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From cobby <ququr...@yahoo.com>
Subject Re: Bucket pruning
Date Fri, 13 Mar 2015 12:22:56 GMT
hi, thanks for the detailed response.
i will experiment with your suggested orc bloom filter solution.

it seems to me the obvious, most straight forward solution is to add support for hash partitioning.
so i can do something like:

create table T()
partitioned by (x into num_partitions,..).

upon insert hash(x) determines which partition to put the record in. upon select, the query
processor can now hash on x and scan only that partition (this optimization will probably
work only on = and other discrete filtering but thats true for partitioning in general).
it seems all of this can be done early in the query plan phase and have no effect on underling
infra.

regards,cobby.



> On 12 במרץ 2015, at 23:05, Gopal Vijayaraghavan <gopalv@apache.org> wrote:
> 
> Hi,
> 
> No and it¹s a shame because we¹re stuck on some compatibility details with
> this.
> 
> The primary issue is the fact that the InputFormat is very generic and
> offers no way to communicate StorageDescriptor or bucketing.
> 
> The split generation for something SequenceFileInputFormat lives inside
> MapReduce, where it has no idea about bucketing.
> 
> So InputFormat.getSplits(conf) returns something relatively arbitrary,
> which contains a mixture of files when CombineInputFormat is turned on.
> 
> I have implemented this twice so far for ORC (for custom Tez jobs, with
> huge wins) by using an MRv2 PathFilter over the regular OrcNewInputFormat
> implementation, by turning off combine input and using Tez grouping
> instead.
> 
> But that has proved to be very fragile for a trunk feature, since with
> schema evolution of partitioned tables older partitions may be bucketed
> with a different count from a newer partition - so the StorageDescriptor
> for each partition has to be fetched across before we can generate a valid
> PathFilter.
> 
> The SARGs are probably a better way to do this eventually as they can
> implement IN_BUCKET(1,2) to indicate 1 of 2 instead of the ³00000_1²
> PathFilter which is fragile.
> 
> 
> Right now, the most fool-proof solution we¹ve hit upon was to apply the
> ORC bloom filter to the bucket columns, which is far safer as it does not
> care about the DDL - but does a membership check on the actual metadata &
> prunes deeper at the stripe-level if it is sorted as well.
> 
> That is somewhat neat since this doesn¹t need any new options for querying
> - it automatically(*) kicks in for your query pattern.
> 
> Cheers,
> Gopal
> (*) - conditions apply - there¹s a threshold for file-size for these
> filters to be evaluated during planning (to prevent HS2 from burning CPU).
> 
> 
> From:  Daniel Haviv <daniel.haviv@veracity-group.com>
> Reply-To:  "user@hive.apache.org" <user@hive.apache.org>
> Date:  Thursday, March 12, 2015 at 2:36 AM
> To:  "user@hive.apache.org" <user@hive.apache.org>
> Subject:  Bucket pruning
> 
> 
> Hi,
> We created a bucketed table and when we select in the following way:
> select * 
> from testtble
> where bucket_col ='X';
> 
> We observe that there all of the table is being read and not just the
> specific bucket.
> 
> Does Hive support such a feature ?
> 
> 
> Thanks,
> Daniel
> 
> 

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