Stamatis Zampetakis reassigned CALCITE-3221:

    Assignee:     (was: Stamatis Zampetakis)

> 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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