[GitHub] [incubator-druid] vogievetsky opened a new issue #8947: Unnecessary cartesian explosion if multi-value column is reused in expression

[GitBox <gi...@apache.org>]

vogievetsky opened a new issue #8947: Unnecessary cartesian explosion if multi-value column is reused in expression
URL: https://github.com/apache/incubator-druid/issues/8947
 
 
   ### Affected Version
   
   Druid 0.16, 0.15
   
   ### Description
   
   With this dataset (as `fs`):
   
   ```json
   {"time":"2019-08-14T00:00:00.000Z","srcGroups":["x","y","z"],"dstGroups":["a","b","c","d"]}
   {"time":"2019-08-14T00:00:00.000Z","srcGroups":["x","y","z"],"dstGroups":["a","c","d"]}
   {"time":"2019-08-14T00:00:00.000Z","srcGroups":["x","y","z"],"dstGroups":["a","g"]}
   ```
   
   Doing this query:
   
   ```sql
   SELECT
     CASE "dstGroups"
       WHEN 'b' THEN 'b'
       WHEN 'g' THEN 'g'
       ELSE 'Other'
     END AS "dst",
     COUNT(*) AS "Count"
   FROM "fs"
   GROUP BY 1
   ORDER BY "Count" DESC
   ```
   
   Yields an unexpected cartesian explosion:
   
   ![image](https://user-images.githubusercontent.com/177816/69681810-1eef6180-1064-11ea-8a08-16a1e8599b45.png)
   
   This is due to this expression in the underlying plan:
   
   `"case_searched((\"dstGroups\" == 'b'),'b',(\"dstGroups\" == 'g'),'g','Other')"`
   
   Which triggers a cartesian product on the same column `dstGroups` which it should not do.

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