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

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.

9.
+ReorderBufferHandleConcurrentAbort(ReorderBuffer *rb, ReorderBufferTXN *txn,
{
..
+ ReorderBufferToastReset(rb, txn);
+ if (specinsert != NULL)
+ ReorderBufferReturnChange(rb, specinsert);
..
}

Why do we need to do these here when we wouldn't have been done for
any exception other than ERRCODE_TRANSACTION_ROLLBACK?

10. I have got the below failure once. I have not investigated this
in detail as the patch is still under progress. See, if you have any
idea?
# Failed test 'check extra columns contain local defaults'
# at t/013_stream_subxact_ddl_abort.pl line 81.
# got: '2|0'
# expected: '1000|500'
# Looks like you failed 1 test of 2.
make[2]: *** [check] Error 1
make[1]: *** [check-subscription-recurse] Error 2
make[1]: *** Waiting for unfinished jobs....
make: *** [check-world-src/test-recurse] Error 2

11. Can we test by introducing a new GUC such that all the
transactions (at least in existing tests) start to stream? Basically,
it will allow us to disregard logical_decoding_work_mem and ensure
that all regression tests pass through new-code. Note, I am
suggesting this just for testing purposes, not for actual integration
in the code.

--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com