Re: [PR] [CALCITE-6431] Implement the SINGLE_VALUE aggregation in HiveSqlDiale… [calcite]

[via GitHub]

YiwenWu commented on code in PR #3817:
URL: https://github.com/apache/calcite/pull/3817#discussion_r1642057291


##########
core/src/main/java/org/apache/calcite/util/RelToSqlConverterUtil.java:
##########
@@ -93,6 +101,41 @@ public static void unparseSparkArrayAndMap(SqlWriter writer,
     writer.endList(frame);
   }
 
+
+  /**
+   * Rewrite SINGLE_VALUE use SparkSQL Mode.
+   *
+   * <blockquote><pre>
+   * CASE COUNT(*)
+   * WHEN 0 THEN NULL
+   * WHEN 1 THEN MIN(&lt;result&gt;)
+   * ELSE (SELECT NULL UNION ALL SELECT NULL))

Review Comment:
   > > 1. SELECT NULL UNION ALL SELECT NULL
   > 
   > this is intentional to make sure when the subquery include more then one 
value ,This SQL will throw exception.
   > 
   > 2: Suggestion: Use COUNT(field) instead of COUNT(*)
   > 
   > Please check 
[CALCITE-6430](https://issues.apache.org/jira/browse/CALCITE-6430).
   
   got it, thanks. Based on count(*) and union all, the result of the subquery 
is guaranteed to be only one row.



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