planner: allow insert default into generated columns

[Deardrops]

What problem does this PR solve?

In MySQL document,

For INSERT, REPLACE, and UPDATE, if a generated column is inserted into, replaced, or updated explicitly, the only permitted value is DEFAULT.

create table t (a int, b int as (a + 1))
insert into t value (1, default)
MySQL: Query OK (0.191 sec)
TiDB: Error 3105: The value specified for generated column 'b' in table 't' is not allowed.

What is changed and how it works?

Allow use the DEFAULT keyword when insert into the generated columns.

When build the insert plan, check if

• the target column is generated column,
• and the insert expression is DEFAULT

Skip add this column's expression into this insert plan.

Because we can't insert value into generated clumns directly, it's acceptable to ignore the generated column in insert plan.

Check List

Tests

• Intergration test

Release note

• Allow use the DEFAULT keyword when insert into the generated columns.

[codecov]

Codecov Report

Merging #11901 into master will not change coverage.
The diff coverage is n/a.

@@             Coverage Diff             @@
##             master     #11901   +/-   ##
===========================================
  Coverage   81.3215%   81.3215%           
===========================================
  Files           443        443           
  Lines         95286      95286           
===========================================
  Hits          77488      77488           
  Misses        12309      12309           
  Partials       5489       5489

[Deardrops]

/run-all-tests

[Deardrops]

/run-all-tests

[wjhuang2016]

LGTM

[tangenta]

LGTM

[tangenta]

@Deardrops Do we have the plan to support insert into t set a = 1, b = default?

[Deardrops]

@tangenta

@Deardrops Do we have the plan to support insert into t set a = 1, b = default?

Yes, we have open a issue for this problem #11939

[Deardrops]

/run-all-tests