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https://issues.apache.org/jira/browse/CALCITE-3221?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17272786#comment-17272786
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Ruben Q L commented on CALCITE-3221:
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I agree with [~julianhyde]: [~vladimirsitnikov]'s suggestion is a valid
proposal for a (separate) optimization of {{EnumerableDefaults#union}}. But
this does not collide with the current ticket's purpose: a new MergeUnion
operator that keeps the collation from its (sorted) inputs.
BTW, I think the PR is in a good shape, if anybody else wants to take a final
look, please go ahead. Otherwise I plan to merge it in the coming days.
> Add a sort-merge union algorithm
> --------------------------------
>
> Key: CALCITE-3221
> URL: https://issues.apache.org/jira/browse/CALCITE-3221
> Project: Calcite
> Issue Type: Improvement
> Components: core
> Affects Versions: 1.19.0
> Reporter: Stamatis Zampetakis
> Assignee: Ruben Q L
> Priority: Minor
> Labels: pull-request-available
> Attachments: screenshot-1.png
>
> Time Spent: 11h 20m
> Remaining Estimate: 0h
>
> Currently, the union operation offered by Calcite is based on a {{HashSet}}
> (see
> [EnumerableDefaults.union|https://github.com/apache/calcite/blob/d98856bf1a5f5c151d004b769e14bdd368a67234/linq4j/src/main/java/org/apache/calcite/linq4j/EnumerableDefaults.java#L2747])
> and necessitates reading in memory all rows before returning a single
> result.
> Apart from increased memory consumption the operator is blocking and also
> destroys the order of its inputs.
> The goal of this issue is to add a new union algorithm (EnumerableMergeUnion
> ?) exploiting the fact that the inputs are sorted which consumes less memory
> and retains the order of its inputs.
> Most likely the implementation of the merge join can be useful.
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