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https://issues.apache.org/jira/browse/CALCITE-5787?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=18053754#comment-18053754
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Julian Hyde commented on CALCITE-5787:
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To answer you question, yes, I think {{getInputFieldsUsed}} should return an
{{{}ImmutableBitSet{}}}, with the fields numbered from field 0 of input 0,
through the last field of input 0, followed by field 0 of input 1, and so forth.
There is a simple means to split that bit set into one bit set per input, if
required, using {{RelNode.getInputs()}} and
{{RelNode.getRowType().getFieldCount()}} for each input.
But there also must be a good specification of {{getInputFieldsUsed}} including
what it means for a field to be "used", javadoc that lays explains that
specification, and tests that cover the interesting parts of the specification.
> The RelMdInputFieldsUsed is introduced to track the usage of input fields
> -------------------------------------------------------------------------
>
> Key: CALCITE-5787
> URL: https://issues.apache.org/jira/browse/CALCITE-5787
> Project: Calcite
> Issue Type: New Feature
> Reporter: Rong Rong
> Assignee: Zhen Chen
> Priority: Major
> Labels: pull-request-available
> Fix For: 1.42.0
>
>
> It would be super useful to detect all the input fields used in a {{RelNode}}
> to indicate what's being used from the inputs.
> We have a similar utility to find all input fields referenced for a
> {{RexNode}} via {{RelOptUtil.InputFinder}}. For a relation we can do one step
> further to automatically keep track of all the relevant relations.
> For example,
> - {{Aggregate}} relations: {{RelNode.getInputFieldsUsed}} should return a
> union of the input fields used by {{groupSet}} and all {{aggregateCall}}
> - for each {{aggregateCall}}, we should include all fields used in
> {{{}argList{}}}, {{{}filterArg{}}}, {{{}distinctKeys{}}}, and
> {{{}collation{}}}.
> see relative discussion in CALCITE-5740
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