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From Gopal Vijayaraghavan <gop...@apache.org>
Subject Re: [Proposal] New decimal encoding
Date Tue, 10 Apr 2018 23:22:05 GMT
Hi,

I agree with your analysis about Decimals. 

Something similar has already gone into patch-available previously, but held back

https://issues.apache.org/jira/browse/ORC-209

This is somewhat stuck behind the Vector type system evolving support for this

https://issues.apache.org/jira/browse/HIVE-17235
+
https://issues.apache.org/jira/browse/HIVE-17433

That works only for Text formats in Hive right now, hope that bolsters your case for implementing
it in ORC.

Definite +1 on the DecimalStatistics impl, the String representation is the slowest part of
the Decimal writer loop in Java (i.e for every row, toString + compare min/max).

Cheers,
Gopal




´╗┐On 4/10/18, 1:24 PM, "Wu Gang" <ustcwg@gmail.com> wrote:

    Hi,
    
    This is Gang Wu and I have proposed this in ORC-161
    <https://issues.apache.org/jira/browse/ORC-161> but got no response
    therefore I put it here.
    
    Recently I have done some benchmarks between ORC and our proprietary file
    format. The result indicates that ORC does not have a good performance on
    decimal type. From the aforementioned discussion in this JIRA, I have a
    proposal for adding a new encoding approach for decimal type (I don't think
    adding another kind of decimal type is a good choose which may confuse
    users). My proposal works as follows:
    
       - As Hive already has precision and scale specified in the type, we can
       totally remove the SECONDARY stream which stores scale of each element
       currently.
    
    
       - Since 128-bit integer is used to represent a decimal value and RLE
       supports at most 64-bit integer, we have two cases here.
       - If precision <= 18, then the whole decimal value can be represented in
          a signed 64-bit integer. Therefore we only need a DATA stream and use
          signed integer RLE to encode it.
          - If precision > 18, then we need to use a signed 128-bit integer. A
          solution is to use a signed 64-bit integer to hold higher 64 bits and an
          unsigned 64-bit integer to hold the lower 64 bits (C++ version is exactly
          doing the same thing). In this way, we can use DATA stream with signed
          integer RLE to store higher 64 bits and SECONDARY stream with unsigned
          integer RLE to store lower 64 bits.
    
    
       - DecimalStatistics uses string type to store min/max/sum. We may also
       replace them with combination of sint64 and uint64 as above to represent a
       128-bit integer. This can help save a lot of space.
    
    Any thoughts?
    
    Best,
    Gang
    



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