Sum results of Windows Search query

Question

I am trying to use PowerShell and Windows Search to get some statistics of my disk usage. This is what I came up for so now:

$sql        = "SELECT System.FileExtension, System.Size FROM SYSTEMINDEX WHERE DIRECTORY = 'C:\Users\...\Documents' AND System.FileExtension = '.docx'"
$provider   = "provider=search.collatordso;extended properties=’application=windows’;" 
$connector  = New-Object System.Data.OleDb.OleDbDataAdapter -Argument $sql, $provider 
$dataset    = New-Object System.Data.DataSet

if ($connector.fill($dataset)) {
    $res = $dataset.Tables[0]

    $res | Group-Object System.FileExtension |
        Select-Object System.FileExtension,
            @{Name='Total';Expression={($_.Group | Measure-Object SYSTEM.Size -Sum).Sum}} |
        Format-Table
}

This works, but it looks kind of ugly. I also found and tried this:

$sql       = "SELECT System.Size FROM SYSTEMINDEX WHERE DIRECTORY = 'C:\Users\...\Documents' AND System.FileExtension = '.docx'"
$provider  = "provider=search.collatordso;extended properties=’application=windows’;" 
$connector = New-Object System.Data.OleDb.OleDbDataAdapter -Argument $sql, $provider 
$dataset   = New-Object System.Data.DataSet

if ($connector.fill($dataset)) {
    $dataset.Tables[0].Compute("Sum(SYSTEM.Size)")
}

which looks a lot nicer, but throws an error:

Cannot find an overload for "Compute" and the argument count: "1"