Subtransactions can be started explicitly using the

<command>SAVEPOINT</command> command, but can also be started in

other ways, such as PL/pgSQL's <command>EXCEPTION</command> clause.

PL/Python and PL/TCL also support explicit subtransactions.

PL/Python and PL/Tcl also support explicit subtransactions.

Subtransactions can also be started from other subtransactions.

The top-level transaction and its child subtransactions form a

hierarchy or tree, which is why we refer to the main transaction as