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https://issues.apache.org/jira/browse/CALCITE-3221?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16897439#comment-16897439
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Stamatis Zampetakis edited comment on CALCITE-3221 at 7/31/19 10:49 PM:
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I thought of a couple of generalizations. (Feel free to disregard.)
First, allow this operator to take more than 2 inputs.
Second, allow it to compute aggregate functions, and allow it to take as few as
1 input. With aggregate functions and 1 input, this becomes the sort-based
aggregate algorithm we've wanted forever (see CALCITE-853). With aggregate
functions and 2 or more inputs, it becomes a combined UNION ALL and GROUP BY
(Union all=true followed by Aggregate).
was (Author: julianhyde):
I thought of a couple of generalizations. (Feel free to disregard.)
First, allow this operator to take more than 2 inputs.
Second, allow it to compute aggregate functions, and allow it to take as few as
1 input. With aggregate functions and 1 input, this becomes the sort-based
aggregate algorithm we've wanted forever (see CALCITE-8530). With aggregate
functions and 2 or more inputs, it becomes a combined UNION ALL and GROUP BY
(Union all=true followed by Aggregate).
> 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: 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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