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https://issues.apache.org/jira/browse/CALCITE-4264?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17250355#comment-17250355
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Thomas Rebele commented on CALCITE-4264:
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The purpose of this ticket is just to break the tie where two plans have the
same row cost, but where one plan has lower CPU cost. I had encountered a
similar case as described by [~rubenql]. Including the CPU cost as a secondary
criterion makes the Volcano planner choose the correct plan. This fix is only a
band aid. I agree, the cost model of Calcite needs a major revision in the long
run.
> The query planner should take CPU cost into account
> ---------------------------------------------------
>
> Key: CALCITE-4264
> URL: https://issues.apache.org/jira/browse/CALCITE-4264
> Project: Calcite
> Issue Type: Improvement
> Reporter: Thomas Rebele
> Priority: Major
> Labels: pull-request-available
> Time Spent: 20m
> Remaining Estimate: 0h
>
> Calcite only takes the row count into account when optimizing the queries.
> See [the relevant lines in
> VolcanoCost|https://github.com/apache/calcite/blob/52a57078ba081b24b9d086ed363c715485d1a519/core/src/main/java/org/apache/calcite/plan/volcano/VolcanoCost.java#L98-L116].
> However, two plans might have the same row count, but differ greatly in CPU
> cost. This happens for example when the limit sort rule
> ([CALCITE-3920|https://issues.apache.org/jira/browse/CALCITE-3920]) is
> activated. The row cost is the same, the EnumerableLimitSort only sorts the
> input partially, so has a lower CPU cost.
> Low impact proposal: Compare first the row cost, and only if the row cost is
> equal, compare by CPU cost.
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