presto-main/src/main/java/io/prestosql/operator/OperatorContext.java/OperatorContext/getNestedOperatorStats
class OperatorContext:    public List<OperatorStats> getNestedOperatorStats()
    {
        Supplier<List<OperatorStats>> nestedOperatorStatsSupplier = this.nestedOperatorStatsSupplier.get();
        return Optional.ofNullable(nestedOperatorStatsSupplier)
                .map(Supplier::get)
                .orElseGet(() -> ImmutableList.of(getOperatorStats()));
    }