[MDEV-8369] Unexpected impossible WHERE for a condition on a ZEROFILL field Created: 2015-06-24  Updated: 2015-09-14  Resolved: 2015-09-11

Status: Closed
Project: MariaDB Server
Component/s: Optimizer
Affects Version/s: 5.5, 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:
Blocks
blocks MDEV-8728 Fix a number of problems in equal fie... Closed

 Description   

This script:

DROP TABLE IF EXISTS t1;
 
CREATE TABLE t1 (a INT ZEROFILL);
 
INSERT INTO t1 VALUES (128);
 
SELECT * FROM t1 WHERE a=128;
 
SELECT * FROM t1 WHERE hex(a)='80';

correctly returns one row for both SELECT queries.

if I join the two conditions using AND:

SELECT * FROM t1 WHERE a=128 AND hex(a)='80';

it unexpectedly returns Empty set.

EXPLAIN for the above query tells "Impossible WHERE":

MariaDB [test]> EXPLAIN SELECT * FROM t1 WHERE a=128 AND hex(a)='80';
 
+------+-------------+-------+------+---------------+------+---------+------+------+------------------+
 
| id   | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra            |
 
+------+-------------+-------+------+---------------+------+---------+------+------+------------------+
 
|    1 | SIMPLE      | NULL  | NULL | NULL          | NULL | NULL    | NULL | NULL | Impossible WHERE |
 
+------+-------------+-------+------+---------------+------+---------+------+------+------------------+



 Comments   
Comment by Jiri Kavalik [ 2015-06-24 ] 

explain extended SELECT * FROM t1 WHERE a=128 AND hex(a)='80';
 
show warnings;
 
+-------+------+--------------------------------------------------------+
 
| Level | Code | Message                                                |
 
+-------+------+--------------------------------------------------------+
 
| Note  | 1003 | select `test`.`t1`.`a` AS `a` from `test`.`t1` where 0 |
 
+-------+------+--------------------------------------------------------+

and without zerofil

explain extended SELECT * FROM t1 WHERE a=128 AND hex(a)='80';
 
+-------+------+------------------------------------------------------------------------------+
 
| Level | Code | Message                                                                      |
 
+-------+------+------------------------------------------------------------------------------+
 
| Note  | 1003 | select `test`.`t1`.`a` AS `a` from `test`.`t1` where (`test`.`t1`.`a` = 128) |
 
+-------+------+------------------------------------------------------------------------------+

Comment by Alexander Barkov [ 2015-08-20 ]

The problem happens because optimize_cond() when propagating equalities replaces the condition to:

WHERE a=128 AND hex('0000000128')='80'

which later evaluates to FALSE because HEX() returns hex representations of the string rather than hex representation of the original number 128.

convert_zerofill_number_to_string() is the function which actually replaces Item_field to Item_string is .

Comment by Alexander Barkov [ 2015-08-21 ]

This script demonstrates a similar problem:

DROP TABLE IF EXISTS t1;
 
CREATE TABLE t1 (a INT(6) ZEROFILL);
 
INSERT INTO t1 VALUES (1);
 
SELECT * FROM t1 WHERE a=1;
 
SELECT * FROM t1 WHERE DES_ENCRYPT('test',a)=_latin1 0x8104A375CC9BD44E7C COLLATE latin1_bin; SELECT * FROM t1 WHERE a=1 AND DES_ENCRYPT('test',a)=_latin1 0x8104A375CC9BD44E7C COLLATE latin1_bin;

The first and the second SELECT return one row. The third SELECT returns empty set.
EXPLAIN for the third SELECT:

EXPLAIN SELECT * FROM t1 WHERE a=1 AND DES_ENCRYPT('test',a)=_latin1 0x8104A375CC9BD44E7C COLLATE latin1_bin;

returns

+------+-------------+-------+------+---------------+------+---------+------+------+------------------+
 
| id   | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra            |
 
+------+-------------+-------+------+---------------+------+---------+------+------+------------------+
 
|    1 | SIMPLE      | NULL  | NULL | NULL          | NULL | NULL    | NULL | NULL | Impossible WHERE |
 
+------+-------------+-------+------+---------------+------+---------+------+------+------------------+

Comment by Alexander Barkov [ 2015-08-21 ]

A similar example with WEIGHT_STRING:

DROP TABLE IF EXISTS t1;
 
CREATE TABLE t1 (a INT(6) ZEROFILL);
 
INSERT INTO t1 VALUES (1);
 
SELECT * FROM t1 WHERE a=1;
 
SELECT * FROM t1 WHERE WEIGHT_STRING(a) IS NULL;
 
SELECT * FROM t1 WHERE a=1 AND WEIGHT_STRING(a) IS  NULL;

Comment by Alexander Barkov [ 2015-08-21 ]

A similar example using FORMAT():

DROP TABLE IF EXISTS t1;
 
CREATE TABLE t1 (a BIGINT(20) ZEROFILL);
 
INSERT INTO t1 VALUES (18446744073709551615);
 
SELECT * FROM t1 WHERE a=18446744073709551615;
 
SELECT * FROM t1 WHERE FORMAT(a,0)='18,446,744,073,709,551,615';
 
SELECT * FROM t1 WHERE a=18446744073709551615 AND FORMAT(a,0)='18,446,744,073,709,551,615';

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