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From Maryann Xue <maryann....@gmail.com>
Subject Re: Partial aggregation in Drill-on-Phoenix
Date Tue, 06 Oct 2015 18:03:14 GMT
The partial aggregate seems to be working now, with one interface extension
and one bug fix in the Phoenix project. Will do some code cleanup and
create a pull request soon.

Still there was a hack in the Drill project which I made to force 2-phase
aggregation. I'll try to fix that.

Jacques, I have one question though, how can I verify that there are more
than one slice and the shuffle happens?


On Mon, Oct 5, 2015 at 2:03 PM, James Taylor <jamestaylor@apache.org> wrote:

> Maryann,
> I believe Jacques mentioned that a little bit of refactoring is required
> for a merge sort to occur - there's something that does that, but it's not
> expected to be used in this context currently.
> IMHO, there's more of a clear value in getting the aggregation to use
> Phoenix first, so I'd recommend going down that road as Jacques mentioned
> above if possible. Once that's working, we can circle back to the partial
> sort.
> Thoughts?
> James
> On Mon, Oct 5, 2015 at 10:40 AM, Maryann Xue <maryann.xue@gmail.com>
> wrote:
>> 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?
>> Thanks,
>> Maryann
>> 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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