presto-main/src/main/java/io/prestosql/sql/planner/iterative/rule/PushProjectionThroughExchange.java/PushProjectionThroughExchange/extractExchangeOutputToInput
class PushProjectionThroughExchange:    private static Map<Symbol, SymbolReference> extractExchangeOutputToInput(ExchangeNode exchange, int sourceIndex)
    {
        Map<Symbol, SymbolReference> outputToInputMap = new HashMap<>();
        for (int i = 0; i < exchange.getOutputSymbols().size(); i++) {
            outputToInputMap.put(exchange.getOutputSymbols().get(i), exchange.getInputs().get(sourceIndex).get(i).toSymbolReference());
        }
        return outputToInputMap;
    }