hint:runFirst throws exception
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	WikidataFacts
	Jun 14 2017, 9:20 AM

Description

According to QueryHints, hint:runFirst should allow specifying that a particular join should run first in a join group, for example:

SELECT ?item WHERE {
  ?item wdt:P131* wd:Q1198. hint:Prior hint:runFirst true.
  ?item wdt:P31/wdt:P279* wd:Q387917.
}

However, it doesn’t work:

com.bigdata.rdf.sparql.ast.hints.QueryHintException: scope=Prior, name=runFirst, value=true, op=PropertyPathNode
at com.bigdata.rdf.sparql.ast.hints.RunFirstHint.handle(RunFirstHint.java:66)
at com.bigdata.rdf.sparql.ast.hints.RunFirstHint.handle(RunFirstHint.java:45)
at com.bigdata.rdf.sparql.ast.optimizers.ASTQueryHintOptimizer._applyQueryHint(ASTQueryHintOptimizer.java:304)
at com.bigdata.rdf.sparql.ast.optimizers.ASTQueryHintOptimizer.applyQueryHint(ASTQueryHintOptimizer.java:701)
at com.bigdata.rdf.sparql.ast.optimizers.ASTQueryHintOptimizer.processGroup(ASTQueryHintOptimizer.java:419)
at com.bigdata.rdf.sparql.ast.optimizers.ASTQueryHintOptimizer.optimize(ASTQueryHintOptimizer.java:155)
…

I also tried "true" and "true"^^xsd:boolean as arguments, with the same result. (The last one should be identical to true AFAIK, but who knows what the optimizer does with them…)

(This example query can be made to run by completely disabling the optimizer, but in general it would be nice to have more fine-grained control over it.)

hint:runLast has the same problem.

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WikidataFacts created this task.Jun 14 2017, 9:20 AM

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What it complains about is that hint:runFirst is applicable only to joins, but ?item wdt:P131* wd:Q1198 is not a join. Not sure why exactly it is the case though, need to check.

It is possible to make this query parse, but it still times out.

This one seems to work:

SELECT ?item WHERE {
  hint:Query hint:optimizer "None" .
  ?item wdt:P131* wd:Q1198. 
  ?item wdt:P31/wdt:P279* wd:Q387917.
}

Smalyshev added a project: Upstream.Jun 23 2017, 7:31 PM

Smalyshev moved this task from Incoming to Blazegraph on the Wikidata-Query-Service board.

Smalyshev triaged this task as Low priority.Jun 23 2017, 8:17 PM

Yes, that’s what I meant with:

(This example query can be made to run by completely disabling the optimizer, but in general it would be nice to have more fine-grained control over it.)

Smalyshev added a project: WDQS-Optimizer.Sep 24 2017, 11:51 PM

hint:runFirst throws exceptionOpen, LowPublicActions

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hint:runFirst throws exception
Open, LowPublic
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