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https://issues.apache.org/jira/browse/CALCITE-4522?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17316148#comment-17316148
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Ruben Q L commented on CALCITE-4522:
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I think this change introduces a regression in EnumerableLimitSort cost
computation, specifically in the rowCount part (issue detected by a test suite
in a downstream project).
EnumerableLimitSort cost formula used to be:
{code:java}
planner.getCostFactory().makeCost(inputRowCount, cpu, 0); // cpu is the nLogM *
bytesPerRow
{code}
After this change, the first parameter (rowCount) of this formula in case of a
Sort with fetch (e.g. an EnumerableLimitSort), will not be the inputRowCount,
but {{readCount=Math.min(inCount, offsetValue + fetchValue);}} (which in
practice in most cases would just be {{offsetValue + fetchValue}}:
{code:java}
planner.getCostFactory().makeCost(offsetValue + fetchValue, cpu, 0); // cpu is
the nLogM * bytesPerRow
{code}
In my understanding this is wrong, since a Sort operator, even with fetch (such
as EnumerableLimitSort) will still need to read and process inputRowCount of
rows (even though it just needs to keep offsetValue + fetchValue rows sorted),
so I think the new formula underestimates the cost of Sort with fetch, and its
first parameter should still be inputRowCount in all cases. Should I create a
ticket to address this issue?
> CPU cost of Sort should be lower if sort keys are empty
> -------------------------------------------------------
>
> Key: CALCITE-4522
> URL: https://issues.apache.org/jira/browse/CALCITE-4522
> Project: Calcite
> Issue Type: Improvement
> Components: core
> Reporter: hqx
> Priority: Minor
> Labels: pull-request-available
> Fix For: 1.27.0
>
> Time Spent: 9h 50m
> Remaining Estimate: 0h
>
> The old method to compute the cost of sort has some problem.
> # When the RelCollation is empty, there is no need to sort, but it still
> compute the cpu cost of sort.
> # use n * log\(n) * row_byte to estimate the cpu cost may be inaccurate,
> where n means the output row count of the sort operator, and row_byte means
> the average bytes of one row .
> Instead, I give follow suggestion.
> # the cpu cost is zero if the RelCollation is empty.
> # let heap_size be min(offset + fetch, input_count), and use input_count *
> max(1, log(heap_size))* row_byte to compute the cpu cost.
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