In this mode, nesting a transaction block will cause the creation of a SAVEPOINT on the current transaction if the options provided to the nested transaction block are compatible with the parent one.
In this mode, nesting a transaction block will always create a new transaction, in a separate connection. This mode is equivalent to setting the "transaction" option to "null" in the nested transaction block.
Be very careful when using this mode, as it can easily lead to transaction deadlocks if used improperly.
In this mode, nesting a transaction block in another will reuse the parent transaction if its options are compatible (or throw an error otherwise).
This is the default mode.