单件为我们提供了一种可以将 SQL 语句中每一个 UDF 进行聚集计算时需要的数据存储在另外的对象当中的方式。

The singleton provides us with the means to store another object that is used for the aggregation done in each UDF in the SQL statement.

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世博会有别于其他一般的展览会，它不是以单件展品为对象，而是以城市为对象，从某种意义上讲，城市是世博会最大的展品。

Expo rather than the exhibition, it is not a single pieces of the object, but in cities as an object, in a sense, the city is the largest World Expo exhibits.

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