Incorrect JSON float compare

[breezewish]

Description

Bug Report

MySQL:

mysql> create table tx(id int key, a double, b double, c double, d double);
Query OK, 0 rows affected (0.02 sec)

mysql> insert into tx values (1, 0.1, 0.2, 0.3, 0.0);
Query OK, 1 row affected (0.00 sec)

mysql> select a+b, c from tx;
+---------------------+------+
| a+b                 | c    |
+---------------------+------+
| 0.30000000000000004 |  0.3 |
+---------------------+------+
1 row in set (0.00 sec)

mysql> select json_array(a+b) = json_array(c) from tx;
+---------------------------------+
| json_array(a+b) = json_array(c) |
+---------------------------------+
|                               0 |
+---------------------------------+
1 row in set (0.00 sec)

TiDB:

mysql> create table tx(id int key, a double, b double, c double, d double);
Query OK, 0 rows affected (0.13 sec)

mysql> insert into tx values (1, 0.1, 0.2, 0.3, 0.0);
Query OK, 1 row affected (0.01 sec)

mysql> select a+b, c from tx;
+---------------------+------+
| a+b                 | c    |
+---------------------+------+
| 0.30000000000000004 |  0.3 |
+---------------------+------+
1 row in set (0.00 sec)

mysql> select json_array(a+b) = json_array(c) from tx;
+---------------------------------+
| json_array(a+b) = json_array(c) |
+---------------------------------+
|                               1 |
+---------------------------------+
1 row in set (0.00 sec)

SIG slack channel

#sig-exec

Score

300

Mentor

• @lzmhhh123

[breezewish]

Notice that the following fact (MySQL) may indicate that it is simply not sound to convert numbers to float when doing the comparison:

mysql> create table tx (a double, b bigint);
Query OK, 0 rows affected (0.02 sec)

mysql> insert into tx values (9007199254740993, 9007199254740993);
Query OK, 1 row affected (0.00 sec)

mysql> insert into tx values (9007199254740993, 9007199254740992);
Query OK, 1 row affected (0.01 sec)

mysql> select json_array(a), json_array(b), json_array(a) = json_array(b) from tx;
+------------------------+--------------------+-------------------------------+
| json_array(a)          | json_array(b)      | json_array(a) = json_array(b) |
+------------------------+--------------------+-------------------------------+
| [9.007199254740992e15] | [9007199254740993] |                             0 |
| [9.007199254740992e15] | [9007199254740992] |                             1 |
+------------------------+--------------------+-------------------------------+
2 rows in set (0.00 sec)

[ghost]

Confirming this issue still exists in master:

DROP TABLE IF EXISTS tx;
create table tx(id int key, a double, b double, c double, d double);
insert into tx values (1, 0.1, 0.2, 0.3, 0.0);
select a+b, c from tx;
select json_array(a+b) = json_array(c) from tx;

..

mysql> select a+b, c from tx;
+---------------------+------+
| a+b                 | c    |
+---------------------+------+
| 0.30000000000000004 |  0.3 |
+---------------------+------+
1 row in set (0.00 sec)

mysql> select json_array(a+b) = json_array(c) from tx;
+---------------------------------+
| json_array(a+b) = json_array(c) |
+---------------------------------+
|                               1 |
+---------------------------------+
1 row in set (0.00 sec)

mysql> select tidb_version()\G
*************************** 1. row ***************************
tidb_version(): Release Version: v4.0.0-beta.2-750-g8a661044c
Edition: Community
Git Commit Hash: 8a661044cedf8daad1de4fbf79a390962b6f6c3b
Git Branch: master
UTC Build Time: 2020-07-10 10:52:37
GoVersion: go1.13
Race Enabled: false
TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306
Check Table Before Drop: false
1 row in set (0.00 sec)

[ti-challenge-bot]

The issue has been removed from the challenge program.

[ti-srebot]

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