Allow to define expression default values MySQL 8.0.13

[oleg-andreyev]

Feature Request

Q	A
New Feature	yes
RFC	no

Summary

As of 8.0.13 it's possible to define DEFAULT as an expression:
https://dev.mysql.com/doc/refman/8.0/en/data-type-defaults.html#data-type-defaults-explicit

Would be nice to allow such behavior.

Context (probably bug):

When we have JSON-type column and field in Entity is nullable=false, MySQL will put default value as literal null and when decoding JsonType will return NULL so my property must be nullable while it's undesirable behavior.

IMO best option how to handle this:

• add new key expression to options
• if expression===true do not enquote default value, but instead add parentheses

AR:

CREATE TABLE t1 (
  j JSON        DEFAULT 'JSON_ARRAY()'
);

ER:

CREATE TABLE t1 (
  j JSON        DEFAULT (JSON_ARRAY())
);