MySQL 8.0: Remove i_s.*variables, i_s.*status

[morgo]

Feature Request

Is your feature request related to a problem? Please describe:

MySQL 8.0 removes the following information_schema tables:

GLOBAL_VARIABLES
SESSION_VARIABLES
GLOBAL_STATUS
SESSION_STATUS

These tables are empty in TiDB with the exception of session_variables, so removing these is not expected to cause incompatibility (see alternatives below).

As a separate issue, these tables are now in performance_schema in MySQL. It would be nice to add support in TiDB.

Describe the feature you'd like:

Remove the following information_schema tables:

GLOBAL_VARIABLES
SESSION_VARIABLES
GLOBAL_STATUS
SESSION_STATUS

Describe alternatives you've considered:

It is possible to include the information_schema.session_variables table in the compatibility mode described in #9141

Teachability, Documentation, Adoption, Migration Strategy:

This is easy to teach, since it makes TiDB behavior closer to MySQL.