Thomas Munro <thomas.munro@enterprisedb.com> writes:
> On Mon, May 2, 2016 at 12:24 PM, <david@andl.org> wrote:
>> The file xact.c contains references to sub-transactions (subxact) and
>> transaction nesting level, but no obvious documentation about what these
>> correspond to in SQL.
> Subtransactions are used to implement SAVEPOINT, and also BEGIN blocks
> with EXCEPTION clauses in plpgsql.
Yeah. The implementation is based on nested subtransactions, and that
concept also applies pretty directly to, eg, BEGIN/EXCEPT blocks in
plpgsql. But what's exposed to SQL is SAVEPOINT/RELEASE SAVEPOINT/
ROLLBACK TO SAVEPOINT, because those operations are what the standard
specifies. If you hold your head at the correct angle you can see those
as nested subtransactions, but it's not exactly obvious --- mainly because
RELEASE and ROLLBACK can exit multiple levels of nested subtransaction in
one command.
regards, tom lane