presto-main/src/main/java/io/prestosql/sql/planner/optimizations/HashGenerationOptimizer.java/HashGenerationOptimizer/getHashExpression
class HashGenerationOptimizer:        private Expression getHashExpression(Metadata metadata, TypeProvider types)
        {
            Expression hashExpression = new GenericLiteral(StandardTypes.BIGINT, Integer.toString(INITIAL_HASH_VALUE));
            for (Symbol field : fields) {
                hashExpression = getHashFunctionCall(hashExpression, field, metadata, types);
            }
            return hashExpression;
        }