Re: PATCH: logical_work_mem and logical streaming of large in-progress transactions

On Thu, Jan 9, 2020 at 9:35 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Wed, Jan 8, 2020 at 1:12 PM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
> >
> > I have observed one more design issue.
> >
>
> Good observation.
>
> > The problem is that when we
> > get a toasted chunks we remember the changes in the memory(hash table)
> > but don't stream until we get the actual change on the main table.
> > Now, the problem is that we might get the change of the toasted table
> > and the main table in different streams. So basically, in a stream,
> > if we have only got the toasted tuples then even after
> > ReorderBufferStreamTXN the memory usage will not be reduced.
> >
>
> I think we can't split such changes in a different stream (unless we
> design an entirely new solution to send partial changes of toast
> data), so we need to send them together. We can keep a flag like
> data_complete in ReorderBufferTxn and mark it complete only when we
> are able to assemble the entire tuple. Now, whenever, we try to
> stream the changes once we reach the memory threshold, we can check
> whether the data_complete flag is true, if so, then only send the
> changes, otherwise, we can pick the next largest transaction. I think
> we can retry it for few times and if we get the incomplete data for
> multiple transactions, then we can decide to spill the transaction or
> maybe we can directly spill the first largest transaction which has
> incomplete data.
>
Yeah, we might do something on this line. Basically, we need to mark
the top-transaction as data-incomplete if any of its subtransaction is
having data-incomplete (it will always be the latest sub-transaction
of the top transaction). Also, for streaming, we are checking the
largest top transaction whereas for spilling we just need the larget
(sub) transaction. So we also need to decide while picking the
largest top transaction for streaming, if we get a few transactions
with in-complete data then how we will go for the spill. Do we spill
all the sub-transactions under this top transaction or we will again
find the larget (sub) transaction for spilling.

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