Re: Skip collecting decoded changes of already-aborted transactions

On Wed, Dec 11, 2024 at 8:21 AM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
>
> On Wed, Dec 11, 2024 at 3:18 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> >
> > On Mon, Dec 9, 2024 at 10:19 PM Dilip Kumar <dilipbalaut(at)gmail(dot)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.
>

Are we recording such changes in the reorder buffer? If so, can you
please share how? AFAICU, the main idea behind skipping aborts is to
avoid sending a lot of data to the client that later needs to be
discarded or cases where we spent resources/time spilling the changes
that later need to be discarded. In that vein, the current idea of the
patch where it truncates and skips aborted xacts before streaming or
spilling them sounds reasonable.

--
With Regards,
Amit Kapila.