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Date: Sun, 29 Nov 2009 02:38:20 +0000 (UTC)
From: "Dag H. Wanvik (JIRA)" <jira@apache.org>
To: derby-dev@db.apache.org
Subject: [jira] Updated: (DERBY-4405) Transformation to inner join not
 performed for certain three-way joins
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     [ https://issues.apache.org/jira/browse/DERBY-4405?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Dag H. Wanvik updated DERBY-4405:
---------------------------------

    Issue & fix info: [Patch Available]

> Transformation to inner join not performed for certain three-way joins
> ----------------------------------------------------------------------
>
>                 Key: DERBY-4405
>                 URL: https://issues.apache.org/jira/browse/DERBY-4405
>             Project: Derby
>          Issue Type: Improvement
>          Components: SQL
>    Affects Versions: 10.6.0.0
>            Reporter: Knut Anders Hatlen
>            Priority: Minor
>         Attachments: derby-4405.diff, derby-4405.stat
>
>
> In the CROSS JOIN section in the reference manual (http://db.apache.org/derby/docs/dev/ref/rrefsqljcrossjoin.html) there are three examples that are supposed to be equivalent. However, the performance differs significantly between the different queries.
> The queries use the tours db and look like this:
> (1)
> SELECT * FROM CITIES LEFT OUTER JOIN
>     (FLIGHTS CROSS JOIN COUNTRIES)
>         ON CITIES.AIRPORT = FLIGHTS.ORIG_AIRPORT
>             WHERE COUNTRIES.COUNTRY_ISO_CODE = 'US'
> (2)
> SELECT * FROM CITIES LEFT OUTER JOIN
>     FLIGHTS INNER JOIN COUNTRIES ON 1=1
>         ON CITIES.AIRPORT = FLIGHTS.ORIG_AIRPORT
>             WHERE COUNTRIES.COUNTRY_ISO_CODE = 'US'
> (3)
> SELECT * FROM CITIES LEFT OUTER JOIN
>     (SELECT * FROM FLIGHTS, COUNTRIES) S
>         ON CITIES.AIRPORT = S.ORIG_AIRPORT
>             WHERE S.COUNTRY_ISO_CODE = 'US'
> When executed in ij, (1) and (2) need 6 seconds to complete, whereas (3) completes in 50 ms.
> The query plans for (1) and (2) use nested loop joins and table scans. (3) uses a combination of hash join and nested loop join, and index scans as well as table scans.
> It looks like (3) has been rewritten from a left outer join to an inner join internally. This is fine because all rows that have the right-side columns filled with NULLs will be filtered out by the predicate S.COUNTRY_ISO_CODE='US', so the extra rows generated by the outer join will not be returned.
> This optimization should also be possible for (1) and (2). We should improve the logic so that those joins are transformed too. The transformation happens in HalfOuterJoinNode.transformOuterJoins().

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