Re: Nested transactions: low level stuff

Tom Lane wrote:
> Bruce Momjian <pgman@candle.pha.pa.us> writes:
> >     Do we want UNDO just for subtransactions?
> > That was pretty easily defeated, though I made an argument that you
> > could do UNDO pretty cheaply when you have WAL ensuring crash recovery.
> 
> That argument was what got us into the early-7.1 WAL bloat problems.
> I don't think it's "pretty cheap" to have to hold the entire WAL for the
> length of your longest-running transactions.

With my idea, you wouldn't have to keep WAL around.  Each backend would
keep a list of tids or the relid (if lots of rows are changed) in local
memory and UNDO on subtransaction abort.

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