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

On Sat, Jul 4, 2020 at 11:35 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Tue, Jun 30, 2020 at 5:20 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> >
> > Let me know what you think about the above changes.
> >
>
> I went ahead and made few changes in
> 0005-Implement-streaming-mode-in-ReorderBuffer which are explained
> below. I have few questions and suggestions for the patch as well
> which are also covered in below points.
>
> 1.
> + if (prev_lsn == InvalidXLogRecPtr)
> + {
> + if (streaming)
> + rb->stream_start(rb, txn, change->lsn);
> + else
> + rb->begin(rb, txn);
> + stream_started = true;
> + }
>
> I don't think we want to move begin callback here that will change the
> existing semantics, so it is better to move begin at its original
> position. I have made the required changes in the attached patch.
>
> 2.
> ReorderBufferTruncateTXN()
> {
> ..
> + dlist_foreach_modify(iter, &txn->changes)
> + {
> + ReorderBufferChange *change;
> +
> + change = dlist_container(ReorderBufferChange, node, iter.cur);
> +
> + /* remove the change from it's containing list */
> + dlist_delete(&change->node);
> +
> + ReorderBufferReturnChange(rb, change);
> + }
> ..
> }
>
> I think here we can add an Assert that we're not mixing changes from
> different transactions. See the changes in the patch.
>
> 3.
> SetupCheckXidLive()
> {
> ..
> + /*
> + * setup CheckXidAlive if it's not committed yet. We don't check if the xid
> + * aborted. That will happen during catalog access. Also, reset the
> + * bsysscan flag.
> + */
> + if (!TransactionIdDidCommit(xid))
> + {
> + CheckXidAlive = xid;
> + bsysscan = false;
> ..
> }
>
> What is the need to reset bsysscan flag here if we are already
> resetting on error (like in the previous patch sent by me)?
>
> 4.
> ReorderBufferProcessTXN()
> {
> ..
> ..
> + /* Reset the CheckXidAlive */
> + if (streaming)
> + CheckXidAlive = InvalidTransactionId;
> ..
> }
>
> Similar to the previous point, we don't need this as well because
> AbortCurrentTransaction would have taken care of this.
>
> 5.
> + * XXX Do we need to check if the transaction has some changes to stream
> + * (maybe it got streamed right before the commit, which attempts to
> + * stream it again before the commit)?
> + */
> +static void
> +ReorderBufferStreamTXN(ReorderBuffer *rb, ReorderBufferTXN *txn)
>
> The above comment doesn't make much sense to me, so I have removed it.
> Basically, if there are no changes before commit, we still need to
> send commit and anyway if there are no more changes
> ReorderBufferProcessTXN will not do anything.
>
> 6.
> +ReorderBufferStreamTXN(ReorderBuffer *rb, ReorderBufferTXN *txn)
> {
> ..
> if (txn->snapshot_now == NULL)
> + {
> + dlist_iter subxact_i;
> +
> + /* make sure this transaction is streamed for the first time */
> + Assert(!rbtxn_is_streamed(txn));
> +
> + /* at the beginning we should have invalid command ID */
> + Assert(txn->command_id == InvalidCommandId);
> +
> + dlist_foreach(subxact_i, &txn->subtxns)
> + {
> + ReorderBufferTXN *subtxn;
> +
> + subtxn = dlist_container(ReorderBufferTXN, node, subxact_i.cur);
> + ReorderBufferTransferSnapToParent(txn, subtxn);
> + }
> ..
> }
>
> Here, it is possible that there is no base_snapshot for txn, so we
> need a check for that similar to ReorderBufferCommit.
>
> 7. Apart from the above, I made few changes in comments and ran pgindent.
>
> 8. We can't stream the transaction before we reach the
> SNAPBUILD_CONSISTENT state because some other output plugin can apply
> those changes unlike what we do with pgoutput plugin (which writes to
> file). And, I think applying the transactions without reaching a
> consistent state would be anyway wrong. So, we should avoid that and
> if do that then we should have an Assert for streamed txns rather than
> sending abort for them in ReorderBufferForget.

I was analyzing this point so currently, we only enable streaming in
StartReplicationSlot so basically in CreateReplicationSlot the
streaming will be always off because by that time plugins are not yet
startup that will happen only on StartReplicationSlot. See below
snippet from patch 0007. However, I agree during start replication
slot we might decode some of the extra walls of the transaction for
which we already got the commit confirmation and we must have a way to
avoid that. But I think we don't need to do anything for the
CONSISTENT snapshot point. What's your thought on this?

@@ -1016,6 +1016,12 @@ CreateReplicationSlot(CreateReplicationSlotCmd *cmd)
WalSndPrepareWrite, WalSndWriteData,
WalSndUpdateProgress);

+ /*
+ * Make sure streaming is disabled here - we may have the methods,
+ * but we don't have anywhere to send the data yet.
+ */
+ ctx->streaming = false;
+

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