Convert ALTER TABLE ... ADD CONSTRAINT ... CHECK SQL to pgroll operation

[andrew-farries]

Convert SQL statements like:

ALTER TABLE foo ADD CONSTRAINT bar CHECK (age > 0)

to the equivalent pgroll operation:

[
  {
    "create_constraint": {
      "check": "age > 0",
      "columns": [
        "placeholder"
      ],
      "down": {
        "placeholder": "TODO: Implement SQL data migration"
      },
      "name": "bar",
      "table": "foo",
      "type": "check",
      "up": {
        "placeholder": "TODO: Implement SQL data migration"
      }
    }
  }
]

As we don't currently have a reliable way of extracting the columns covered by the constraint from the the CHECK SQL expression, the converted pgroll operation uses placeholders for the columns, up and down fields.

Some forms of the statement are not currently representable by the create_constraint operation. For these a raw SQL migration is generated:

"ALTER TABLE foo ADD CONSTRAINT bar CHECK (age > 0) NO INHERIT",
"ALTER TABLE foo ADD CONSTRAINT bar CHECK (age > 0) NOT VALID",