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From Jacques Nadeau <jacq...@dremio.com>
Subject Re: Partial aggregation in Drill-on-Phoenix
Date Wed, 07 Oct 2015 15:54:47 GMT
Drill doesn't currently have a merge-sort operation available outside the
context of an exchange. See here:

https://github.com/apache/drill/tree/master/exec/java-exec/src/main/java/org/apache/drill/exec/physical/impl/mergereceiver

We'll need to do a bit of refactoring to provide this functionality outside
the context of an exchange. The one other thing we'll have to think about
in this context is how do we avoid doing a n-way merge in the case that the
we're not using the collation.

--
Jacques Nadeau
CTO and Co-Founder, Dremio

On Wed, Oct 7, 2015 at 8:18 AM, Maryann Xue <maryann.xue@gmail.com> wrote:

> One thing from what I asked James offline yesterday, and maybe we can
> discuss a little bit in today's meeting:
>
> Phoenix uses a list of lists of Scan objects to indicate Region boundaries
> and guideposts, and if the top-level list contains more than one element it
> means that the results from different Scanner/ResultIterator should be
> merge-sorted. We now use this list in Drill integration to generate
> different batches or slices. I see from the Drill plan of a simple select
> like "SELECT * FROM A.BEER" that a Drill Sort node sits on top of the
> PhoenixTableScan. I guess this is a real sort rather than a merge-sort.
> So optimally,
> 1) this should be a merge-sort (to be more accurate, a merge)
> 2) furthermore, if Drill has something to indicate the order among slices
> and batches, we could even turn it into a concat.
>
> The structure of this Scan list might be helpful for 2), or we may have
> some Logical representation for this. Otherwise, we can simply flatten this
> list to a one-dimensional list as we do now (in my ci yesterday).
>
>
>
> Thanks,
> Maryann
>
> On Tue, Oct 6, 2015 at 9:52 PM, Maryann Xue <maryann.xue@gmail.com> wrote:
>
>> Yes, but the partially aggregated results will not contain any duplicate
>> rowkeys, since they are also group-by keys. What we need is the aggregators
>> and call aggregate for each row. We can write a new simpler ResultIterator
>> to replace this, but for now it should work correctly.
>>
>> On Tue, Oct 6, 2015 at 9:45 PM, James Taylor <jamestaylor@apache.org>
>> wrote:
>>
>>> The results we get back from the server-side scan are already the
>>> partial aggregated values we need. GroupedAggregatingResultIterator
>>> will collapse adjacent Tuples together which happen to have the same row
>>> key. I'm not sure we want/need this to happen. Instead I think we just need
>>> to decode the aggregated values directly from the result of the scan.
>>>
>>> On Tue, Oct 6, 2015 at 6:07 PM, Maryann Xue <maryann.xue@gmail.com>
>>> wrote:
>>>
>>>> Hi James,
>>>>
>>>> bq. A few questions for you: not sure I understand the changes you made
>>>> to PhoenixRecordReader. Is it necessary to wrap the server-side scan
>>>> results in a GroupedAggregatingResultIterator? Each server-side scan will
>>>> produce results with a single tuple per group by key. In Phoenix, the
>>>> GroupedAggregatingResultIterator's function in life is to do the final
>>>> merge. Note too that the results aren't sorted that come back from the
>>>> aggregated scan (while GroupedAggregatingResultIterator needs tuples sorted
>>>> by the group by key). Or is this just to help in decoding the values coming
>>>> back from the scan?
>>>>
>>>> It is necessary. I suppose what we should return as a partial result
>>>> from PhoenixRecordReader is exactly the same as what we do in standalone
>>>> Phoenix+Calcite, except that the result is partial or say incomplete. For
>>>> example, we have "select a, count(*) from t group by a", we should return
>>>> rows that have "a" as the first expression value, and "count(*)" as the
>>>> second expression value. For this "count" expression, it actually needs a
>>>> ClientAggregator for evaluation, and that's what this
>>>> GroupedAggregatingResultIterator is used for.
>>>> Since "each server-side scan will produce results with a single tuple
>>>> per group by key", and PhoenixRecordReader is only dealing with one
>>>> server-side result each time, we don't care how the group-by keys are
>>>> arranged (ordered or unordered"). Actually
>>>> GroupedAggregatingResultIterator is not the group-by iterator we use
>>>> for AggregatePlan. It does not "combine". It treats every row as a
>>>> different group, by returning its rowkey as the group-by key (
>>>> GroupedAggregatingResultIterator.java:56).
>>>>
>>>> In short, this iterator is for decoding the server-side values. So we
>>>> may want to optimize this logic by removing this serialization and
>>>> deserialization and having only one set of aggregators in future.
>>>>
>>>> bq. Also, not sure what impact it has in the way we "combine" the scans
>>>> in our Drill parallelization code (PhoenixGroupScan.applyAssignments()),
>>>> as each of our scans could include duplicate group by keys. Is it ok to
>>>> combine them in this case?
>>>>
>>>> It should not matter, or at least is not related to the problem I'm now
>>>> having.
>>>>
>>>> bq. One more question: how is the group by key communicated back to
>>>> Drill?
>>>>
>>>> According to the HashAggPrule, if it decides to create a two-phase
>>>> aggregate, the first phase is now handled by Phoenix (after applying the
>>>> PhoenixHashAggPrule). I assume then the partial results gets shuffled based
>>>> on the hash of their group-by keys (returned by PhoenixRecordReader). The
>>>> final step is the Drill hash aggregation.
>>>>
>>>>
>>>> This is my test table "A.BEER", which has for columns: "B", "E1", "E2",
>>>> "R", all of INTEGER types. And the data is generated like this:
>>>> for (x=1 to N) { //currently N=1000
>>>>  UPSERT INTO A.BEER VALUES (x, x % 10, x % 100, x);
>>>> }
>>>>
>>>> The group-by query for testing is "SELECT e1, count(*) FROM a.beer
>>>> GROUP BY e1".
>>>> The expected result should be:
>>>> 0 100
>>>> 1 100
>>>> 2 100
>>>> 3 100
>>>> 4 100
>>>> 5 100
>>>> 6 100
>>>> 7 100
>>>> 8 100
>>>> 9 100
>>>> The actual result was:
>>>> 6 0
>>>> 7 0
>>>> 8 0
>>>> 9 0
>>>> 0 0
>>>> 1 100
>>>> 2 100
>>>> 3 100
>>>> 4 100
>>>> 5 100
>>>>
>>>> Here I just tried another one "SELECT e2, count(*) FROM a.beer GROUP
>>>> BY e2".
>>>> Similarly, the expected result should have group-by keys from 0 to 99,
>>>> each having a value of 10 as the count, while the actual result was:
>>>> from group-by key 86 to 99, together with 0, their count values were
>>>> all 0; the rest (1 to 85) all had the correct value 10.
>>>>
>>>> Looks to me that the scans were good but there was a problem with one
>>>> of the hash buckets.
>>>>
>>>> Thanks,
>>>> Maryann
>>>>
>>>>
>>>> On Tue, Oct 6, 2015 at 6:45 PM, James Taylor <jamestaylor@apache.org>
>>>> wrote:
>>>>
>>>>> Nice progress, Maryann.
>>>>>
>>>>> A few questions for you: not sure I understand the changes you made to
>>>>> PhoenixRecordReader. Is it necessary to wrap the server-side scan results
>>>>> in a GroupedAggregatingResultIterator? Each server-side scan will produce
>>>>> results with a single tuple per group by key. In Phoenix, the
>>>>> GroupedAggregatingResultIterator's function in life is to do the final
>>>>> merge. Note too that the results aren't sorted that come back from the
>>>>> aggregated scan (while GroupedAggregatingResultIterator needs tuples
sorted
>>>>> by the group by key). Or is this just to help in decoding the values
coming
>>>>> back from the scan?
>>>>>
>>>>> Also, not sure what impact it has in the way we "combine" the scans in
>>>>> our Drill parallelization code (PhoenixGroupScan.applyAssignments()),
as
>>>>> each of our scans could include duplicate group by keys. Is it ok to
>>>>> combine them in this case?
>>>>>
>>>>> One more question: how is the group by key communicated back to Drill?
>>>>>
>>>>> Thanks,
>>>>> James
>>>>>
>>>>>
>>>>> On Tue, Oct 6, 2015 at 2:10 PM, Maryann Xue <maryann.xue@gmail.com>
>>>>> wrote:
>>>>>
>>>>>> Added a few fixes in the pull request. Tested with two regions,
>>>>>> turned out that half of the result is empty (count = 0).
>>>>>> Not sure if there's anything wrong with
>>>>>> https://github.com/maryannxue/drill/blob/phoenix_plugin/contrib/storage-phoenix/src/main/java/org/apache/drill/exec/store/phoenix/rel/PhoenixHashAggPrule.java
>>>>>> .
>>>>>> Like Julian said, this rule looks a bit hacky.
>>>>>>
>>>>>> To force a 2-phase HashAgg, I made a temporary change as well:
>>>>>>
>>>>>> diff --git
>>>>>> a/exec/java-exec/src/main/java/org/apache/drill/exec/planner/physical/AggPruleBase.java
>>>>>> b/exec/java-exec/src/main/java/org/apache/drill/exec/planner/physical/AggPruleBase.java
>>>>>>
>>>>>> index b911f6b..58bc918 100644
>>>>>>
>>>>>> ---
>>>>>> a/exec/java-exec/src/main/java/org/apache/drill/exec/planner/physical/AggPruleBase.java
>>>>>>
>>>>>> +++
>>>>>> b/exec/java-exec/src/main/java/org/apache/drill/exec/planner/physical/AggPruleBase.java
>>>>>>
>>>>>> @@ -60,12 +60,12 @@ public abstract class AggPruleBase extends Prule
>>>>>> {
>>>>>>
>>>>>>    // If any of the aggregate functions are not one of these, then
we
>>>>>>
>>>>>>    // currently won't generate a 2 phase plan.
>>>>>>
>>>>>>    protected boolean create2PhasePlan(RelOptRuleCall call,
>>>>>> DrillAggregateRel aggregate) {
>>>>>>
>>>>>> -    PlannerSettings settings =
>>>>>> PrelUtil.getPlannerSettings(call.getPlanner());
>>>>>>
>>>>>> -    RelNode child = call.rel(0).getInputs().get(0);
>>>>>>
>>>>>> -    boolean smallInput = child.getRows() < settings.getSliceTarget();
>>>>>>
>>>>>> -    if (! settings.isMultiPhaseAggEnabled() ||
>>>>>> settings.isSingleMode() || smallInput) {
>>>>>>
>>>>>> -      return false;
>>>>>>
>>>>>> -    }
>>>>>>
>>>>>> +//    PlannerSettings settings =
>>>>>> PrelUtil.getPlannerSettings(call.getPlanner());
>>>>>>
>>>>>> +//    RelNode child = call.rel(0).getInputs().get(0);
>>>>>>
>>>>>> +//    boolean smallInput = child.getRows() <
>>>>>> settings.getSliceTarget();
>>>>>>
>>>>>> +//    if (! settings.isMultiPhaseAggEnabled() ||
>>>>>> settings.isSingleMode() || smallInput) {
>>>>>>
>>>>>> +//      return false;
>>>>>>
>>>>>> +//    }
>>>>>>
>>>>>>
>>>>>>      for (AggregateCall aggCall : aggregate.getAggCallList()) {
>>>>>>
>>>>>>        String name = aggCall.getAggregation().getName();
>>>>>>
>>>>>>
>>>>>> Thanks,
>>>>>> Maryann
>>>>>>
>>>>>>
>>>>>>
>>>>>> On Tue, Oct 6, 2015 at 2:31 PM, Julian Hyde <jhyde@apache.org>
wrote:
>>>>>>
>>>>>>> Drill's current approach seems adequate for Drill alone but extending
>>>>>>> it to a heterogenous system that includes Phoenix seems like
a hack.
>>>>>>>
>>>>>>> I think you should only create Prels for algebra nodes that you
know
>>>>>>> for sure are going to run on the Drill engine. If there's a
>>>>>>> possibility that it would run in another engine such as Phoenix
then
>>>>>>> they should still be logical.
>>>>>>>
>>>>>>> On Tue, Oct 6, 2015 at 11:03 AM, Maryann Xue <maryann.xue@gmail.com>
>>>>>>> wrote:
>>>>>>> > 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?
>>>>>>> >
>>>>>>> >
>>>>>>> > Thanks,
>>>>>>> > Maryann
>>>>>>> >
>>>>>>> > 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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