非索引字将从 bts 索引中排除，不可搜索。
非索引字可以减少执行搜索的时间、减小索引大小并帮助避免虚假结果。可以为数据中的常见单词创建定制非索引字列表，也可以使用缺省非索引字列表。要创建定制的非索引字列表，请使用 stopwords 索引参数。
并非每个 Basic Text Search 分析器都支持非索引字。
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