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  1. MariaDB Server
  2. MDEV-8661

Wrong result for SELECT..WHERE a='a' AND a='a' COLLATE latin1_bin

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    Description

      This bug is similar to http://bugs.mysql.com/bug.php?id=5134
      Note, the patch for MySQL bug#5134 fixed only a particular case of the problem when the BINARY keyword is used. The problem is in fact more general.

      This script:

      SET NAMES latin1;
      		 
      DROP TABLE IF EXISTS t1;
      		 
      CREATE TABLE t1 (a CHAR(10));
      		 
      INSERT INTO t1 VALUES ('a'),('A');
      		 
      SELECT * FROM t1 WHERE a = 'a' COLLATE latin1_bin;

      correctly returns one row:

      +------+
      		 
      | a    |
      		 
      +------+
      		 
      | a    |
      		 
      +------+

      Now if I add an extra part into the condition:

      SELECT * FROM t1 WHERE a='a' AND a='a' COLLATE latin1_bin;

      it returns two rows:

      +------+
      		 
      | a    |
      		 
      +------+
      		 
      | a    |
      		 
      | A    |
      		 
      +------+

      The expected result is to return one row in both cases.

      The problem happens because "AND a='a' COLLATE latin1_bin" gets erroneously replaced to "AND 'a'='a' COLLATE latin1_bin" which is further evaluates to TRUE and gets removed from the WHERE condition. So, the query gets rewritten to:

      SELECT * FROM t1 WHERE a='a';

      The method which actually replaces the field to the constant is Item_field::equal_fields_propagator() in item.cc.

      This condition is not strict enough:

        if (!item || !has_compatible_context(item))
      		 
          item= this;

      It should also take into account the collations of the two operations that the field "a" appears in.

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        Issue Links

          blocks

          Bug - A problem which impairs or prevents the functions of the product. MDEV-8728 Fix a number of problems in equal field and equal expression propagation

          • Critical - Crashes, loss of data, severe memory leak.
          • Closed

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            bar Alexander Barkov added a comment - - edited

            Another example:

            SET NAMES latin1;
            		 
            DROP TABLE IF EXISTS t1;
            		 
            CREATE TABLE t1 (a VARCHAR(10) CHARACTER SET latin1 COLLATE latin1_swedish_ci);
            		 
            INSERT INTO t1 VALUES ('a'),('A');
            		 
            SELECT * FROM t1 WHERE a='a' COLLATE latin1_bin;
            		 
            SELECT * FROM t1 WHERE a='A' COLLATE latin1_swedish_ci;

            The above script returns one row in the first SELECT and two rows in the second SELECT:

            +------+
            		 
            | a    |
            		 
            +------+
            		 
            | a    |
            		 
            +------+
            		 
            1 row in set (0.00 sec)
            		 
             
            		 
            +------+
            		 
            | a    |
            		 
            +------+
            		 
            | a    |
            		 
            | A    |
            		 
            +------+
            		 
            2 rows in set (0.00 sec)

            Now if I join the above two condition using AND:

            SELECT * FROM t1 WHERE a='a' COLLATE latin1_bin AND a='A' COLLATE latin1_swedish_ci;

            I get empty set. This is wrong. The expected result is to return one row, the same result as in the first SELECT.

            bar Alexander Barkov added a comment - - edited Another example: SET NAMES latin1; DROP TABLE IF EXISTS t1; CREATE TABLE t1 (a VARCHAR(10) CHARACTER SET latin1 COLLATE latin1_swedish_ci); INSERT INTO t1 VALUES ('a'),('A'); SELECT * FROM t1 WHERE a='a' COLLATE latin1_bin; SELECT * FROM t1 WHERE a='A' COLLATE latin1_swedish_ci; The above script returns one row in the first SELECT and two rows in the second SELECT: +------+ | a | +------+ | a | +------+ 1 row in set (0.00 sec)   +------+ | a | +------+ | a | | A | +------+ 2 rows in set (0.00 sec) Now if I join the above two condition using AND: SELECT * FROM t1 WHERE a='a' COLLATE latin1_bin AND a='A' COLLATE latin1_swedish_ci; I get empty set. This is wrong. The expected result is to return one row, the same result as in the first SELECT.
            bar Alexander Barkov added a comment -

            The problem is repeatable in MySQL-5.7.8

            bar Alexander Barkov added a comment - The problem is repeatable in MySQL-5.7.8

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              bar Alexander Barkov
              bar Alexander Barkov
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                Updated:
                Resolved:

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