Nested Transactions in CockroachDB 20.1

CockroachDB 20.1 introduces support for nested transactions, a SQL feature that simplifies the work of programmers of certain client applications. Nested transactions are additional transactions that occur within a regular transaction or other nested transactions, like Russian dolls. They are invisible to concurrent clients due to the atomicity and isolation of their surrounding, "outermost" transaction. Nested transactions have been invented for the benefit of software engineers of client applications in component-based design. CockroachDB's support for nested transactions is limited in three noticeable ways: locking semantics, canceling schema changes, and recovering from serializability conflicts.