SharePoint Object model code to delete items from large lists

Scenario: You have thousands of items in a SharePoint list and you want to delete them, in a way that query should not take longer to execute.

Solution:
1. Construct a CAML query for each item in a list you want to delete
2.  You can then pass on the aggregated CAML query for all the items to OOB ProcessBatchData() function as below:-

private static StringBuilder BuildBatchDeleteCommand(SPList list)

{

StringBuilder sbDelete = new StringBuilder();

 

                sbDelete.Append("<?xml version=\"1.0\" encoding=\"UTF-8\"?><Batch>");

                string command = "<Method><SetList Scope=\"Request\">" + list.ID +

                    "</SetList><SetVar Name=\"ID\">{0}</SetVar><SetVar Name=\"Cmd\">Delete</SetVar></Method>";

  

foreach (SPListItem item in list.Items)

{

 sbDelete.Append(string.Format(command, item.ID.ToString())); 
} 
 sbDelete.Append("</Batch>");
 return sbDelete;
}

  string strBatchDelete = BuildBatchDeleteCommand(list).ToString();

string strProcessBatch = site.RootWeb.ProcessBatchData(strBatchDelete);

This will place the items in a recycle bin. So to delete items from the recycle bin use
web.RecycleBin.Delete()

 Refer to below form post to do it using powershell:-
http://sharepoint.stackexchange.com/questions/26542/deleting-all-the-items-from-a-large-list-in-sharepoint