I have

• notablescans=true in mongod.conf
• sparse index on {cl:1, lc:1}
• index on {clExists:1}

Running on the console, neither:

• db.Example.update({clExists:{$exists:false}, cl: {$exists:false}}, {$set : { clExists: false}});
• db.Example.update({clExists:{$exists:false}}, {$set : { clExists: false}});
  gives no output but exits very quickly

Running:

• db.Example.find({clExists:{$exists:false}, cl:{$exists:false}}).limit(10).forEach(printjson);
  gives "table scans not allowed:jerome5.Example"