried to extrapolate or interpolate data, tried to find the line that best fits a series of data points, or used Microsoft Excel’s LINEST or LOGEST functions. Regression analyzes the relationship between several predictive inputs, or independent variables, and a dependent variable that is to be predicted. Regression finds the line that best fits the data, with a minimum of error. For example, you have a dataset ...
    1 comment
    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...
    Post a Comment
    Read more