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https://issues.apache.org/jira/browse/DERBY-4422?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12775923#action_12775923
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Bryan Pendleton commented on DERBY-4422:
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Does this indicate a weakness in the Derby query-flattening algorithm? I read
through
http://db.apache.org/derby/docs/10.5/tuning/ctuntransform36368.html
and was unsure whether Derby should have been able to flatten this query
automatically or not. It seems like the crucial bit is:
"There is a uniqueness condition that ensures that the subquery does not
introduce
any duplicates if it is flattened into the outer query block"
But I think that the DISTINCT in the *outer* query should have provided such a
guarantee.
I think that the DISTINCT in the *subquery* is unnecessary, due to:
http://db.apache.org/derby/docs/10.5/tuning/ctuntransform867165.html
> Extremely slow subqueries when subquerying on strings
> -----------------------------------------------------
>
> Key: DERBY-4422
> URL: https://issues.apache.org/jira/browse/DERBY-4422
> Project: Derby
> Issue Type: Bug
> Affects Versions: 10.5.3.0
> Environment: Suse Unix 10.3.
> Reporter: Tony Brusseau
> Attachments: badqp.txt, derby.log, fixed-time-derby.log, goodqp.txt,
> SQLFile1.sqlaa.gz, SQLFile1.sqlab.gz, SQLFile2.sqlaa.gz, SQLFile2.sqlab.gz,
> SQLFile2.sqlac.gz, tmp2.sql, unlimited-time-derby.log
>
>
> SELECT DISTINCT C.INTERNAL_ID
> FROM OC_CONCEPTS C
> WHERE
> (C.INTERNAL_ID IN (SELECT DISTINCT OCS.CONCEPT_ID FROM OC_CONCEPT_STRINGS
> OCS WHERE OCS.CONCEPT_ID=1))
> Both queries above are trivial and they are both acting on fully indexed
> primary key columns. This query runs just fine.
> However, when I make the small modification of searching on an indexed text
> column in the sub query, it takes over 6 seconds to run, even though both
> queries independently take < 1ms run.
> SELECT DISTINCT C.INTERNAL_ID
> FROM OC_CONCEPTS C
> WHERE
> (C.INTERNAL_ID IN (SELECT DISTINCT OCS.CONCEPT_ID FROM OC_CONCEPT_STRINGS
> OCS WHERE OCS.NL_LC = 'dash'))
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