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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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                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.

                  Show
                  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.
                  Hide
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                  bar Alexander Barkov added a comment -

                  The problem is repeatable in MySQL-5.7.8

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

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                    • Assignee:
                      bar Alexander Barkov
                      Reporter:
                      bar Alexander Barkov
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                      • Created:
                        Updated:
                        Resolved: