liyang created KYLIN-2617:
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Summary: SUM when rewritten as TOPN does not give consistent
(correct) result?
Key: KYLIN-2617
URL: https://issues.apache.org/jira/browse/KYLIN-2617
Project: Kylin
Issue Type: Bug
Reporter: liyang
>From Tingmao Lin <z...@msn.com>
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.
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