Re: Skip collecting decoded changes of already-aborted transactions - Mailing list pgsql-hackers

On Wed, Dec 11, 2024 at 3:18 AM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
>
> On Mon, Dec 9, 2024 at 10:19 PM Dilip Kumar <dilipbalaut@gmail.com> wrote:

> > >
> > > If the largest transaction is non-streamable, won't the transaction
> > > returned by ReorderBufferLargestTXN() in the other case already
> > > suffice the need?
> >
> > I see your point, but I don’t think it’s quite the same. When
> > ReorderBufferCanStartStreaming() is true, the function
> > ReorderBufferLargestStreamableTopTXN() looks for the largest
> > transaction among those that have a base_snapshot. So, if the largest
> > transaction is aborted but hasn’t yet received a base_snapshot, it
> > will instead select the largest transaction that does have a
> > base_snapshot, which could be significantly smaller than the largest
> > aborted transaction.
>
> IIUC the transaction entries in reorderbuffer have the base snapshot
> before decoding the first change (see SnapBuildProcessChange()). In
> which case the transaction doesn't have the base snapshot and has the
> largest amount of changes? Subtransaction entries could transfer its
> base snapshot to its parent transaction entry but such subtransactions
> will be picked by ReorderBufferLargestTXN().
>
IIRC, there could be cases where reorder buffers of transactions can
grow in size without having a base snapshot, I think transactions
doing DDLs and generating a lot of INVALIDATION messages could fall in
such a category.  And that was one of the reasons why we were using
txns_by_base_snapshot_lsn inside
ReorderBufferLargestStreamableTopTXN().

--
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com