Yeah, we looked into the TopNCounter source code and found that for low cardinality scenario, in TopNCounter, m1=m2=0 so it just sum up the values. But the result still goes wrong. I will collect more information for investigation ASAP :)
I have a question: as Billy Liu said in this thread kylin will check the ORDER BY clause to determine whether to rewrite. But I didn't find any access to SQLdigest.sortColumns in TopNMeasureType.influenceCapabilityCheck(). Does kylin check it elsewhere ? It seems that if sum(measure) is the only measure in the query and the group by column matches, then TopNMeasureType.isTopNCompatibleSum() will pass and the query get rewritten. This confuses the user since they may expect a accurate result for every distinct value of group by column(s). ________________________________ From: ShaoFeng Shi <[email protected]> Sent: Sunday, May 14, 2017 2:13 PM To: user Subject: Re: Questions about SUM behavior when rewritten as TOPN 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 <[email protected]<mailto:[email protected]>>: 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 <[email protected]<mailto:[email protected]>> 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 <[email protected]<mailto:[email protected]>>: 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
