Precision controll on Pi() seems not work as mariadb does

[hnes]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

select PI(), PI()+0.000000000000000000;
select PI(), PI()+0.0000000000000;
select PI(), PI()+0.0000000;

2. What did you expect to see? (Required)

MariaDB [testdb]> select version();
+----------------+
| version()      |
+----------------+
| 5.5.68-MariaDB |
+----------------+
1 row in set (0.00 sec)

MariaDB [testdb]> select PI(), PI()+0.000000000000000000;
+----------+---------------------------+
| PI()     | PI()+0.000000000000000000 |
+----------+---------------------------+
| 3.141593 |      3.141592653589793000 |
+----------+---------------------------+
1 row in set (0.00 sec)

MariaDB [testdb]> select PI(), PI()+0.0000000000000;
+----------+----------------------+
| PI()     | PI()+0.0000000000000 |
+----------+----------------------+
| 3.141593 |      3.1415926535898 |
+----------+----------------------+
1 row in set (0.00 sec)

MariaDB [testdb]> select PI(), PI()+0.0000000;
+----------+----------------+
| PI()     | PI()+0.0000000 |
+----------+----------------+
| 3.141593 |      3.1415927 |
+----------+----------------+
1 row in set (0.00 sec)

3. What did you see instead (Required)

MySQL [testdb]> select PI(), PI()+0.000000000000000000;
+----------+---------------------------+
| PI()     | PI()+0.000000000000000000 |
+----------+---------------------------+
| 3.141593 |         3.141592653589793 |
+----------+---------------------------+
1 row in set (0.00 sec)

MySQL [testdb]> select PI(), PI()+0.0000000000000;
+----------+----------------------+
| PI()     | PI()+0.0000000000000 |
+----------+----------------------+
| 3.141593 |    3.141592653589793 |
+----------+----------------------+
1 row in set (0.00 sec)

MySQL [testdb]> select PI(), PI()+0.0000000;
+----------+-------------------+
| PI()     | PI()+0.0000000    |
+----------+-------------------+
| 3.141593 | 3.141592653589793 |
+----------+-------------------+
1 row in set (0.00 sec)

4. What is your TiDB version? (Required)

MySQL [testdb]> SELECT tidb_version();
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| tidb_version()                                                                                                                                                                                                                                                                                                       |
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Release Version: v5.1.0
Edition: Community
Git Commit Hash: 8acd5c88471cb7b4d4c4a8ed73b4d53d6833f13e
Git Branch: heads/refs/tags/v5.1.0
UTC Build Time: 2021-06-24 07:10:32
GoVersion: go1.16.4
Race Enabled: false
TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306
Check Table Before Drop: false |
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

[dveeden]

I don't think one should rely on this behavior and instead use ROUND()/FLOOR() instead.

Comparing this with MySQL 9.1.0:

mysql-9.1.0> select PI(), PI()+0.000000000000000000;
+----------+---------------------------+
| PI()     | PI()+0.000000000000000000 |
+----------+---------------------------+
| 3.141593 |      3.141592653589793000 |
+----------+---------------------------+
1 row in set (0.00 sec)

mysql-9.1.0> select PI(), PI()+0.0000000000000;
+----------+----------------------+
| PI()     | PI()+0.0000000000000 |
+----------+----------------------+
| 3.141593 |      3.1415926535898 |
+----------+----------------------+
1 row in set (0.00 sec)

mysql-9.1.0> select PI(), PI()+0.0000000;
+----------+----------------+
| PI()     | PI()+0.0000000 |
+----------+----------------+
| 3.141593 |      3.1415927 |
+----------+----------------+
1 row in set (0.01 sec)

mysql-9.1.0> SELECT ROUND(PI(),5);
+---------------+
| ROUND(PI(),5) |
+---------------+
|       3.14159 |
+---------------+
1 row in set (0.00 sec)