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https://issues.apache.org/jira/browse/CALCITE-4334?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17437548#comment-17437548
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Julian Hyde commented on CALCITE-4334:
--------------------------------------
Dev branch:
[julianhyde/4334-group-id|https://github.com/julianhyde/calcite/tree/4334-group-id].
> TRUE_AGG, an aggregate function that always returns TRUE
> --------------------------------------------------------
>
> Key: CALCITE-4334
> URL: https://issues.apache.org/jira/browse/CALCITE-4334
> Project: Calcite
> Issue Type: Bug
> Reporter: Julian Hyde
> Priority: Major
>
> It would be useful to have an (internal) aggregate function that has no
> arguments and returns a constant value, regardless of how many rows are in
> the group. We suggest {{TRUE_AGG}}, which always returns the {{BOOLEAN}}
> value {{TRUE}}.
> If there are reasons why it would be better to return other constants (such
> as 0, 1, FALSE or the empty string) we will consider them.
> For example, when rewriting sub-queries (see SubQueryRemoveRule) we introduce
> add "true as indicator" to the SELECT clause of sub-queries. It can be used
> to detect rows generated by an outer join. If it is an aggregate query, we
> would have to write "min(true) as indicator", which necessitates an extra
> {{Project}} below the {{Aggregate}} to provide the "true" value. A TRUE_AGG
> aggregate function allows us to avoid the extra {{Project}}.
> Another example came up during CALCITE-4317. We would like to make
> {{RelBuilder.aggregate(groupKey())}} throw when given an empty group key and
> no aggregate calls. (Because it would create an {{Aggregate}} that has zero
> fields, and that is problematic elsewhere in Calcite.) But we would also like
> a pattern where an aggregate with an empty group key becomes a constant
> single-row relational expression. So, {{RelBulder.aggregate(groupKey(),
> aggregateCall(TRUE_AGG))}} should generate {{VALUES TRUE}}.
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