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https://issues.apache.org/jira/browse/JENA-885?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14350175#comment-14350175
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Andy Seaborne commented on JENA-885:
------------------------------------
The query would be better written as:
{noformat}
PREFIX ex: <http://example.com/>
SELECT ?s (COALESCE(?labelA, ?a) AS ?valueA)
WHERE
{
OPTIONAL
{ ?s ex:propA ?a
OPTIONAL
{ ?a ex:label ?labelA}
}
} LIMIT 1000
{noformat}
The form {{OPTIONAL\{\}/BIND}} where {{BIND}} refers to variables from inside
the {{OPTIONAL}} is what stops the optimizer using the more efficient form.
What is more this is itself still inside the outer {{OPTIONAL}}. Nest
{{OPTIONALs}} in SPARQL can lead to confusion when variable are mentioned in
the inner most {{OPTIONAL}} and not the intermediate one. The optimizer is
careful - only applying known safe transformations and the use of intermediate
{{BIND}} with a deep inner variable is not covered.
A general point: {{COALESCE}} is a more direct way to write
{{IF(BOUND(...)...)}}.
> Poor performance and timeout failure with BIND in nested OPTIONALs
> ------------------------------------------------------------------
>
> Key: JENA-885
> URL: https://issues.apache.org/jira/browse/JENA-885
> Project: Apache Jena
> Issue Type: Bug
> Affects Versions: Jena 2.11.2
> Reporter: Mark Buquor
> Attachments: ExecuteTestQueries.java, GenerateTestDataset.java
>
>
> There appears to be a performance issue with BIND when used inside nested
> OPTIONALs. Affected queries fail to time out.
> The following patterns appear to be affected:
> {noformat}
> OPTIONAL { ... OPTIONAL { ... BIND ( ... ) } }
> OPTIONAL { ... OPTIONAL { ... } BIND ( ... ) }
> {noformat}
> The following patterns appear to be unaffected:
> {noformat}
> OPTIONAL { ... OPTIONAL { ... } } BIND ( ... )
> OPTIONAL { ... BIND ( ... ) }
> OPTIONAL { ... } BIND ( ... )
> {noformat}
> So far, users have been able to work around the performance issue by
> rewriting their queries. However, the timeout failure is still a significant
> reliability issue, since affected queries consume resources and can run
> indefinitely. I've attached a testcase that exhibits the performance and
> timeout problems. Reproduced with a recent 2.13.0-SNAPSHOT build.
> {noformat}
> Execution Timeout (ms): 30000
> Query: PREFIX ex: <http://example.com/> SELECT ?s ?valueA { OPTIONAL { ?s
> ex:propA ?a . OPTIONAL { ?a ex:label ?labelA . } } BIND ( IF ( BOUND
> (?labelA), ?labelA, ?a) as ?valueA) }
> Execution time (ms): 586
> Execution time (ms): 143
> Query: PREFIX ex: <http://example.com/> SELECT ?s ?valueA { OPTIONAL { ?s
> ex:propA ?a . OPTIONAL { ?a ex:label ?labelA . } BIND ( IF ( BOUND (?labelA),
> ?labelA, ?a) as ?valueA) } }
> Execution time (ms): 110922
> Execution time (ms): 41004
> {noformat}
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