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From "Knut Anders Hatlen (JIRA)" <j...@apache.org>
Subject [jira] Commented: (DERBY-4660) Expose merge join through optimizer override
Date Fri, 21 May 2010 10:49:17 GMT

    [ https://issues.apache.org/jira/browse/DERBY-4660?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12869937#action_12869937
] 

Knut Anders Hatlen commented on DERBY-4660:
-------------------------------------------

I think there may be a bug MergeJoinResultSet. We should look into it once we have it enabled
and can test it.

Here's what I think is wrong: In getNextRowCore() we move forward in the right result until
we find a row with a key that matches the current row in the left result. If that row also
satisfies the restriction in JoinResultSet.restriction, the row is returned. Otherwise, we
move to the next row in the left result. But we have no guarantee that there are no more rows
in the right result that match the current row in the left result, and that do satisfy the
restriction, even if we found one match that didn't satisfy the restriction. So I think we
may skip some rows in the right result.

This won't be a problem if the join key is known to be unique in the right result, but the
comments in the class suggest that only the left result has to be unique.

> Expose merge join through optimizer override
> --------------------------------------------
>
>                 Key: DERBY-4660
>                 URL: https://issues.apache.org/jira/browse/DERBY-4660
>             Project: Derby
>          Issue Type: Improvement
>          Components: SQL
>    Affects Versions: 10.7.0.0
>            Reporter: Knut Anders Hatlen
>            Priority: Minor
>
> Derby comes with a class called MergeJoinResultSet which is not currently used. As a
first step towards full support for merge joins, we should expose the functionality provided
by this class through an optimizer override, so that we can start testing it.

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