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https://issues.apache.org/jira/browse/JENA-615?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13863006#comment-13863006
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Rob Vesse commented on JENA-615:
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I am getting some fairly mixed figures to be honest. Depending on the dataset
and the query for simple queries like my aforementioned example the difference
is typically a few hundredths of a milliseconds at best and whether the
original/optimized query is better varies.
I'm going to try out a slightly more complex query with multiple {{!=}} clauses
and see if there is more benefit to be had there. If not then this could
perhaps be put in as an off by default optimisation since for some storage
systems there may be a more marked performance difference.
> Possible optimisation for FILTER(?var != <constant>)
> ----------------------------------------------------
>
> Key: JENA-615
> URL: https://issues.apache.org/jira/browse/JENA-615
> Project: Apache Jena
> Issue Type: Improvement
> Components: ARQ
> Reporter: Rob Vesse
> Assignee: Rob Vesse
> Priority: Minor
> Labels: algebra, optimization, sparql
>
> I have an idea for a possible optimisation for queries of the following
> general form:
> {noformat}
> SELECT *
> WHERE
> {
> # Some Patterns
> FILTER(?var != <http://constant>)
> }
> {noformat}
> This pattern crops up surprisingly often in real SPARQL workloads since it is
> often used to either limit a variable to exclude certain possibilities or to
> avoid self referential links in the data.
> In some cases it seems like this could be safely rewritten as follows:
> {noformat}
> SELECT *
> WHERE
> {
> # Some Patterns
> MINUS { BIND(<http://constant> AS ?var) }
> }
> {noformat}
> Or perhaps in a more generalised form like so:
> {noformat}
> SELECT * WHERE
> {
> # Some patterns
> MINUS { VALUES ?var { <http://constant/1> <http://constant/2> } }
> }
> {noformat}
> Which would nicely deal with the case of stating that a variable is not equal
> to multiple constant values.
> As I pointed out earlier this would not apply in every case, specifically I
> think at least the following must be true:
> - The variable must be guaranteed to be bound (similar to existing filter
> equality and implicit join optimisations)
> There is also the potential to spot cases where the variable will always be
> unbound and thus the expression is always an error and replace the entire
> sub-tree with {{table empty}} as we already do for equality and implicit join
> filters.
> I plan on taking a look at implementing this in the new year, if anyone has
> any thoughts on this (especially wrt to restrictions that should apply to
> when the optimisation is considered safe) then please comment.
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