1.One example of a symbolic tree is a representation of an SQL query plan, which has operations like filter, join, union, and project.
符号树的一个示例就是SQL查询计划的表示形式，该查询提供筛选、连接、联合和project等操作。
2.Ability to join, union, aggregate, and otherwise correlate data from multiple sources in a single query
在一个查询中连接、联合、聚集来自多个数据源的数据并使它们相关的能力。