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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