presto-main/src/main/java/io/prestosql/sql/planner/optimizations/UnaliasSymbolReferences.java/UnaliasSymbolReferences/mapExchangeNodeOutputToInputSymbols
class UnaliasSymbolReferences:        private void mapExchangeNodeOutputToInputSymbols(ExchangeNode node)
        {
            checkState(node.getInputs().size() == 1);

            for (int symbolIndex = 0; symbolIndex < node.getOutputSymbols().size(); symbolIndex++) {
                Symbol canonicalOutput = canonicalize(node.getOutputSymbols().get(symbolIndex));
                Symbol canonicalInput = canonicalize(node.getInputs().get(0).get(symbolIndex));

                if (!canonicalOutput.equals(canonicalInput)) {
                    map(canonicalOutput, canonicalInput);
                }
            }
        }