Apache Calcite解析自身输出的BigQuery格式SQL失败及疑问

我是Apache Calcite新手，遇到了一个奇怪的问题：

给定简单SQL查询：

select * from orders

使用以下Java代码解析：

SqlParser.Config sqlParserConfig = SqlParser
    .configBuilder()
    .setConformance(SqlConformanceEnum.LENIENT)
    .build();
SqlParser parser = SqlParser.create(sqlQuery, sqlParserConfig);
SqlNode parseAst = parser.parseQuery();
CalciteCatalogReader catalogReader = buildCatalogReader(schema, typeFactory);
SqlValidator.Config validatorConf = SqlValidator.Config.DEFAULT.withSqlConformance(SqlConformanceEnum.BIG_QUERY);
validatorConf = validatorConf.withIdentifierExpansion(true);
SqlValidator validator = SqlValidatorUtil.newValidator(SqlStdOperatorTable.instance(),
    catalogReader, typeFactory,
    validatorConf);
validator.validate(parseAst).toSqlString(BigQuerySqlDialect.DEFAULT).toString();

得到的输出结果：

SELECT ORDERS.o_orderkey, ORDERS.o_custkey, ORDERS.o_orderstatus, ORDERS.o_totalprice, ORDERS.`o_order date`, ORDERS.o_orderpriority, ORDERS.o_clerk, ORDERS.o_shippriority, ORDERS.o_comment
FROM ORDERS AS ORDERS

（注：含空格的列名o_order date使用反引号引用，这部分逻辑合理）

但如果将上述输出的查询字符串用SqlConformanceEnum.BIG_QUERY配置重新解析，会抛出如下错误：

org.apache.calcite.sql.parser.SqlParseException: Lexical error at line 1, column 95.  Encountered: "`" (96), after : ""

我尝试修改解析配置，强制设置处理反引号引用的标识符：

SqlParser.Config sqlParserConfig = SqlParser
    .configBuilder()
    .setConformance(getConformance(SqlConformanceEnum.BIG_QUERY))
    .setQuoting(Quoting.BACK_TICK_BACKSLASH)
    .build();

调整后得到可用的查询结果：

SELECT ORDERS.o_orderkey AS O_ORDERKEY, ORDERS.o_custkey AS O_CUSTKEY, ORDERS.o_orderstatus AS O_ORDERSTATUS, ORDERS.o_totalprice AS O_TOTALPRICE, ORDERS.`o_order date`, ORDERS.o_orderpriority AS O_ORDERPRIORITY, ORDERS.o_clerk AS O_CLERK, ORDERS.o_shippriority AS O_SHIPPRIORITY, ORDERS.o_comment AS O_COMMENT
FROM ORDERS AS ORDERS

现在有两个疑问：

• BigQuery本身使用反引号作为标识符的引用符号，但Calcite的BigQuery默认一致性配置为何采用双引号？
• 循环解析过程中，第一次生成的查询与原始输入不一致，第二次解析后结果才趋于稳定，这种初始不一致的现象原因是什么？

内容的提问来源于stack exchange，提问作者Michael Crampon