Hyunsik Choi created TAJO-601:
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Summary: Improve distinct aggregation query processing
Key: TAJO-601
URL: https://issues.apache.org/jira/browse/TAJO-601
Project: Tajo
Issue Type: Improvement
Components: planner/optimizer
Reporter: Hyunsik Choi
Assignee: Hyunsik Choi
Fix For: 0.8-incubating
Currently, distinct aggregation queries are executed as follows:
* the first stage: it just shuffles tuples by hashing grouping keys.
* the second stage: it sorts them and executes sort aggregation.
This way executes queries including distinct aggregation functions with only
two stages. But, it leads to large intermediate data during shuffle phase.
This kind of query can be rewritten as two queries:
{code:title=original query}
SELECT grp1, grp2, count(*) as total, count(distinct grp3) as distinct_col from
rel1 group by grp1, grp2;
{code}
{code:title=rewritten query}
SELECT grp1, grp2, sum(cnt) as total, count(grp3) as distinct_col from (
SELECT grp1, grp2, grp3, count(*) as cnt from rel1 group by grp1, grp2, grp3)
tmp1 group by grp1, grp2;
{code}
I'm expecting that this rewrite will significantly reduce the intermediate data
volume and query response time in most cases.
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