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https://issues.apache.org/jira/browse/CALCITE-7031?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=18035757#comment-18035757
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Zhen Chen commented on CALCITE-7031:
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Thank you very much, [~dongsl] , for submitting a new draft proposal for
decorrelation. I've also seen your summary comparing the differences between
the new approach and the existing implementation. I'm not very familiar with
this area, but I'm very willing to learn. If these points in the new
implementation are indeed difficult to achieve with the current framework, then
I believe it's necessary to have a new decorrelation algorithm. Of course, our
conventional thinking is to fix the flaws in the existing framework bit by bit,
but we shouldn't outright oppose a new implementation. If compatibility with
the older version is a concern, having both algorithms coexist and allowing
users to choose could also be a good option.
> Implement the general decorrelation algorithm (Neumann & Kemper)
> ----------------------------------------------------------------
>
> Key: CALCITE-7031
> URL: https://issues.apache.org/jira/browse/CALCITE-7031
> Project: Calcite
> Issue Type: Wish
> Components: core
> Affects Versions: 1.39.0
> Reporter: Mihai Budiu
> Priority: Minor
> Labels: pull-request-available
>
> Today Calcite uses a heuristic decorrelator which looks for some specific
> patterns and replaces them with equivalent ones. This can handle a restricted
> set of queries.
> There is a general algorithm for doing this, at least for the standard
> operators:
> https://github.com/lonng/db-papers/blob/main/papers/nested-query/unnesting-arbitrary-queries.pdf
> The algorithm is a series of rewrites rules, each of which pushes the
> decorrelation operators down in the plan towards leaves, until they can be
> eliminated.
> It would be great if Calcite had an implementation of this algorithm. This
> could be contributed in pieces, where each rewrite rule is a separate PR. I
> think these rules could be easily implemented in the existing planner rewrite
> framework.
> There are three design problems that I can foresee:
> 1) The plans generated by this algorithm are in general DAGs and not trees. I
> think trees would work, but they have the potential to be much larger than
> the corresponding DAGs (perhaps exponentially larger in the size of the
> original plan). I understand that there is a Calcite operator for
> representing DAGs; I don't know if that's the goal of the Spool operator - it
> seems to imply some kind of materialization of the result. The most important
> decision is how to represent DAG plans such that the rewriting framework
> continues to operate correctly.
> 2) A second issue is that the rewrite rules in the paper use a restricted
> form of relation D which has some nice properties (e.g., it is a set). I am
> not sure how such information can be represented in a Calcite plan, but I
> suspect this can be done.
> 3) Third, while the algorithm in the paper handles many SQL-like operators,
> the Calcite IR is even richer, supporting operators like Window, Cubes,
> Unnest, recursive queries, etc. I don't know how the algorithm would extend
> to plans containing such operators. But even if it doesn't handle all such
> operators, a general-purpose decorrelator would be a significant improvement
> over the existing one.
> This project would also give us the chance to close many issues related to
> the current decorrelator, some of which have been unsolved over many years.
> Please comment here if you are interested in this project. I think the most
> important problem to address is no 1 above, the handling of DAGs.
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