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https://issues.apache.org/jira/browse/CALCITE-3221?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17115001#comment-17115001
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Julian Hyde commented on CALCITE-3221:
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[~hyuan], I agree with [~zabetak], and I'm a bit surprised that you don't think
that a new sub-class of RelNode is appropriate if we are using a new algorithm.
We've never really had a discussion about when to create a new sub-class of
RelNode, but I'll point out that a union based on merge will have a different
cost model than a union based on concatenate-then-hash-aggregate. I think that
is sufficient reason to split into different RelNode sub-classes.
> 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
> Priority: Minor
>
> 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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