Re: PATCH: logical_work_mem and logical streaming of largein-progress transactions

On Sun, Oct 13, 2019 at 12:25 PM Dilip Kumar <dilipbalaut@gmail.com> wrote:
>
> On Thu, Oct 3, 2019 at 2:43 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> >
> > 3.
> > + *
> > + * While updating the existing change with detoasted tuple data, we need to
> > + * update the memory accounting info, because the change size will differ.
> > + * Otherwise the accounting may get out of sync, triggering serialization
> > + * at unexpected times.
> > + *
> > + * We simply subtract size of the change before rejiggering the tuple, and
> > + * then adding the new size. This makes it look like the change was removed
> > + * and then added back, except it only tweaks the accounting info.
> > + *
> > + * In particular it can't trigger serialization, which would be pointless
> > + * anyway as it happens during commit processing right before handing
> > + * the change to the output plugin.
> >   */
> >  static void
> >  ReorderBufferToastReplace(ReorderBuffer *rb, ReorderBufferTXN *txn,
> > @@ -3023,6 +3281,13 @@ ReorderBufferToastReplace(ReorderBuffer *rb, ReorderBufferTXN *txn,
> >   if (txn->toast_hash == NULL)
> >   return;
> >
> > + /*
> > + * We're going modify the size of the change, so to make sure the
> > + * accounting is correct we'll make it look like we're removing the
> > + * change now (with the old size), and then re-add it at the end.
> > + */
> > + ReorderBufferChangeMemoryUpdate(rb, change, false);
> >
> > It is not very clear why this change is required.  Basically, this is done at commit time after which actually we shouldn't attempt to spill these changes.  This is mentioned in comments as well, but it is not clear if that is the case, then how and when accounting can create a problem.  If possible, can you explain it with an example?
> >
> IIUC, we are keeping the track of the memory in ReorderBuffer which is
> common across the transactions.  So even if this transaction is
> committing and will not spill to dis but we need to keep the memory
> accounting correct for the future changes in other transactions.
>

You are right.  I somehow missed that we need to keep the size
computation in sync even during commit for other in-progress
transactions in the ReorderBuffer.  You can ignore this point or maybe
slightly adjust the comment to make it explicit.