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https://issues.apache.org/jira/browse/CALCITE-6020?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17873642#comment-17873642
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Julian Hyde commented on CALCITE-6020:
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I regard SUM0 as a more fundamental operation than SUM - it matches the "+"
operator in algebra, for instance - and suspect that we should use it whenever
applicable.
{{AggregateReduceFunctionsRule}} doesn't seem to be a good place for this logic
to live. AggregateReduceFunctionsRule matches an {{Aggregate}}, and window
functions never occur in an Aggregate. Maybe we also need a
{{WindowReduceFunctionsRule}} (because window aggregates usually occur in a
{{Window}} - although they also occur in a {{Project}}, at a different stage of
planning). And then move the logic for particular window/aggregate functions to
a neutral location (where whether-this-function-is-applied-to-a-window or
whether-this-function-is-applied-to-a-group-that-may-be-empty are part of the
context of the rewrite).
> SqlToRelConverter should not replace windowed SUM with equivalent expression
> using SUM0
> ---------------------------------------------------------------------------------------
>
> Key: CALCITE-6020
> URL: https://issues.apache.org/jira/browse/CALCITE-6020
> Project: Calcite
> Issue Type: Improvement
> Reporter: Zoltan Haindrich
> Assignee: Zoltan Haindrich
> Priority: Major
> Labels: pull-request-available
>
> {{SqlToRelConverter}} replaces {{SUM}} with {{SUM0}} around
> [here|https://github.com/apache/calcite/blob/e1991e08a225ef08c2402ab35c310d88fff3c222/core/src/main/java/org/apache/calcite/sql2rel/SqlToRelConverter.java#L5885]
> This might have been needed at some point in the past - but I think it will
> be better to leave it as {{SUM}} - as in case there is no {{SUM0}} in the
> system that will be replaced with a {{COALESCE(SUM(...) , 0 )}} to provide it
> - as see
> [here|https://github.com/apache/calcite/blob/e1991e08a225ef08c2402ab35c310d88fff3c222/core/src/test/java/org/apache/calcite/rel/rel2sql/RelToSqlConverterTest.java#L1288]
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