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https://issues.apache.org/jira/browse/JENA-2020?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17256628#comment-17256628
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Andy Seaborne commented on JENA-2020:
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Note the return type - {{QueryIterator}}.
Most of the main query engine is streaming - the results are calculated when
the iterator moves forward ({{.hasNext}}/{{.next}}). If you want to make it
happen, you have to consume the iterator.
Sometimes, the results are calculated immediately and/or completely on first
result (e.g. a sort, or the related `OpTopN`; group+aggregation in subqueries).
The reference engine calculates everything immediately and returns an iterator
over the table of calculated results. Needs more memory to do that.
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What does {{OpExistence}} aim to do? Is it related to {{FILTER EXISTS}}?
> Purpose of EvaluatorSimple and OpExecutor
> -----------------------------------------
>
> Key: JENA-2020
> URL: https://issues.apache.org/jira/browse/JENA-2020
> Project: Apache Jena
> Issue Type: Question
> Components: ARQ
> Reporter: Martin Pekár
> Priority: Major
> Original Estimate: 1h
> Remaining Estimate: 1h
>
> I am in the midst of adding a new operator in the transformation of OpBGP. I
> am now trying to implement the execution of the operator in the query plan,
> but I am now slightly confused about the purpose of EvaluatorSimple used in
> EvaluatorDispatch and OpExecutor. At the moment, it seems like they are doing
> the same thing. The difference seems to be that OpExecutor stores the result
> of applying the operator in a QueryIterator, whereas EvaluatorSimple stores
> its result in a Table of bindings.
> Can someone give me an explanation of purposes of these two classes and how
> they should be used?
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