Hi Štěpán
For a stable/consistent pagination, an ORDER BY is always necessary. Even if
the data is constant and the statistics of indexes are not refreshed between
fetching different pages, the optimizer can chose different ordering depending
on number of rows to skip and return. In fact no ORDER BY clause implies that
the optimizer may do whatever with the order, including different order for
different runs of same query on same data.
Best Regards
Hugh Williams
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On 22 Apr 2014, at 14:24, Bc. Štěpán Pilař <xpil...@vse.cz> wrote:
> Hi,
>
> I'm working with Silk to find links within a dataset. I have several graphs,
> each with tens of thousands of resources for linking in a Virtuoso instance.
> When given Virtuoso SPARQL endpoint parameters to Silk it is able to load in
> batches (default by 1000) all resources needed, even though the total number
> exceeds MaxSortedTopRows directive. As the name suggests, I assume, it
> applies only when using ORDER BY and not using it avoids the need for nested
> query as described in
> http://virtuoso.openlinksw.com/dataspace/doc/dav/wiki/Main/VirtTipsAndTricksHowToHandleBandwidthLimitExceed.
>
> As far as I've been able to find out, the result set ordering when omitting
> ORDER BY is unspecified, but my attempts (and Silk's assumption) point to
> some implicit ordering in Virtuoso so that repeated query like:
>
> SELECT *
> WHERE {
> ?s ?p ?o .
> }
> OFFSET 100000
> LIMIT 1000
>
> returns the same results again and again.
>
> Am I right in thinking that it is just coincidence resulting from use of some
> data structure in Virtuoso? Is it possible to describe this implicit order
> (like most RDBMS return records in order of creation when no specific order
> applied)? Do operations modifying data between two same queries alter the
> implicit order?
>
> Thanks
> Štěpán
> Pilař------------------------------------------------------------------------------
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