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https://issues.apache.org/jira/browse/CALCITE-2204?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16385600#comment-16385600
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LeoWangLZ commented on CALCITE-2204:
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[~julianhyde] In function propagateCostImprovements0, This subset is already in
the chain being propagated to. This means that the graph is cyclic, and
therefore the cost of this relational expression - not this subset - must be
infinite. it may miss the best plan. I think we should change the way of
propagation. now the algorithm like pre-order, it may use Breadth-first Search.
like
{code:java}
parent21,parent22
parent11, parent12
input{code}
now the order of propagation is input->parent11->parent22->parent12.
use BFS, it maybe input->parent11->parent12->parent21->parent22,
and so, it only concern the relNode in RelSubset instead of RelSubset. it will
no have a cycle in RelSubset, the cost will be propagated normally
> Volcano Planner may not choose the cheapest cost of plan wrongly
> ----------------------------------------------------------------
>
> Key: CALCITE-2204
> URL: https://issues.apache.org/jira/browse/CALCITE-2204
> Project: Calcite
> Issue Type: Bug
> Reporter: LeoWangLZ
> Assignee: Julian Hyde
> Priority: Major
>
> Volcano Planner will propagate the cost improvement to parents that one of
> the inputs has the best plan. But it not propagate to all parents firstly, it
> propagate one parent and go to the parent s of parent. In the way, if one
> parent may propagate to other parent on the same level, and the cost maybe
> less than others, but when propagate the parent again, it will not propagate
> because the cost is already calculated, and it's can't be less then the cost
> of the self.
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