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https://issues.apache.org/jira/browse/CALCITE-3221?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17273118#comment-17273118
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Julian Hyde commented on CALCITE-3221:
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[~amaliujia], Yes, that sounds about right.
> 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: 12h
> Remaining Estimate: 0h
>
> Currently, the union operation offered by Calcite (see
> [EnumerableDefaults.union|https://github.com/apache/calcite/blob/d98856bf1a5f5c151d004b769e14bdd368a67234/linq4j/src/main/java/org/apache/calcite/linq4j/EnumerableDefaults.java#L2747])
> "breaks" the collation (if any) of its inputs.
> The goal of this issue is to create a new union algorithm
> (EnumerableMergeUnion) that, given the fact that its inputs are sorted by the
> same collation, will return the union / union all result respecting this
> collation.
> Most likely the implementation of the merge join can be useful.
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