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https://issues.apache.org/jira/browse/CALCITE-1172?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16788403#comment-16788403
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Julian Hyde commented on CALCITE-1172:
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It should be safe to merge SUM onto COUNT. For example, {{SELECT SUM(x) FROM
(SELECT COUNT(y) AS x FROM t GROUP BY a, b) GROUP BY a}} is equivalent to
{{SELECT COUNT(y) FROM t GROUP BY a}}.
Maybe it's not possible to apply the rule if the top query has {{GROUP BY ()}},
e.g. testAggregateMerge6. But otherwise it should be OK to merge SUM-COUNT into
COUNT.
In RelOptRulesTest.java, can you format the SQL statements and add \n inside
the strings? Then the SQL strings remain readable. More effort goes into
maintaining these tests than into writing them, so we want to be nice to the
maintainers.
> Add rule to flatten two Aggregate operators into one
> ----------------------------------------------------
>
> Key: CALCITE-1172
> URL: https://issues.apache.org/jira/browse/CALCITE-1172
> Project: Calcite
> Issue Type: Bug
> Reporter: Julian Hyde
> Priority: Major
> Labels: pull-request-available
> Time Spent: 1h
> Remaining Estimate: 0h
>
> If there is an Aggregate on top of an Aggregate, and the top aggregate's
> group key is a subset of the lower aggregate's group key, and the aggregates
> are expansions of rollups, then a rule would convert into a single aggregate.
> For example, SUM of SUM becomes SUM; SUM of COUNT becomes COUNT; MAX of MAX
> becomes MAX; MIN of MIN becomes MIN. AVG of AVG would not match, nor would
> COUNT of COUNT.
> The rule would convert
> {noformat}
> Aggregate({x}, SUM(c1), MIN(c2), MAX(c3))
> Aggregate({x, y}, SUM(c1), MIN(c2), MAX(c3))
> R
> {noformat}
> into
> {noformat}
> Aggregate({x}, SUM(c1), MIN(c2), MAX(c3))
> R
> {noformat}
> The rule would not work if there was anything between the two {{Aggregate}}
> operators.
> The implementation would use {{SqlSplittableAggFunction}} to deduce rollups,
> and in fact would be practically the converse of
> {{AggregateExchangeTransposeRule}} proposed in CALCITE-907.
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