Add support for expression as index key part

[jvliwanag]

In MySQL 8.0.17+, there's now support for indices defined on expressions.

This is currently used on MySQL's multi valued index.

Sample schema:

CREATE TABLE customers (
    id BIGINT NOT NULL AUTO_INCREMENT PRIMARY KEY,
    modified DATETIME DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
    custinfo JSON,
    INDEX zips( (CAST(custinfo->'$.zipcode' AS UNSIGNED ARRAY)) )
    );

Taken from: mysql docs

Currently, sea-query only supports indices on column names.

[jvliwanag]

Found a cheat/workaround for now by creating defining a struct and implementing Iden for it that overrides the quotes.

Example to render:

((CAST(JSON_KEYS(`option_set_refs`) AS char(255) array)))

I use:

struct JsonKeysRefIndexExpr(&'static str);

impl Iden for JsonKeysRefIndexExpr {
    fn prepare(&self, s: &mut dyn std::fmt::Write, q: sq::Quote) {
        write!(s, "{}", self.quoted(q)).unwrap();
    }

    fn quoted(&self, q: sq::Quote) -> String {
        format!(
            "(CAST(JSON_KEYS({}) AS char(255) array))",
            iden(self.0).quoted(q)
        )
    }

    fn unquoted(&self, s: &mut dyn std::fmt::Write) {
        write!(s, "(CAST(JSON_KEYS({}) AS char(255) array))", self.0).unwrap();
    }
}