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From ShaoFeng Shi <shaofeng...@apache.org>
Subject Re: select column returns null values
Date Mon, 24 Sep 2018 06:28:55 GMT
Hi Hari,

You're right that Cube has joined data (from fact and lookup tables), and
then aggregate them by the dimensions. But Kylin doesn't keep the raw data
after the cube be built, only keep the cuboid (group by certain
dimensions), so couldn't answer the "select * " without "group by" query.

The logic to pushdown is quite simple today. Please look at
QueryService#executeRequest();
When Cube throws an exception, it redirects to the pushdown engine.

hosur narahari <hnr1992@gmail.com> 于2018年9月21日周五 上午11:11写道:

> Hi ShaoFeng,
>
> Thanks for your response. I've few follow up questions.
>
> 1. In FAQ, it's mentioned that "The cube only has aggregated data, so all
> your queries should be aggregated queries". But since cube is created by
> joining based on various dimensions, shouldn't joins without aggregation be
> valid cube querying scenarios(I hope it's already being done).
>
> 2. "select price from " statement should also be pushed down, the rule can
> be optimized for it. Can you point me to documentation for specifying
> pushdown rules. It would be of big help.
>
> Thanks & Regards,
> Hari
>
> On Thu, Sep 20, 2018 at 5:27 PM ShaoFeng Shi <shaofengshi@apache.org>
> wrote:
>
>> Hi Hosur,
>>
>> Firstly, please check Kylin's FAQ, search "Why I got an error when
>> running a “select * “ query?":
>> https://kylin.apache.org/docs/gettingstarted/faq.html
>>
>> Maybe the "select price from " statement should also be pushed down, the
>> rule of pushdown can be optimized for it.
>>
>> hosur narahari <hnr1992@gmail.com> 于2018年9月20日周四 上午11:15写道:
>>
>>> Hi,
>>>
>>> I was trying out kylin with sample-cube example. And when I run
>>> "select * from kylin_sales" query, it returns all the values correctly. But
>>> when
>>> I run "select price from kylin_sales", it returns null in all rows.
>>> Price also was coming correctly in first scenario.
>>>
>>> One more thing is, first query is pushed down to hive and second one is
>>> fetched from cube. And yes, I've enabled pushdown.
>>>
>>> Please let me know if I'm missing something.
>>>
>>> Best Regards,
>>> Hari
>>>
>>
>>
>> --
>> Best regards,
>>
>> Shaofeng Shi 史少锋
>>
>>

-- 
Best regards,

Shaofeng Shi 史少锋

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