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From Maryann Xue <maryann....@gmail.com>
Subject Re: Partial aggregation in Drill-on-Phoenix
Date Mon, 05 Oct 2015 17:40:46 GMT
I actually tried implementing partial sort with
https://github.com/jacques-n/drill/pull/4, which I figured might be a
little easier to start with than partial aggregation. But I found that even
though the code worked (returned the right results), the Drill side sort
turned out to be a ordinary sort instead of a merge which it should have
been. Any idea of how to fix that?


On Mon, Oct 5, 2015 at 12:52 PM, Jacques Nadeau <jacques@dremio.com> wrote:

> Right now this type of work is done here:
> https://github.com/apache/drill/blob/master/exec/java-exec/src/main/java/org/apache/drill/exec/planner/physical/HashAggPrule.java
> https://github.com/apache/drill/blob/master/exec/java-exec/src/main/java/org/apache/drill/exec/planner/physical/AggPruleBase.java
> With Distribution Trait application here:
> https://github.com/apache/drill/blob/master/exec/java-exec/src/main/java/org/apache/drill/exec/planner/physical/DrillDistributionTraitDef.java
> To me, the easiest way to solve the Phoenix issue is by providing a rule
> that matches HashAgg and StreamAgg but requires Phoenix convention as
> input. It would replace everywhere but would only be plannable when it is
> the first phase of aggregation.
> Thoughts?
> --
> Jacques Nadeau
> CTO and Co-Founder, Dremio
> On Thu, Oct 1, 2015 at 2:30 PM, Julian Hyde <jhyde@apache.org> wrote:
>> Phoenix is able to perform quite a few relational operations on the
>> region server: scan, filter, project, aggregate, sort (optionally with
>> limit). However, the sort and aggregate are necessarily "local". They
>> can only deal with data on that region server, and there needs to be a
>> further operation to combine the results from the region servers.
>> The question is how to plan such queries. I think the answer is an
>> AggregateExchangeTransposeRule.
>> The rule would spot an Aggregate on a data source that is split into
>> multiple locations (partitions) and split it into a partial Aggregate
>> that computes sub-totals and a summarizing Aggregate that combines
>> those totals.
>> How does the planner know that the Aggregate needs to be split? Since
>> the data's distribution has changed, there would need to be an
>> Exchange operator. It is the Exchange operator that triggers the rule
>> to fire.
>> There are some special cases. If the data is sorted as well as
>> partitioned (say because the local aggregate uses a sort-based
>> algorithm) we could maybe use a more efficient plan. And if the
>> partition key is the same as the aggregation key we don't need a
>> summarizing Aggregate, just a Union.
>> It turns out not to be very Phoenix-specific. In the Drill-on-Phoenix
>> scenario, once the Aggregate has been pushed through the Exchange
>> (i.e. onto the drill-bit residing on the region server) we can then
>> push the DrillAggregate across the drill-to-phoenix membrane and make
>> it into a PhoenixServerAggregate that executes in the region server.
>> Related issues:
>> * https://issues.apache.org/jira/browse/DRILL-3840
>> * https://issues.apache.org/jira/browse/CALCITE-751
>> Julian

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