presto-main/src/main/java/io/prestosql/sql/planner/LocalExecutionPlanner.java/LocalExecutionPlanner/makeLayoutFromOutputSymbols
class LocalExecutionPlanner:        private ImmutableMap<Symbol, Integer> makeLayoutFromOutputSymbols(List<Symbol> outputSymbols)
        {
            ImmutableMap.Builder<Symbol, Integer> outputMappings = ImmutableMap.builder();
            int channel = 0;
            for (Symbol symbol : outputSymbols) {
                outputMappings.put(symbol, channel);
                channel++;
            }
            return outputMappings.build();
        }