Design tidb_enable_strict_compatibility_80 = BOOLEAN

[morgo]

Feature Request

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

In planning MySQL 8.0 compatibility, we would like to offer a smoother upgrade than MySQL 5.7 -> 8.0 offers.

For example:

• set tx_isolation = x is removed.
• GRANT auto-creating users is removed (the NO_AUTO_CREATE_USER sql mode is meaningless).

Describe the feature you'd like:

I propose we create a new setting, roughly named tidb_enable_strict_compatibility_80, default FALSE. This means by default there will be better compatibility with 5.7 where possible, but a user can opt-in to MySQL 8.0 behavior.

This has the advantage of future smoother upgrades and sometimes improved usability, security etc. A future release may enable strict compatibility by default.

Describe alternatives you've considered:

It could also be possible to enable strict compatibility by default.

Teachability, Documentation, Adoption, Migration Strategy:

It will need some design to work out what conditions require compatibility. Trivial items like removing unused variables should not provide compatibility.

[morgo]

I've renamed the variable tidb_strict_compatibility_80 to fit with existing tidb_ namespace. 80 is also used since it matches the MySQL variable show_compatibility_56.

[ghost]

I have updated the proposed variable to tidb_enable_XX to match an established convention of sysvars.