Deletes all workflow history list items from a specific workflow history list. Also deletes everything from the recycle bin. Note that this script has a bug where it continues after it should be done (infinite loop).

Delete-WorkflowHistoryItems.ps1

if ((Get-PSSnapIn -Name Microsoft.SharePoint.PowerShell -ErrorAction SilentlyContinue) -eq $null){
	Add-PSSnapin -Name Microsoft.SharePoint.PowerShell
}

$url = "http://ccsdev/sites/psc2"
$site = New-Object Microsoft.SharePoint.SPSite($url)
$web = $site.OpenWeb()

try{
	$list = $web.GetList("/sites/psc2/WorkflowHistory")
	
	$cmd = [System.String]::Format("<Method><SetList Scope=`"Request`">{0}</SetList><SetVar Name=`"ID`">{{0}}</SetVar><SetVar Name=`"Cmd`">Delete</SetVar><SetVar Name=`"owsfileref`">{{1}}</SetVar></Method>", $list.ID);
	$q = New-Object Microsoft.SharePoint.SPQuery
	$q.RowLimit = 100
	
	while($list.ItemCount -gt 0){
		$items = $list.GetItems($q)
		Write-Host "$(Get-Date): Deleting $($items.Count) items starting with ID $($items[0].ID)"
		$sbDel = New-Object System.Text.StringBuilder
		$sbDel.Append("<?xml version=`"1.0`" encoding=`"UTF-8`"?><Batch>") | Out-Null
		$ids = New-Object "System.Collections.Generic.List``1[System.Guid]"
		for ($i=0; $i -lt $items.Count; $i++){
			$item = $items[$i]
			$delLine = [System.String]::Format($cmd, $item.ID.ToString(), $item.File.ServerRelativeUrl)
			$sbDel.Append($delLine) | Out-Null
			$ids.Add($item.UniqueId)
		}
		$sbDel.Append("</Batch>") | Out-Null
		
		$web.ProcessBatchData($sbDel.ToString()) | Out-Null
		
		# The line below always failed so we will manually clear the recycle bin
		#$web.RecycleBin.Delete($ids.ToArray())
		$web.RecycleBin.DeleteAll()

		$list.Update()
	}
}
finally{
	if ($web -ne $null){
		$web.Close()
	}
	if ($site -ne $null){
		$site.Close()
	}
}