Embeddable, Inheritance & Secondary Tables

@SecondaryTable maps one entity across multiple tables sharing the primary key; route fields with @Column(table="...") and Hibernate joins on read and splits on write automatically.

Random UUIDv4 as a PK scatters inserts across the B-tree causing page splits, fragmentation, and cache misses; fix with time-ordered UUIDv7, ULID, or sequential bigint so inserts append.

@Embeddable marks a value object class stored inline in the owning entity's table; @Embedded includes it in the entity; @AttributeOverride renames columns when the same embeddable type appears twice.

@MappedSuperclass shares columns with subclasses but is not an entity and cannot be queried; @Inheritance creates a real entity hierarchy with polymorphic queries and associations.

SINGLE_TABLE stores the whole hierarchy in one table with a discriminator column; JOINED normalizes into base plus per-subclass tables joined by key; TABLE_PER_CLASS gives each concrete class a full standalone table.