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COMMENT? Re: Pagination in SPARQL OFFSET and LIMIT needs ORDER BY

From: Jerven Bolleman <jerven.bolleman@isb-sib.ch>
Date: Thu, 6 Oct 2011 22:09:54 +0200
Message-Id: <04957385-E8FB-4D8B-B041-1671F49C741B@isb-sib.ch>
To: jerven.bolleman@isb-sib.ch
Dear Workgroup,

I have not seen any discussion of my suggestion the OFFSET works deterministically. Has this been discussed in the workgroup and been disregarded as a bad idea or breaking compatibility? Or has my comment/question slipped through the cracks? Or is it now to late as July 29th has passed?

Regards,
Jerven



On May 13, 2011, at 10:01 AM, Jerven Bolleman wrote:

> Dear workgroup,
> 
> I realized that I might not have been so clear in describing the problem.
> 
> Assume that you maintain a publicly available SPARQL endpoint.
> You want to support both a HTML view and the official SPARQL formats.
> 
> Lets say a user executes the query
> SELECT * WHERE {?s ?p ?o}
> This will download every triple in your store. In my store this will mean trying to download 160gb of triples via a single HTTP connection.
> This is not likely to work and if it did most browser will crash on the HTML view.
> 
> Therefore I would like to always put a LIMIT on the query to make sure that the result will match the capabilities of a common HTTP connection.
> e.g. default LIMIT 1000
> 
> But I do want people to download more than just the first 1000 results to their query. I just want them to do it in multiple requests that are likely to complete and not crash their browsers.
> 
> So I need pagination i.e. OFFSET. In practical terms this does exactly what I need (having briefly tested OWLIM and Virtuoso).
> i.e. page 1 SELECT * WHERE {?s ?p ?o} OFFSET 0 LIMIT 1000
>     page 2 SELECT * WHERE {?s ?p ?o} OFFSET 1000 LIMIT 1000
> Until there are no more results. However, this is not specified to work in the current public draft.
> 
> Having the following 2 triples in a store.
> <_:1> <lala> "hi"
> <_:1> <lala> "by"
> 
> The following query
> SELECT * WHERE {?s ?p ?o}
> Can evaluate to either a)
> <_:1> <lala> "hi"
> <_:1> <lala> "by"
> or b)
> <_:1> <lala> "by"
> <_:1> <lala> "hi"
> 
> i.e. ordering is random but all results are returned.
> 
> The following query, assume the implementation always returns ordering a)
> 
> SELECT * WHERE {?s ?p ?o} OFFSET 0 LIMIT 1
> 
> Can return
> <_:1> <lala> "hi"
> And in the same store it is valid to return this for
> SELECT * WHERE {?s ?p ?o} OFFSET 1 LIMIT 1
> As well.
> 
> So while the chunks are small I am not guaranteed to get all valid results. I need to add an ORDER BY clause. However, I can't without changing the query as you can not add ORDER BY *. Nor is this always desired because ORDER BY actually means that you need to ORDER the results. This can be very expensive relative to executing the query.
> 
> Therefore, I would define OFFSET more specifically.
> 
> When a implementation returns a result set for a query. Then it should do so in a deterministic manner. i.e. executing the same query twice on a store with constant data will return results in the same order.
> The OFFSET parameter is then interpreted as discard the first X results that a the same query without OFFSET would have generated.
> 
> This means that for a query A with N results. The concatenation results of queries A OFFSET 0..N LIMIT 1 is equal to the result of the query A.
> 
> Regards,
> Jerven Bolleman
> 
> P.S. the original source of this discussion is.
> http://answers.semanticweb.com/questions/9456/jena-pagination-for-sparql
> 
> 
> On 05/12/2011 04:32 PM, Jerven Bolleman wrote:
>> Dear workgroup,
>> 
>> I was recently made aware that there is no easy way to get a guaranteed working pagination.
>> 
>> i.e. QUERY OFFSET 0 LIMIT 5 page 1
>>       QUERY OFFSET 5 LIMIT 5 page 2
>>       QUERY OFFSET 10 LIMIT 5 page 3
>> 
>> Without adding an ORDER BY clause. Adding any kind of ORDER BY clause would be enough to ensure pagination worked. I would therefore like to see an  ORDER BY * or ORDER BY ANY option. To ensure that the results come in some implementation specific order and that this can be used to show all possible results.
>> 
>> Trying a few public current SPARQL implementations. With ORDER BY * showed that this is currently not implemented. Although pagination with OFFSET and LIMIT without an ORDER BY clause  seems to work as a naive user (e.g. me) would expect. Meaning that for current SPARQL implementers it is no work at all other than dealing with a slightly different SPARQL grammar.
>> 
>> Pagination guaranteed to succeed would then be
>> 
>> i.e. QUERY OFFSET 0 LIMIT 5 ORDER BY ANY page 1
>>       QUERY OFFSET 5 LIMIT 5 ORDER BY ANY page 2
>>       QUERY OFFSET 10 LIMIT 5 ORDER BY ANY page 3
>> 
>> The other option is to expand the description of the OFFSET clause. For example the use of the OFFSET clause should guarantee that query results come back in a consistent order.
>> 
>> I hope this concern makes sense.
>> 
>> Regards,
>> Jerven
>> 
>> 
> 
> <jerven_bolleman.vcf>
Received on Thursday, 6 October 2011 20:10:35 GMT

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