presto-main/src/main/java/io/prestosql/operator/DriverContext.java/DriverContext/addOperatorContext
class DriverContext:    public OperatorContext addOperatorContext(int operatorId, PlanNodeId planNodeId, String operatorType)
    {
        checkArgument(operatorId >= 0, "operatorId is negative");

        for (OperatorContext operatorContext : operatorContexts) {
            checkArgument(operatorId != operatorContext.getOperatorId(), "A context already exists for operatorId %s", operatorId);
        }

        OperatorContext operatorContext = new OperatorContext(
                operatorId,
                planNodeId,
                operatorType,
                this,
                notificationExecutor,
                driverMemoryContext.newMemoryTrackingContext());
        operatorContexts.add(operatorContext);
        return operatorContext;
    }