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https://issues.apache.org/jira/browse/CALCITE-3671?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17008646#comment-17008646
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Stamatis Zampetakis commented on CALCITE-3671:
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Regarding (c) should we really generate a hash/merge join when there are no
equality predicates? Is there any benefit in doing so?
> Join cost computation should consider join condition (equi vs non-equi)
> -----------------------------------------------------------------------
>
> Key: CALCITE-3671
> URL: https://issues.apache.org/jira/browse/CALCITE-3671
> Project: Calcite
> Issue Type: Improvement
> Affects Versions: 1.21.0
> Reporter: Ruben Q L
> Priority: Major
>
> In some Join algorithms, the actual cost of performing the join would depend
> on whether or not the join conditions is an equi-join or not, therefore
> computeSelfCost should reflect that.
> This would be the case for example of HashJoin (which now supports all type
> of join condition, see CALCITE-2973) or MergeJoin (idem, CALCITE-3285).
> To sump up, we can have three different scenarios:
> a) The condition is a "complete equi-join condition"; this is the best case
> scenario, the join is performed purely on a hash/merge based algorithm and no
> extra predicate is required.
> b) The condition is a "partial equi-join conditiom", i.e. the condition
> contains some equi-join items, but also some non-equi-join items; in this
> case the join is performed on a hash/merge based algorithm (for the equi-join
> items) + an extra predicate (for the non-equi-join ones).
> c) The join condition is a "complete non-equi-join-condition", i.e. there are
> no equi-join elements to build a hash/merge based solution, so the algorithm
> is performed based on a predicate which evaluates the whole condition. This
> is the worst-case scenario, since the Hash/Merge Join actually behaves as a
> kind of de-facto nested loop join.
> Currently, since the condition nature is not evaluated in the
> computeSelfCost, cases a-b-c would have an equivalent cost; we should reflect
> somehow that: cost a < cost b < cost c
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