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From Herman van Hövell tot Westerflier <hvanhov...@questtec.nl>
Subject Re: multiple count distinct in SQL/DataFrame?
Date Wed, 07 Oct 2015 20:56:10 GMT
We could also fallback to approximate count distincts when the user
requests multiple count distincts. This is less invasive than throwing an
AnalysisException, but it could violate the principle of least surprise.



Met vriendelijke groet/Kind regards,

Herman van Hövell tot Westerflier

QuestTec B.V.
Torenwacht 98
2353 DC Leiderdorp
hvanhovell@questtec.nl
+599 9 521 4402


2015-10-07 22:43 GMT+02:00 Reynold Xin <rxin@databricks.com>:

> Adding user list too.
>
>
>
> ---------- Forwarded message ----------
> From: Reynold Xin <rxin@databricks.com>
> Date: Tue, Oct 6, 2015 at 5:54 PM
> Subject: Re: multiple count distinct in SQL/DataFrame?
> To: "dev@spark.apache.org" <dev@spark.apache.org>
>
>
> To provide more context, if we do remove this feature, the following SQL
> query would throw an AnalysisException:
>
> select count(distinct colA), count(distinct colB) from foo;
>
> The following should still work:
>
> select count(distinct colA) from foo;
>
> The following should also work:
>
> select count(distinct colA, colB) from foo;
>
>
> On Tue, Oct 6, 2015 at 5:51 PM, Reynold Xin <rxin@databricks.com> wrote:
>
>> The current implementation of multiple count distinct in a single query
>> is very inferior in terms of performance and robustness, and it is also
>> hard to guarantee correctness of the implementation in some of the
>> refactorings for Tungsten. Supporting a better version of it is possible in
>> the future, but will take a lot of engineering efforts. Most other
>> Hadoop-based SQL systems (e.g. Hive, Impala) don't support this feature.
>>
>> As a result, we are considering removing support for multiple count
>> distinct in a single query in the next Spark release (1.6). If you use this
>> feature, please reply to this email. Thanks.
>>
>> Note that if you don't care about null values, it is relatively easy to
>> reconstruct a query using joins to support multiple distincts.
>>
>>
>>
>
>

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