Retrieving Data Using the AdomdDataReader

Article
10/31/2023

When retrieving analytical data, the AdomdDataReader object provides a good balance between overhead and interactivity. The AdomdDataReader object retrieves a read-only, forward-only, flattened stream of data from an analytical data source. This unbuffered stream of data enables procedural logic to efficiently process results from an analytical data source sequentially. This makes the AdomdDataReader a good choice when retrieving large amounts of data for display purposes because the data is not cached in memory.

The AdomdDataReader can also increase application performance by retrieving data as soon as it is available, instead of waiting for the complete results of the query to be returned. The AdomdDataReader also reduces system overhead because, by default, this reader stores only one row at a time in memory.

The tradeoff for optimized performance is that the AdomdDataReader object provides less information about retrieved data than other data retrieval methods. The AdomdDataReader object does not support a large object model for representing data or metadata, nor does this object model allow for more complex analytical features like cell writeback. However, the AdomdDataReader object does provide a set of strongly typed methods for retrieving cellset data and a method for retrieving cellset metadata in a tabular format. Additionally, AdomdDataReader implements the IDbDataReader interface to support data binding and for retrieving data using the SelectCommand method, from the System.Data namespace of the Microsoft .NET Framework Class Library.

Retrieving Data from the AdomdDataReader

To use the AdomdDataReader object to retrieve data, you follow these steps:

Create a new instance of the object.

To create a new instance of the AdomdDataReader class, you call the Execute or ExecuteReader method of the AdomdCommand object.
Retrieve data.

As the command runs the query, ADOMD.NET returns the results in the Resultset format, a tabular format as described in the XML for Analysis specification, to flatten the data for the AdomdDataReader object. A tabular format is unusual when querying analytical data considering the variable dimensionality in such data.

ADOMD.NET stores these tabular results in the network buffer on the client until you request them by using one of the following methods:
- Call the Read method of the AdomdDataReader object.
  
  The Read method obtains a row from the query results. You can then pass the name, or the ordinal reference, of the column to the Item property to access each column of the returned row. For example, the first column in the current row is named, ColumnName. Then, either reader[0].ToString() or reader["ColumnName"].ToString() will return the contents of the first column in the current row.
- Call one of the typed accessor methods.
  
  The AdomdDataReader provides a series of typed accessor methods—methods that let you access column values in their native data types. When you know the underlying data type of a column value, a typed accessor method reduces the amount of type conversion required when retrieving the column value, and thereby, provides the highest performance.
  
  Some of the typed accessor methods that are available include GetDateTime, GetDouble, and GetInt32. For a complete list of typed accessor methods, see AdomdDataReader.
Close the reader.

You should always call the Close method when you have finished using the AdomdDataReader object. While an instance of an AdomdDataReader `object is open, the AdomdConnection is being used exclusively by that AdomdDataReader. You will not be able to run any commands on the instance of the AdomdConnection, including creating another AdomdDataReader or System.Xml.XmlReader, until you close the original AdomdDataReader.

Example of Retrieving Data from the AdomdDataReader

The following code example iterates through a AdomdDataReader object, and returns the first two values, as strings, from each row.

If Reader.HasRows Then  
    Do While objReader.Read()  
        Console.WriteLine(vbTab & "{0}" & vbTab & "{1}", _  
            objReader.GetString(0), objReader.GetString(1))  
    Loop  
Else  
  Console.WriteLine("No rows returned.")  
End If  
  
objReader.Close()

if (objReader.HasRows)  
  while (objReader.Read())  
    Console.WriteLine("\t{0}\t{1}", _  
        objReader.GetString(0), objReader.GetString(1));  
else  
  Console.WriteLine("No rows returned.");  
  
objReader.Close();

Retrieving Metadata from the AdomdDataReader

While an instance of an AdomdDataReader object is open, you can retrieve schema information, or metadata, about the current recordset using the GetSchemaTable method. GetSchemaTable returns a DataTable object that is populated with the schema information for the current recordset. The DataTable will contain one row for each column of the recordset. Each column of the schema table row maps to a property of the column returned in the cellset, where ColumnName is the name of the property and the value of the column is the value of the property.

Example of Retrieving Metadata from the AdomdDataReader

The following code example writes out the schema information for an AdomdDataReader object.

Dim schemaTable As DataTable = objReader.GetSchemaTable()  
  
Dim objRow As DataRow  
Dim objColumn As DataColumn  
  
For Each objRow In schemaTable.Rows  
  For Each objColumn In schemaTable.Columns  
    Console.WriteLine(objColumn.ColumnName & " = " & objRow(objColumn).ToString())  
  Next  
  Console.WriteLine()  
Next  
DataTable schemaTable = objReader.GetSchemaTable();

foreach (DataRow objRow in schemaTable.Rows)  
{  
  foreach (DataColumn objColumn in schemaTable.Columns)  
    Console.WriteLine(objColumn.ColumnName + " = " + objRow[objColumn]);  
  Console.WriteLine();  
}

Retrieving Multiple Result Sets

Data mining supports the concept of nested tables, which ADOMD.NET exposes as nested rowsets. To retrieve the nested rowset associated with each row, you call the GetDataReader method.

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