private ImmutableMap<Symbol, Integer> makeLayoutFromOutputSymbols(List<Symbol> outputSymbols) {
    if (outputSymbols == null) {
      return ImmutableMap.of();
    }
    ImmutableMap.Builder<Symbol, Integer> outputMappings = ImmutableMap.builder();
    int channel = 0;
    for (Symbol symbol : outputSymbols) {
      outputMappings.put(symbol, channel);
      channel++;
    }
    return outputMappings.buildOrThrow();
  }
This Visitor is responsible for transferring Table PlanNode Tree to Table Operator Tree.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/plan/planner/TableOperatorGenerator.java