Re: nested transactions

On Fri, 22 Nov 2002 00:32:46 -0500 (EST), Bruce Momjian
<pgman(at)candle(dot)pha(dot)pa(dot)us> wrote:
>I am going to work on nested transactions for 7.4.
> [...]
>And finally, I must abort tuple changes made by the aborted
>subtransaction. One way of doing that is to keep all relation id's
>modified by the transaction, and do a sequential scan of the tables on
>abort, changing the transaction id's to a fixed aborted transaction id.
>However, this could be slow. (We could store tids if only a few rows
>are updated by a subtransaction. That would speed it up considerably.)

Depends on your definition of "few". I don't expect problems for up
to several thousand tids. If there are more modified tuples, we could
first reduce the list to page numbers, before finally falling back to
table scans.

>Another idea is to use new transaction id's for the subtransactions, and
>[...]
>would increase the clog size per transaction from 2 bits to 4 bytes
>(two bits for status, 30 bits for offset to parent).

Nice idea, this 30 bit offset. But one could argue that increased
clog size even hurts users who don't use nested transactions at all.
If parent/child dependency is kept separate from status bits (in
pg_subtransxxxx files), additional I/O cost is only paid if
subtransactions are actually used. New status bits (XMIN_IS_SUB,
XMAX_IS_SUB) in tuple headers can avoid unnecessary parent xid
lookups.

I also thought of subtransaction xids in tuple headers as short lived
information. Under certain conditions they can be replaced with the
parent xid as soon as the parent transaction has finished. I proposed
this to be done on the next tuple access just like we set
committed/aborted flags now, though I'm not sure anymore that it is
safe to do this.

Old pg_subtrans files can be removed by VACUUM.

One more difference between the two proposals: The former (locally
remember modified tuples) can be used for recovery after a failed
command. The latter (subtrans tree) can only help, if we give a new
xid to each command, which I'm sure we don't want to do.

Servus
Manfred