code: def call(self, step):
      if step.reduce != "union":
          raise LogicError(f"Unknown MapReduceStep type: {step.reduce}")
  
      step_data = self.steps_data[step.values.step_num]
      vars = []
      for row in step_data.get_records():
          var_group = {}
          vars.append(var_group)
          for name, value in row.items():
              if name != "__mindsdb_row_id":
                  var_group[name] = value
  
      data = ResultSet()
  
      substep = step.step
      if type(substep) == FetchDataframeStep:
          query = substep.query
          for var_group in vars:
              markQueryVar(query.where)
              for name, value in var_group.items():
                  replaceQueryVar(query.where, value, name)
              sub_data = self._fetch_dataframe_step(substep)
              if len(data.columns) == 0:
                  data = sub_data
              else:
                  data.add_records(sub_data.get_records())
  
              unmarkQueryVar(query.where)
      elif type(substep) == MultipleSteps:
          data = self._multiple_steps_reduce(substep, vars)
      else:
          raise LogicError(f"Unknown step type: {step.step}")
      return data
  
file_path: mindsdb/mindsdb/api/executor/sql_query/steps/map_reduce_step.py
imports: import copy
  from mindsdb_sql.parser.ast import BinaryOperation, UnaryOperation, Constant
  from mindsdb_sql.planner.steps import MapReduceStep, FetchDataframeStep, MultipleSteps
  from mindsdb.api.executor.sql_query.result_set import ResultSet
  from mindsdb.api.executor.exceptions import LogicError
  from .base import BaseStepCall
  from .fetch_dataframe import FetchDataframeStepCall
label_desc: Use image node_id 1 for calling the MapReduceStepCall obj's underlying member method code with example usage: obj.call(step) and returns: data