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From ShaoFeng Shi <shaofeng...@apache.org>
Subject Re: Questions about SUM behavior when rewritten as TOPN
Date Sun, 14 May 2017 14:13:33 GMT
Agree with Yang's points; When cardinality is small than the TopN counter's
capacity, the result should be accurate. I checked the 1.6 source code and
didn't find clue. Please share more information (cube definition and query
logs) for investigation, thanks!

For negtive number in TopN, actually that isn't recommended, as it goes
against TopN's purpose, which is to counting something happened. When
merging two TopN counters, one counter will use another's last element's
number to accumulate (if another is full) on its elements (as a
supplement). If the last element is close to 0, its impaction will be
minor. But if the last element is a big negative value, you know it's
impaction will be considerable!  It doesn't make sense to reduce existing
element's counting value if the last element's value is negative. So please
use it properly in your scenario. Ofcourse, I think Kylin should also add
more checks there.

2017-05-14 17:18 GMT+08:00 Tingmao Lin <Z089@msn.com>:

> The SQL in the original email is exactly what we input in the "insight"
> tab in kylin admin UI.
>
> I do not have access to the host running kylin now ,and I will post the
> detailed log output tomorrow.
>
>
> We reproduced the inaccurate result behavior using a source table with <10
> rows and  I should be able to write a reproduce step tomorrow.
>
> ------------------------------
> *From:* Billy Liu <billyliu@apache.org>
> *Sent:* Sunday, May 14, 2017 12:21 AM
> *To:* user
> *Subject:* Re: Questions about SUM behavior when rewritten as TOPN
>
> Thanks Tingmao for the report.
>
> Could you show us the complete SQL? In your SQL, there is no order by
> statement. If no ORDER BY, the query should not be rewritten into TopN
> measure.
>
> 2017-05-12 23:52 GMT+08:00 Tingmao Lin <Z089@msn.com>:
>
>> Hi,
>>
>> We found that SUM() query on a cardinality 1 dimension is not accurate
>> (or "not correct") when automatically  rewritten as TOPN.
>> Is that the expected behavior of kylin or there are any other issue?
>>
>> We built a cube on a table ( measure1: bigint, dim1_id:varchar,
>> dim2_id:varchar, ... ) using kylin 1.6.0 (Kafka streaming source)
>>
>> The cube has two measures: SUM(measure1) and
>> TOPN(10,sum-orderby(measure1),group by dim2_id) . (other measures
>> omitted)
>> and two dimensions  dim1_id, dim2_id   (other dims omitted)
>>
>> About the source table data:
>> The cardinality of dim1_id  is 1 (same dim1_id for all rows in the
>> source table)
>> The cardinality of dim2_id  is 1 (same dim2_id for all rows in the source
>> table)
>> The possible value of measure1 is [1,0,-1]
>>
>> When we query
>>     "select SUM(measure1) FROM table GROUP BY dim2_id"
>>  =>     the result has one row:"sum=7",
>>       from the kylin logs we found that the query has been automatically  rewritten
>> as TOPN(measure1,sum-orderby(measure1),group by dim2_id)
>>
>> When we write another query to prevent TOPN rewrite, for example:
>>
>>    "select SUM(measure1),count(*) FROM table GROUP BY dim2_id"     =>   one
>> row -- "sum=-2,count=24576"
>>
>>    "select SUM(measure1),count(*) FROM table"
>>              =>   one row -- "sum=-2,count=24576"
>>
>>
>> The result is different (7 and -2) when rewritting to TOPN or not.
>>
>>
>> My question is: are the following behavior "works as expected" ,or TOPN
>> algorithm does not support negative counter values very well , or any issue
>> there?
>>
>>
>> 1. SUM() query  automatically rewritten as TOPN and gives approximated
>> result when no TOPN present in the query.
>>
>> 2. When cardinality is 1, TOPN does not give accurate result.
>>
>>
>>
>>
>> Thanks.
>>
>>
>>
>>
>


-- 
Best regards,

Shaofeng Shi 史少锋

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