Cases where Lookup Vindexes are not required to enforce global uniqueness.

example.sql

# All tables have a user_id column, and the keyspace is sharded by this column.

CREATE TABLE `products` (
  `id` BIGINT NOT NULL,
  `user_id` BIGINT NOT NULL,
  PRIMARY KEY (`id`)
)

CREATE TABLE `orders` (
  `id` BIGINT NOT NULL,
  `user_id` BIGINT NOT NULL,
  `product_id` BIGINT NOT NULL,
  `line_item_id` BIGINT NOT NULL,
  PRIMARY KEY (`id`),

  # Not unique across shards. We would require a Lookup Vindex.
  UNIQUE KEY `index_on_line_item_id` (`line_item_id`),

  # Unique across shards, because each 'user_id' is only in one shard.
  UNIQUE KEY `index_orders_on_user_id_and_line_item_id` (`user_id`,`line_item_id`),

  # Unique across shards, because the 'products' table is sharded by 'user_id'. Uniqueness is guaranteed as a 'product_id' will only exist in a single shard.
  UNIQUE KEY `index_orders_on_product_id_and_line_item_id` (`product_id`,`line_item_id`),
)