[jira] [Created] (CALCITE-4334) TRUE_AGG, an aggregate function that always returns TRUE

["Julian Hyde (Jira)" <ji...@apache.org>]

Julian Hyde created CALCITE-4334:
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             Summary: 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


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.

Another example. 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 to generate a constant single-row relational expression. So, {{RelBulder.aggregate(groupKey(), aggregateCall(TRUE_AGG))}} should generate {{VALUES TRUE}}. 



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