Re: Savepoints

Mikheev, Vadim wrote:
> > I have talked in the past about a possible implementation of
> > savepoints/nested transactions.  I would like to more formally outline
> > my ideas below.
> 
> Well, I would like to do the same -:)

Good.

> > ...
> > There is no reason for other backend to be able to see savepoint undo
> > information, and keeping it private greatly simplifies the
> > implementation.
> 
> Yes... and requires additional memory/disk space: we keep old records
> in data files and we'll store them again...

I was suggesting keeping only relid/tid or in some cases only relid. 
Seems like one or the other will fit all needs:  relid/tid for update of
a few rows, relid for many rows updated in the same table.  I saw no
need to store the actual data.

> How about: use overwriting smgr + put old records into rollback
> segments - RS - (you have to keep them somewhere till TX's running
> anyway) + use WAL only as REDO log (RS will be used to rollback TX'
> changes and WAL will be used for RS/data files recovery).
> Something like what Oracle does.

Why record the old data rows rather than the tids?  While the
transaction is running, the rows can't be moved anyway.  Also, why store
them in a shared area.  That has additional requirements because one old
transaction can require all transactions to keep their stuff around. 
Why not just make it a private data file for each backend?

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