Regression with MariaDB 10.5.1+ with old temporal types in CDbColumnSchema $dbType

Problem

Starting from MariaDB 10.5.1 columns with old temporal formats are marked with a / mariadb-5.3 / comment in the output of SHOW CREATE TABLE, SHOW COLUMNS, DESCRIBE statements, as well as in the COLUMN_TYPE column of the INFORMATION_SCHEMA.COLUMNS Table. (https://mariadb.com/kb/en/datetime/#internal-format). MySQL 5.6.24+ has a similar feature, but it is disabled by default (https://dev.mysql.com/doc/refman/8.0/en/server-system-variables.html#sysvar_show_old_temporals. This affects column types DATETIME, TIME and TIMESTAMP for tables that have been created with the old temporal type and haven't been manually upgraded.

Yii 1 uses SHOW COLUMNS to extract the column type into CDbColumnSchema->dbType. Before the change: $dbType = 'datetime'; After the change: $dbType = 'datetime /* mariadb-5.3 */';

CDbColumnSchema defines $dbType as: "the DB type of this column.". This is not very strict and values vary per database system. However, problems occur where code relies on exact values of dbType:

CMysqlColumnSchema fails to set a null a default value for these columns in combination with a default expression of CURRENT_TIMESTAMP(): https://github.com/yiisoft/yii/blob/master/framework/db/schema/mysql/CMysqlColumnSchema.php#L47
CTimestampBehavior fails to map a default value expression for these columns: https://github.com/yiisoft/yii/blob/master/framework/zii/behaviors/CTimestampBehavior.php#L130
User code may have similar problems if they are expecting the same values as these framework classes.

Possible solutions

Do nothing. Users can find and convert their columns from the old temporal type to the new one, but this must be done manually with ALTER TABLE statements: https://mariadb.com/kb/en/datetime/#internal-format:~:text=In%20order%20to%20update%20table%20columns%20from%20the%20older%20format%20to%20the%20newer%20format%2C%20execute%20an%20ALTER%20TABLE...%20MODIFY%20COLUMN%20statement%20that%20changes%20the%20column%20to%20the%20*same*%20data%20type, https://dev.mysql.com/blog-archive/mysql-8-0-removing-support-for-old-temporal-datatypes/#:~:text=How%20to%20Fix%20Old%20Temporals
Keep dbType as-is, but modify CTimestampBehavior and CMySQLColumnSchema to deal with the problem. This is easy to apply but users still have to check their application code.
Strip the comments out of dbType to restore the old behavior. Other framework usages and user code can stay the same, but dbType no longer is a 1:1 value taken from the database engine.

Additional info

Q	A
Yii version	1.1.*
PHP version	any
Operating system	any
Database	MariaDB 10.5.1+ or MySQL 5.6.24+ with option `show_old_temporals` enabled.

yiisoft / yii