MongoDB gets from indexes information about whether a path is multikey and uses it to determine if this patch can have array data. It uses this information to eliminate traverseF and make the computations faster. If the path is not a multikey, PathArrayness concludes that it cannot be an array and omits traverseF. In geospatial indexes like 2dsphere, a field storing coordinates as [lon,lat] is treated as a single point and the path is not marked as multikey. Hence, PathArrayness is false and traverseF is omitted. However, the field is really an array and traverseF should be used. In addition to geo indexes we also exclude all non-BTREE indexes from contributing to PathArrayness because only BTREE indexes reliably reflect array semantics through their multikey metadata.