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From ShaoFeng Shi <shaofeng...@apache.org>
Subject Re: Extend Column Is Empty
Date Thu, 08 Nov 2018 02:58:30 GMT
Kaisen introduced extended column in his blog:
https://blog.bcmeng.com/post/kylin-dimension.html#extended-column

ShaoFeng Shi <shaofengshi@apache.org> 于2018年11月7日周三 上午9:15写道:

> One correction: from the code I see, when the extended column is not
> determined, it will use the first appeared value; In your case, the first
> value might be all blank, so it appeared like all empty.
> The related code is:
> https://github.com/apache/kylin/blob/master/core-metadata/src/main/java/org/apache/kylin/measure/extendedcolumn/ExtendedColumnMeasureType.java#L281
>
> This measure has such limitations, please not use it if you are not sure
> about it. I'm thinking whether we need to disable it.
>
> ShaoFeng Shi <shaofengshi@apache.org> 于2018年11月7日周三 上午9:04写道:
>
>> Hi Leo,
>>
>> "Extended column" is a special measure, which is to store a dimension
>> value as a measure. It doesn't support aggregation, so if there are
>> multiple values in this "measure", it will be empty.
>>
>> Let me give an example.
>>
>> Say you have a table, which has dimensions like "*user_id*", "*user_name*",
>> "*age*"; Here the "*user_id*" is unique, which will be used to do
>> filtering/searching; The "user_name" will not be used for filtering, but it
>> is nice to show the value in the result. So, in this case, we can declare
>> "user_id" and "age" as dimensions, but define "user_name" as an extended
>> column. Then the cube will have 2 dimensions instead of 3, its size will be
>> smaller.
>>
>> When you run a query like "select user_id, user_name, count(*) from
>> fact_table group by user_id, user_name", it will be expected, because the
>> "user_name" value is decided (明确的) under the context of a "user_id";
>>
>> But if you run a query without "user_id", for example, "select age,
>> user_name, count(*) from fact_table group by age, user_name", this
>> column will be empty, because it has no "user_id" in the row. The extended
>> column can not work in the query independently
>>
>> So, the conclusion is, the extended column(s) need to appear together
>> with its "host" column. If its "host" column is absent, the value will be
>> empty.
>>
>> We will add this to Kylin document.
>>
>>
>>
>> Francis Liang <soulvagrant@hotmail.com> 于2018年11月5日周一 下午5:21写道:
>>
>>> Actually you need at least one aggregation, such as sum(lost_day_cnt),
>>> or you may try select a, b from table group by a, b, which should not be
>>> the normal case however. Hope this makes sense. Best, Feng.
>>>
>>>
>>>
>>> *发件人: *宋海涛 <htsong@gaojihealth.com>
>>> *发送时间: *2018年11月5日 17:16
>>> *收件人: *user@kylin.apache.org
>>> *主题: *答复: Extend Column Is Empty
>>>
>>>
>>>
>>> select zone, data_from,ent_name,org_no, org_name, goods_id,goods_spec,
>>> big_mc,mid_mc,sma_mc,mini_mc,lost_day_cnt,
>>>
>>> lost_amt,avg_paid_in_amt,dt
>>>
>>> from gjst.st_goods_lost_monitor_detail_day  where dt='20181102' and
>>>
>>> zone='西南平台' and data_from='haitang' and org_no='16208'
>>>
>>>
>>>
>>> *发件人:* Francis Liang [mailto:soulvagrant@hotmail.com]
>>> *发送时间:* 2018年11月5日 17:14
>>> *收件人:* user@kylin.apache.org
>>> *主题:* 答复: Extend Column Is Empty
>>>
>>>
>>>
>>> Could you share your sql statement? Thanks. Best, Feng.
>>>
>>>
>>>
>>> *发件人**:* 宋海涛 <htsong@gaojihealth.com>
>>> *发送时间**:* Monday, November 5, 2018 4:58:53 PM
>>> *收件人**:* user@kylin.apache.org
>>> *主题**:* Extend Column Is Empty
>>>
>>>
>>>
>>> Hi guys,
>>>
>>>
>>>
>>>          When I run a SQL which contains extended columns, I got empty
>>> values for extended columns . Anyone knows why?
>>>
>>> For example , ENT_NAME and ORG_NAME are extended column based on
>>> DATA_FROM and ORG_NO:
>>>
>>> [image: cid:image001.png@01D4752B.489B5550]
>>>
>>>
>>>
>>> Thanks a lot.
>>>
>>>
>>>
>>> Leo
>>>
>>>
>>>
>>
>>
>> --
>> Best regards,
>>
>> Shaofeng Shi 史少锋
>>
>>
>
> --
> Best regards,
>
> Shaofeng Shi 史少锋
>
>

-- 
Best regards,

Shaofeng Shi 史少锋

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