Designing Aggregations (XMLA)

Aggregation designs are associated with the partitions of a particular measure group to make sure that the partitions use the same structure when storing aggregations. Using the same storage structure for partitions lets you to easily define partitions that can be later merged using the MergePartitions command. For more information about aggregation designs, see Aggregazioni e relativa progettazione (SSAS).

To define aggregations for an aggregation design, you can use the DesignAggregations command in XML for Analysis (XMLA). The DesignAggregations command has properties that identify which aggregation design to use as a reference and how to control the design process based upon that reference. Using the DesignAggregations command and its properties, you can design aggregations iteratively or in batch, and then view the resulting design statistics to evaluate the design process.

Specifying an Aggregation Design

The Object property of the DesignAggregations command must contain an object reference to an existing aggregation design. The object reference contains a database identifier, cube identifier, measure group identifier, and aggregation design identifier. If the aggregation design does not already exist, an error occurs.

Controlling the Design Process

You can use the following properties of the DesignAggregations command to control the algorithm used to define aggregations for the aggregation design:

• The Steps property determines how many iterations the DesignAggregations command should take before it returns control to the client application.
• The Time property determines how many milliseconds the DesignAggregations command should take before it returns control to the client application.
• The Optimization property determines the estimated percentage of performance improvement the DesignAggregations command should try to achieve. If you are iteratively designing aggregations, you only have to send this property on the first command.
• The Storage property determines the estimated amount of disk storage, in bytes, used by the DesignAggregations command. If you are iteratively designing aggregations, you only have to send this property on the first command.
• The Materialize property determines whether the DesignAggregations command should create the aggregations defined during the design process. If you are iteratively designing aggregations, this property should be set to false until you are ready to save the designed aggregations. When set to true, the current design process ends and the defined aggregations are added to the specified aggregation design.

Specifying Queries

The DesignAggregations command supports usage-based optimization command by including one or more Query elements in the Queries property. The Queries property can contain one or more Query elements. If the Queries property does not contain any Query elements, the aggregation design specified in the Object element uses a default structure that contains a general set of aggregations. This general set of aggregations is designed to meet the criteria specified in the Optimization and Storage properties of the DesignAggregations command.

Each Query element represents a goal query that the design process uses to define aggregations that target the most frequently used queries. You can either specify your own goal queries, or you can use the information stored by an instance of Microsoft SQL Server 2005 Analysis Services (SSAS) in the query log to retrieve information about the most frequently used queries. The Usage-Based Optimization Wizard uses the query log to retrieve goal queries based on time, usage, or a specified user when it sends a DesignAggregations command. For more information, see Guida sensibile al contesto di Ottimizzazione guidata basata sulle statistiche di utilizzo (SSAS).

If you are iteratively designing aggregations, you only have to pass goal queries in the first DesignAggregations command because the Analysis Services instance stores these goal queries and uses these queries during subsequent DesignAggregations commands. After you pass goal queries in the first DesignAggregations command of an iterative process, any subsequent DesignAggregations command that contains goal queries in the Queries property generates an error.

The Query element contains a comma-delimited value that contains the following arguments:

Frequency,Dataset[,Dataset...]

• Frequency
  A weighting factor that corresponds to the number of times that the query has previously been executed. If the Query element represents a new query, the Frequency value represents the weighting factor used by the design process to evaluate the query. As the frequency value becomes larger, the weight that is put on the query during the design process increases.

• Dataset
  A numeric string that specifies which attributes from a dimension are to be included in the query. This string must have the same number of characters as the number of attributes in the dimension. Zero (0) indicates that the attribute in the specified ordinal position is not included in the query for the specified dimension, while one (1) indicates that the attribute in the specified ordinal position is included in the query for the specified dimension.

  For example, the string "011" would refer to a query involving a dimension with three attributes, from which the second and third attributes are included in the query.

  [!NOTA] Some attributes are excluded from consideration in the dataset. For more information about excluded attributes, see Query Element (XMLA).

  Each dimension in the measure group that contains the aggregation design is represented by a Dataset value in the Query element. The order of Dataset values must match the order of dimensions included in the measure group.

Designing Aggregations Using Iterative or Batch Processes

You can use the DesignAggregations command as part of an iterative process or a batch process, depending on the interactivity required by the design process.

Designing Aggregations Using an Iterative Process

To iteratively design aggregations, you send multiple DesignAggregations commands to provide fine control over the design process. The Aggregation Design Wizard uses this same approach to provide fine control over the design process. For more information, see Guida sensibile al contesto di Progettazione guidata aggregazioni (SSAS).

[!NOTA] An explicit session is required to iteratively design aggregations. For more information about explicit sessions, see Managing Connections and Sessions (XMLA).

To start the iterative process, you first send a DesignAggregations command that contains the following information:

• The Storage and Optimization property values on which the whole design process is targeted.
• The Steps and Time property values on which the first step of the design process is limited.
• If you want usage-based optimization, the Queries property that contains the goal queries on which the whole design process is targeted.
• The Materialize property set to false. Setting this property to false indicates that the design process does not save the defined aggregations to the aggregation design when the command is completed.

When the first DesignAggregations command finishes, the command returns a rowset that contains design statistics. You can evaluate these design statistics to determine whether the design process should continue or whether the design process is finished. If the process should continue, you then send another DesignAggregations command that contains the Steps and Time values with which this step of the design process is limited. You evaluate the resulting statistics and then determine whether the design process should continue. This iterative process of sending DesignAggregations commands and evaluating the results continues until you reach your goals and have a appropriate set of aggregations defined.

After you have reached the set of aggregations that you want, you send one final DesignAggregations command. This final DesignAggregations command should have its Steps property set to 1 and its Materialize property set to true. By using these settings, this final DesignAggregations command completes the design process and saves the defined aggregation to the aggregation design.

Designing Aggregations Using a Batch Process

You can also design aggregations in a batch process by sending a single DesignAggregations command that contains the Steps, Time, Storage, and Optimization property values on which the whole design process is targeted and limited. If you want usage-based optimization, the goal queries on which the design process is targeted should also be included in the Queries property. Also make sure that the Materialize property is set to true, so that the design process saves the defined aggregations to the aggregation design when the command finishes.

You can design aggregations using a batch process in either an implicit or explicit session. For more information about implicit and explicit sessions, see Managing Connections and Sessions (XMLA).

Returning Design Statistics

When the DesignAggregations command returns control to the client application, the command returns a rowset that contains a single row representing the design statistics for the command. The rowset contains the columns listed in the following table.

Column	Data type	Description
Steps	Integer	The number of steps taken by the command before returning control to the client application.
Time	Long integer	The number of milliseconds taken by the command before returning control to the client application.
Optimization	Double	The estimated percentage of performance improvement achieved by the command before returning control to the client application.
Storage	Long integer	The estimated number of bytes taken by the command before returning control to the client application.
Aggregations	Long integer	The number of aggregations defined by the command before returning control to the client application.
LastStep	Boolean	Indicates whether the data in the rowset represents the last step in the design process. If the Materialize property of the command was set to true, the value of this column is set to true.

You can use the design statistics that are contained in the rowset returned after each DesignAggregations command in both iterative and batch design. In iterative design, you can use the design statistics to determine and display progress. When you are designing aggregations in batch, you can use the design statistics to determine the number of aggregations created by the command.

Vedere anche

Concetti

Using XML for Analysis in Analysis Services (XMLA)

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