[ http://issues.apache.org/jira/browse/DERBY-504?page=all ]
Knut Anders Hatlen updated DERBY-504:
-------------------------------------
Attachment: DERBY-504.distinct.diff
Fixes the optimization bug which caused the trouble. Disables some
optimization, so maybe we should fix it in another way?
> SELECT DISTINCT returns duplicates when selecting from subselects
> -----------------------------------------------------------------
>
> Key: DERBY-504
> URL: http://issues.apache.org/jira/browse/DERBY-504
> Project: Derby
> Type: Bug
> Components: SQL
> Versions: 10.2.0.0
> Environment: Latest development sources (SVN revision 232227), Sun JDK 1.5,
> Solaris/x86
> Reporter: Knut Anders Hatlen
> Assignee: Knut Anders Hatlen
> Priority: Minor
> Attachments: DERBY-504.distinct.diff
>
> When one performs a select distinct on a table generated by a subselect,
> there sometimes are duplicates in the result. The following example shows the
> problem:
> ij> CREATE TABLE names (id INT PRIMARY KEY, name VARCHAR(10));
> 0 rows inserted/updated/deleted
> ij> INSERT INTO names (id, name) VALUES
> (1, 'Anna'), (2, 'Ben'), (3, 'Carl'),
> (4, 'Carl'), (5, 'Ben'), (6, 'Anna');
> 6 rows inserted/updated/deleted
> ij> SELECT DISTINCT(name) FROM (SELECT name, id FROM names) AS n;
> NAME
> ----------
> Anna
> Ben
> Carl
> Carl
> Ben
> Anna
> Six names are returned, although only three names should have been returned.
> When the result is explicitly sorted (using ORDER BY) or the id column is
> removed from the subselect, the query returns three names as expected:
> ij> SELECT DISTINCT(name) FROM (SELECT name, id FROM names) AS n ORDER BY
> name;
> NAME
> ----------
> Anna
> Ben
> Carl
> 3 rows selected
> ij> SELECT DISTINCT(name) FROM (SELECT name FROM names) AS n;
> NAME
> ----------
> Anna
> Ben
> Carl
> 3 rows selected
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