[jira] [Created] (CALCITE-3181) Support limit per group in Window

["Haisheng Yuan (JIRA)" <ji...@apache.org>]

Haisheng Yuan created CALCITE-3181:
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             Summary: Support limit per group in Window
                 Key: CALCITE-3181
                 URL: https://issues.apache.org/jira/browse/CALCITE-3181
             Project: Calcite
          Issue Type: Improvement
          Components: core
            Reporter: Haisheng Yuan


We have a lot of queries like the following to retrieve top N tuples per group:

{code:java}
SELECT x, y FROM
     (SELECT x, y, ROW_NUMBER() OVER (PARTITION BY x ORDER BY y) 
     AS rn FROM t1) t2 WHERE rn <= 3;
{code}

The performance is not good if each group has a lot more tuples than wanted, because we will retrieve and sort all the tuples, instead of just doing a top-N heap sort.

In order to do optimization for this kind of query, we need to extend window to support limit, if and only if there is only 1 window function, and it is {{row_number()}}. We also need a substitute rule to push the limit into window. Of course, we also need to modify executor to support this optimization (can be later).

Thoughts? Objections?



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