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Re: Berlin SPARQL Benchmark V2 - Results for Sesame, Virtuoso, Jena TDB, D2R Server, and MySQL

From: Kingsley Idehen <kidehen@openlinksw.com>
Date: Wed, 24 Sep 2008 14:56:32 -0400
Message-ID: <48DA8D60.5010304@openlinksw.com>
To: Paul Gearon <gearon@ieee.org>
CC: semantic-web@w3.org, "public-lod@w3.org" <public-lod@w3.org>

Paul Gearon wrote:
> On Mon, Sep 22, 2008 at 3:47 AM, Eyal Oren <eyal@cs.vu.nl> wrote:
>   
>> On 09/19/08/09/08 23:12 +0200, Orri Erling wrote:
>>     
>>>> Has has there been any analysis on whether there is a *fundamental*
>>>> reason for such performance difference? Or is it simply a question of
>>>> "maturity"; in other words, relational db technology has been around for a
>>>> very long time and is very mature, whereas RDF implementations are still
>>>> quite recent, so this gap will surely narrow ...?
>>>>         
>>> This is a very complex subject.  I will offer some analysis below, but
>>> this I fear will only raise further questions.  This is not the end of the
>>> road, far from it.
>>>       
>> As far as I understand, another issue is relevant: this benchmark is
>> somewhat unfair as the relational stores have one advantage compared to the
>> native triple stores: the relational data structure is fixed (Products,
>> Producers, Reviews, etc with given columns), while the triple representation
>> is generic (arbitrary s,p,o).
>>     
>
> This point has an effect on several levels.
>
> For instance, the flexibility afforded by triples means that objects
> stored in this structure require processing just to piece it all
> together, whereas the RDBMS has already encoded the structure into the
> table. Ironically, this is exactly the reason we
> (Tucana/Kowari/Mulgara) ended up building an RDF database instead of
> building on top of an RDBMS: The flexibility in table structure was
> less efficient that a system that just "knew" it only had to deal with
> 3 columns. Obviously the shape of the data (among other things)
> dictates what it is the better type of storage to use.
>
> A related point is that processing RDF to create an object means you
> have to move around a lot in the graph. This could mean a lot of
> seeking on disk, while an RDBMS will usually find the entire object in
> one place on the disk. And seeks kill performance.
>
> This leads to the operations used to build objects from an RDF store.
> A single object often requires the traversal of several statements,
> where the object of one statement becomes the subject of the next.
> Since the tables are typically represented as
> Subject/Predicate/Object, this means that the main table will be
> "joined" against itself. Even RDBMSs are notorious for not doing this
> efficiently.
>
> One of the problems with self-joins is that efficient operations like
> merge-joins (when they can be identified) will still result in lots of
> seeking, since simple iteration on both sides of the join means
> seeking around in the same data. Of course, there ARE ways to optimize
> some of this, but the various stores are only just starting to get to
> these optimizations now.
>
> Relational databases suffer similar problems, but joins are usually
> only required for complex structures between different tables, which
> can be stored on different spindles. Contrast this to RDF, which needs
> to do do many of these joins for all but the simplest of data.
>
>   
>> One can question whether such flexibility is relevant in practice, and if
>> so, one may try to extract such structured patterns from data on-the-fly.
>> Still, it's important to note that we're comparing somewhat different things
>> here between the relational and the triple representation of the benchmark.
>>     
>
> This is why I think it is very important to consider the type of data
> being stored before choosing the type of storage to use. For some
> applications an RDBMS is going to win hands down every time. For other
> applications, an RDF store is definitely the way to go. Understanding
> the flexibility and performance constraints of each is important. This
> kind of benchmarking helps with that. It also helps identify where RDF
> databases need to pick up their act.
>
> Regards,
> Paul Gearon
>
>
>   
Paul,

You make valid points, the problem here is that the benchmark has been 
released without enough clarity about it's prime purpose. To even 
compare RDF Quads Stores with an RDBMS engine when the schema is 
Relational in itself is kinda twisted.

The role of mappers (DR2Q & Virtuoso RDF Views) for instance,  should 
have been made much clearer, maybe in separate results tables. I say 
this because these mappers offer different approaches to projecting 
RDBMS based data in RDF Linked Data form, on the fly, and their purpose 
in this benchmark is all about raw performance and scalability as it 
relates to following RDF Linked Data generation and deployment conditions:

1. Schema is Relational
2. RDF warehouse is impractical

As I am sure you know, we could invert this whole benchmark "Open World" 
style, and then bring RDBMS engines to their knees by incorporating 
SPARQL query patterns comprised of ?p's and subclasses .

To conclude, the quad store numbers should simply be a conparisons of 
the quad stores themselves, and not the quad stores vs the mappers or 
native SQL. This clarification really needs to make it's way into the 
benchmark narrative.


-- 


Regards,

Kingsley Idehen	      Weblog: http://www.openlinksw.com/blog/~kidehen
President & CEO 
OpenLink Software     Web: http://www.openlinksw.com
Received on Wednesday, 24 September 2008 18:57:14 GMT

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