[MDEV-8369] Unexpected impossible WHERE for a condition on a ZEROFILL field - JIRA

Details

Type: Bug
Status: Closed
Priority: Major
Resolution: Fixed
Affects Version/s: 10.1, 10.0, 5.5
Fix Version/s: 10.1.8
Component/s: Optimizer
Labels:
None

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 |
+------+-------------+-------+------+---------------+------+---------+------+------+------------------+

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MDEV-8728 Fix a number of problems in equal field and equal expression propagation

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Jiri Kavalik added a comment - 2015-06-24 13:21 - edited

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) |
+-------+------+------------------------------------------------------------------------------+

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Jiri Kavalik added a comment - 2015-06-24 13:21 - edited 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) | +-------+------+------------------------------------------------------------------------------+

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Alexander Barkov added a comment - 2015-08-20 13:36 - edited

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 .

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Alexander Barkov added a comment - 2015-08-20 13:36 - edited 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 .

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Alexander Barkov added a comment - 2015-08-21 09:30

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 |
+------+-------------+-------+------+---------------+------+---------+------+------+------------------+

Show

Alexander Barkov added a comment - 2015-08-21 09:30 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 | +------+-------------+-------+------+---------------+------+---------+------+------+------------------+

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Alexander Barkov added a comment - 2015-08-21 09:54

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;

Show

Alexander Barkov added a comment - 2015-08-21 09:54 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;

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Alexander Barkov added a comment - 2015-08-21 10:02 - edited

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';

Show

Alexander Barkov added a comment - 2015-08-21 10:02 - edited 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';

Unexpected impossible WHERE for a condition on a ZEROFILL field

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