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https://issues.apache.org/jira/browse/CALCITE-2216?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16402239#comment-16402239
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Julian Hyde commented on CALCITE-2216:
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In my experience "hidden fields" (especially implicit time fields) are an
anti-pattern. Consider making an the group key or accumulator a tuple that
includes all of the fields you need, then throw away those fields when
producing a result.
> Improve extensibility of AggregateReduceFunctionsRule
> -----------------------------------------------------
>
> Key: CALCITE-2216
> URL: https://issues.apache.org/jira/browse/CALCITE-2216
> Project: Calcite
> Issue Type: Improvement
> Components: core
> Affects Versions: 1.15.0
> Reporter: Fabian Hueske
> Assignee: Julian Hyde
> Priority: Minor
>
> I'm proposing to improve the extensibility of
> {{AggregateReduceFunctionsRule}}. The purpose of the rule is to decompose
> complex aggregation functions like {{VAR_POP}} and {{STDDEV_SAMP}} into
> {{COUNT}} and {{SUM}} functions and compute the original functions in a
> subsequent Calc operator.
> Right now, the rule class provides a {{protected}} method that can be
> overridden to create an {{Aggregate}} with the updated aggregate calls.
> We are using the rule in Flink and have a special {{Aggregate}} Rel for
> group-windowed aggregations ({{GROUP BY TUMBLE/HOP/SESSION}}). Our
> implementation requires to forward some additional fields from the
> {{Aggregate}} for window properties like {{TUMBLE_START}} or {{HOP_END}}. In
> the current form, we cannot extend the rule, because these fields are striped
> off by the {{Calc}} node that is automatically added by the rule.
> I'm proposing to also move the code to create the {{Calc}} into a
> {{protected}} method just like the code to create the new {{Aggregate}}.
> I know, this is a fairly Flink-specific issue, but the code changes are
> minimal (no change in functionality) and it would help us, because we would
> not need to copy the rule and maintain it in Flink.
> I'll open a PR for this. Looking forward to your comments.
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