In SERVER-113679, we calibrated the cost of spilling to disk for SORT operations. However, we only calibrated for the SORT_SIMPLE and SORT_DEFAULT implementations.

When using a $sort + $limit, the specific topN algorithm is used, which theoretically could also spill to disk. We initially didn't take this into account because:

• The cost formula for topN is already complex and hard to extend for the spilling case
• Usually the $limit values are quite small, and they would have to be really huge (order of 100K) to actually cause spilling