Type-safe representation for generated column with default value

[leiysky]

Generated column with default value is not allowed in Postgres, but we can not enforce this with current type constraint.

[tisonkun]

Ref

• feat: support generated column #9 (comment)
• https://www.postgresql.org/docs/current/ddl-generated-columns.html

[tisonkun]

@leiysky I misinterpret the requirement as ensuring the type of default value and the type of column are consistent.

Now I know that a generated column cannot have a default value, I can see two possible solutions:

1. Distinguish "normal" column spec and generated column spec
2. Do not check it

Option 1 is possible, and I'll try. But pqb and any SQL builder, for convenience, do not check strict syntax correctness - or else it would introduce quite a lot of type noise.

[tisonkun]

For example, sea-query doesn't check this:

#[derive(Debug, Clone, Default)]
pub struct ColumnSpec {
    pub nullable: Option<bool>,
    pub default: Option<Expr>, // <-- default value
    pub auto_increment: bool,
    pub unique: bool,
    pub primary_key: bool,
    pub check: Option<Check>,
    pub generated: Option<Generated>, // <-- generated options
    pub extra: Option<String>,
    pub comment: Option<String>,
    pub using: Option<Expr>,
}

#[derive(Debug, Clone)]
pub struct Generated {
    pub expr: Expr,
    pub stored: bool,
}