The application probably still treats id as unique, but nothing in the schema guarantees it. And you can’t recover the guarantee with a separate UNIQUE (id) constraint: both MySQL and PostgreSQL require every unique constraint on a partitioned table to include the partition key columns. The uniqueness property has effectively been traded away.
And the post did mention it, argueing it may be better to partition by auto-incremented ID with the support of catch-all range and partution monitoring.
> The primary key already exists. For tables using BIGINT AUTO_INCREMENT, it’s monotonically increasing: newer rows have larger IDs. That’s the property range partitioning needs. The primary key is the partition key.
> The primary key already exists. For tables using BIGINT AUTO_INCREMENT, it’s monotonically increasing: newer rows have larger IDs. That’s the property range partitioning needs. The primary key is the partition key.