The queryStats in memory structure need not allocate memory for the structures that are not used. Since optimizer metrics collection in queryStats is not enabled by default the optimizer will not allocate the memory for those unless internalCollectQueryOptimizerMetrics is set.
Instead QueryStatsEntry will have a pointer to QueryOptimizerMetrics which will be allocated only if internalCollectQueryOptimizerMetrics is set; QueryOptimizerMetrics in turn is an unordered_map<std::string, OptimizerMetricsBucket>. The query key in the map encodes the properties of the collected data, such as “cqf-collscan_only” etc.