[MDEV-13172] Wrong result / SELECT ... WHERE EXISTS ... (with UNIQUE Key) - Jira

Sorry, but I'm not sure if it's a bug ... but I think so:

CREATE TABLE `tab1` (

  `Id` int(11) NOT NULL,

  PRIMARY KEY (`Id`)

) ENGINE=InnoDB DEFAULT CHARSET=latin1;

INSERT INTO `tab1` (`Id`) VALUES (1);

CREATE TABLE `tab2` (

  `tab1_Id` int(11) NOT NULL DEFAULT 0,

  `col1` int(11) DEFAULT NULL,

  UNIQUE KEY `col1` (`col1`)

) ENGINE=InnoDB DEFAULT CHARSET=latin1;

INSERT INTO `tab2` (`tab1_Id`, `col1`) VALUES (1, NULL), (1, NULL);

Now two Statements:

SELECT Id FROM tab1 WHERE EXISTS

  (SELECT 1 AS `C1` FROM tab2 WHERE tab1.Id = tab2.tab1_Id AND tab2.col1 IS NULL);

SELECT Id FROM tab1 WHERE EXISTS

  (SELECT 1 AS `C1` FROM tab2 WHERE tab1.Id = tab2.tab1_Id);

Statement 1 gets a result with 2 identical rows, Statement 2 gets a result with (the correct, I mean) 1 row.

If I delete the UNIQUE Key on tab2:

ALTER TABLE `tab2` DROP INDEX `col1`;

both Statements gets a result with only 1 row. Why does the UNIQUE Key have such a effect? It's a bug?

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