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https://issues.apache.org/jira/browse/JENA-780?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Rob Vesse reassigned JENA-780:
------------------------------
Assignee: Rob Vesse
> Single use extend expressions could be substituted directly for their later
> usage
> ---------------------------------------------------------------------------------
>
> Key: JENA-780
> URL: https://issues.apache.org/jira/browse/JENA-780
> Project: Apache Jena
> Issue Type: Improvement
> Components: ARQ, Optimizer
> Affects Versions: Jena 2.12.0
> Reporter: Rob Vesse
> Assignee: Rob Vesse
> Priority: Minor
>
> This RFE is a follow on from JENA-779, the query with a sub-optimal plan
> there uses a {{BIND}} to create a value which is then only used once in a
> subsequent filter.
> Actually that query uses it twice but I think the general approach I am
> trying to describe in this RFE bears consideration. In this case it seems
> like it would be possible to substitute the extend expression for the bound
> variable in the filter expression.
> Simplified variant of original query such that the bound value is only used
> once:
> {noformat}
> SELECT DISTINCT ?uri
> {
> { ?uri ?p ?o }
> UNION
> {
> ?sub ?p ?uri
> FILTER(isIRI(?uri))
> }
> BIND(str(?uri) as ?s)
> FILTER(STRSTARTS(?s, "http://";))
> }
> {noformat}
> Rewritten query:
> {noformat}
> SELECT DISTINCT ?domainName
> {
> { ?uri ?p ?o }
> UNION
> {
> ?sub ?p ?uri
> FILTER(isIRI(?uri))
> }
> FILTER(STRSTARTS(str(?uri), "http://";))
> }
> {noformat}
> Which avoids an extend expression whose value is only used once and will
> ultimately be thrown away.
> From a {{Transform}} standpoint this is likely awkward to implement in a pure
> transform since it requires knowledge about the query structure above the
> {{FILTER}} i.e. whether the bound variable is used elsewhere and so would
> need to use before and after visitors to track that additional state but I
> think this is a feasible optimisation.
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