impala在一个select中执行多个count distinct时会报错，比如执行

select key, count(distinct column_a), count(distinct column_b) from test_table group by key

Query submitted at: 2019-09-28 00:34:20 (Coordinator: http://DataOne-001:25000)
ERROR: AnalysisException: all DISTINCT aggregate functions need to have the same set of parameters as count(DISTINCT column_a);
deviating function: count(DISTINCT column_b)
Consider using NDV() instead of COUNT(DISTINCT) if estimated counts are acceptable. Enable the APPX_COUNT_DISTINCT query option to
perform this rewrite automatically.

这时有几种方法：

1 使用近似值

1.1 set APPX_COUNT_DISTINCT = true
1.2 count distinct改为ndv，即ndv(column_a)
这两种方法底层实现是一样的，设置APPX_COUNT_DISTINCT会自动将count distinct改写为ndv，ndv全称为（number of distinct values），用到
Cardinality（基数计数），底层实现是类似HLLC（Hyper LogLog Counting）这种概率算法，详见参考；

An aggregate function that returns an approximate value similar to the result of COUNT(DISTINCT col ) , the "number of distinct values". It is much faster than the combination of COUNT and DISTINCT , and uses a constant amount of memory and thus is less memory-intensive for columns with high cardinality.

2 使用精确值

改写为多个子查询然后join，比如

select a.key, a.count_a, b.count_b from
(select key, count(distinct column_a) count_a from test_table group by key) a join
(select key, count(distinct column_b) count_b from test_table group by key) b on a.key = b.key

http://impala.apache.org/docs/build/html/topics/impala_ndv.html#ndv

APPX_COUNT_DISTINCT

http://impala.apache.org/docs/build/html/topics/impala_appx_count_distinct.html

https://stackoverflow.com/questions/39236076/impala-all-distinct-aggregate-functions-need-to-have-the-same-set-of-parameters

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