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https://issues.apache.org/jira/browse/PHOENIX-2989?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15326914#comment-15326914
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James Taylor commented on PHOENIX-2989:
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You should also override the addExpression method as shown here[1] because
otherwise an aggregator will end up being pushed to server and executed row by
row when it's not needed:
{code}
SELECT count(distinct pk1) FROM t ORDER BY count(distinct pk2)
{code}
[1]
https://issues.apache.org/jira/browse/PHOENIX-2989?focusedCommentId=15326744&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-15326744
> Allow DistinctPrefixFilter optimization when HAVING clause only reference
> COUNT(DISTINCT)
> -----------------------------------------------------------------------------------------
>
> Key: PHOENIX-2989
> URL: https://issues.apache.org/jira/browse/PHOENIX-2989
> Project: Phoenix
> Issue Type: Sub-task
> Reporter: James Taylor
> Fix For: 4.8.0
>
> Attachments: 2989-orderby.txt
>
>
> The DistinctPrefixFilter optimization can still be used if a HAVING clause
> only references COUNT(DISTINCT) expressions. One way to detect this is to
> collect a Set<ParseNode> using a visitor for the SELECT and HAVING which only
> collects COUNT(DISTINCT) expressions. This set will then be used as the GROUP
> BY nodes if there's no existing GROUP BY.
> The check for whether or not to add the filter can then change to something
> like this:
> {code}
> if (... &&
> ( context.getAggregationManager().isEmpty() ||
> ( plan.getGroupBy().isUngroupedAggregate() &&
> plan.getGroupBy().getKeyExpressions().size() ==
> context.getAggregationManager().getAggregators().getAggregatorCount()
> ) ) )
> {code}
> That way, it'll only add the filter if all expressions pulled in as a GROUP
> BY expression (only the count distinct ones) account for all of the
> aggregators.
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