[MDEV-8712] Wrong result for SELECT..WHERE latin1_bin_column=_latin1'a' AND latin1_bin_column='A' Created: 2015-08-31  Updated: 2015-09-11  Resolved: 2015-09-11

Status: Closed
Project: MariaDB Server
Component/s: Optimizer
Affects Version/s: 10.0, 10.1
Fix Version/s: 10.1.8

Type: Bug Priority: Major
Reporter: Alexander Barkov Assignee: Alexander Barkov
Resolution: Fixed Votes: 0
Labels: None

Issue Links:
Duplicate
duplicates MDEV-8705 Wrong result for SELECT..WHERE latin1... Closed

 Description   

This script:

SET NAMES latin1;
 
DROP TABLE IF EXISTS t1;
 
CREATE TABLE t1 (a VARCHAR(20) COLLATE latin1_bin);
 
INSERT INTO t1 VALUES ('a');
 
SELECT * FROM t1 WHERE a='A';
 
SELECT * FROM t1 WHERE a='A' AND a=_latin1'a';

correctly returns empty set for both SELECT queries.

If I now change the order of the two conditions in the second SELECT query:

SELECT * FROM t1 WHERE a=_latin1'a' AND a='A';

it erroneously returns one row:

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

The same problem is repeatable with character set introducers
(SET NAMES latin1 is not needed in this case):

DROP TABLE IF EXISTS t1;
 
CREATE TABLE t1 (a VARCHAR(20) COLLATE latin1_bin);
 
INSERT INTO t1 VALUES ('a');
 
SELECT * FROM t1 WHERE a=_latin1'A';
 
SELECT * FROM t1 WHERE a=_latin1'A' AND a=_latin1'a';
 
SELECT * FROM t1 WHERE a=_latin1'a' AND a=_latin1'A';

The problem happens in Item_equal::add_const(), in this code:

  else
 
  {
 
    Item_func_eq *func= new (thd->mem_root) Item_func_eq(thd, c, const_item);
 
    if (func->set_cmp_func())
 
    {
 
      /*
 
        Setting a comparison function fails when trying to compare
 
        incompatible charsets. Charset compatibility is checked earlier,
 
        except for constant subqueries where we may do it here.
 
      */
 
      return;
 
    }
 
    func->quick_fix_field();
 
    cond_false= !func->val_int();
 
  }

It does not take into account the column collation (latin1_bin) and compares the two constants as latin1_swedish_ci, which gives TRUE. The second part of the AND is then eliminated from the query.

So:

SELECT * FROM t1 WHERE a=_latin1'A' AND a=_latin1'a';

gets rewritten to:

SELECT * FROM t1 WHERE a=_latin1'A';

which returns no rows, while:

SELECT * FROM t1 WHERE a=_latin1'a' AND a=_latin1'A';

gets rewritten as

SELECT * FROM t1 WHERE a=_latin1'a';

which returns one row.
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