Multiple Active Result Sets

SQL Server 2005 has so many new features , One of those is Multiple Active Result Sets or MARS. Multiple Active Result Sets is a new SQL Server 2005 feature that, putting it simply, allows the user to run more than one SQL batch on an open connection at the same time.

Pre-SQL 2005 era

In SQL Server's prior to 2005, you could only run one batch per connection. This means simply that you could only do this:

private void MARS_Off()
{
    SqlConnection conn = new SqlConnection("Server=serverName;
        Database=adventureworks;Trusted_Connection=yes;");

    string sql1 = "SELECT * FROM [Person].[Address]";
    string sql2 = "SELECT * FROM [Production].[TransactionHistory]";

    SqlCommand cmd1 = new SqlCommand(sql1, conn);
    SqlCommand cmd2 = new SqlCommand(sql2, conn);
    cmd1.CommandTimeout = 500;
    cmd2.CommandTimeout = 500;
    conn.Open();
    SqlDataReader dr1 = cmd1.ExecuteReader();
    // do stuff with dr1 data
    conn.Close();

    conn.Open();
    SqlDataReader dr2 = cmd2.ExecuteReader();
    // do stuff with dr2 data
    conn.Close();
}

And the accompanying profiler trace:

This example shows that you could use the same connection with the second SqlDataReader only when you finished using the connection with first one. The connection must be closed and reopened as it is shown with Audit Login and Audit Logout events. Opening and closing a connection is an expensive operation so this can hurt performance, even if your connection is stored in the connection pool.

If you for instance wanted to do some processing of the data in your data reader and updating the processed data back to the database you had to use another connection object which again hurts performance. There was no way to use the same opened connection easily for more than one batch at the time. There are of course server side cursors but they have drawbacks like performance and ability to operate only on a single select statement at the time.

SQL 2005 era

SQL Server 2005 team recognized the above mentioned drawback and introduced MARS. So now it is possible to use a single opened connection for more than one batch. A simple way of demonstrating MARS in action is with this code:

private void MARS_On()
{
    SqlConnection conn = new SqlConnection("Server= serverName;Database=adventureworks;
        Trusted_Connection=yes;MultipleActiveResultSets=true;");
    
    string sql1 = "SELECT * FROM [Person].[Address]";
    string sql2 = "SELECT * FROM [Production].[TransactionHistory]";

    SqlCommand cmd1 = new SqlCommand(sql1, conn);
    SqlCommand cmd2 = new SqlCommand(sql2, conn);
    cmd1.CommandTimeout = 500;
    cmd2.CommandTimeout = 500;
    conn.Open();
    SqlDataReader dr1 = cmd1.ExecuteReader();
    SqlDataReader dr2 = cmd2.ExecuteReader();
    conn.Close(); 
}

And the accompanying profiler trace:

MARS is disabled by default on the Connection object. You have to enable it with the addition of MultipleActiveResultSets=true in your connection string.