RE: Avoid streaming the transaction which are skipped (in corner cases) - Mailing list pgsql-hackers

On Sun, Nov 27, 2022 1:33 PM Dilip Kumar <dilipbalaut@gmail.com> wrote:
> 
> On Sat, Nov 26, 2022 at 12:15 PM Amit Kapila <amit.kapila16@gmail.com>
> wrote:
> >
> > On Fri, Nov 25, 2022 at 5:38 PM Amit Kapila <amit.kapila16@gmail.com>
> wrote:
> > >
> > > On Fri, Nov 25, 2022 at 1:35 PM Dilip Kumar <dilipbalaut@gmail.com>
> wrote:
> > > >
> > > > During DecodeCommit() for skipping a transaction we use ReadRecPtr to
> > > > check whether to skip this transaction or not.  Whereas in
> > > > ReorderBufferCanStartStreaming() we use EndRecPtr to check whether
> to
> > > > stream or not. Generally it will not create a problem but if the
> > > > commit record itself is adding some changes to the transaction(e.g.
> > > > snapshot) and if the "start_decoding_at" is in between ReadRecPtr and
> > > > EndRecPtr then streaming will decide to stream the transaction where
> > > > as DecodeCommit will decide to skip it.  And for handling this case in
> > > > ReorderBufferForget() we call stream_abort().
> > > >
> > >
> > > The other cases are probably where we don't have FilterByOrigin or
> > > dbid check, for example,
> XLOG_HEAP2_NEW_CID/XLOG_XACT_INVALIDATIONS.
> > > We anyway actually don't send anything for such cases except empty
> > > start/stop messages. Can we add some flag to txn which says that there
> > > is at least one change like DML that we want to stream?
> > >
> >
> > We can probably think of using txn_flags for this purpose.
> 
> In the attached patch I have used txn_flags to identify whether it has
> any streamable change or not and the transaction will not be selected
> for streaming unless it has at least one streamable change.
> 

Thanks for your patch.

I saw that the patch added a check when selecting largest transaction, but in
addition to ReorderBufferCheckMemoryLimit(), the transaction can also be
streamed in ReorderBufferProcessPartialChange(). Should we add the check in
this function, too?

diff --git a/src/backend/replication/logical/reorderbuffer.c b/src/backend/replication/logical/reorderbuffer.c
index 9a58c4bfb9..108737b02f 100644
--- a/src/backend/replication/logical/reorderbuffer.c
+++ b/src/backend/replication/logical/reorderbuffer.c
@@ -768,7 +768,8 @@ ReorderBufferProcessPartialChange(ReorderBuffer *rb, ReorderBufferTXN *txn,
      */
     if (ReorderBufferCanStartStreaming(rb) &&
         !(rbtxn_has_partial_change(toptxn)) &&
-        rbtxn_is_serialized(txn))
+        rbtxn_is_serialized(txn) &&
+        rbtxn_has_streamable_change(txn))
         ReorderBufferStreamTXN(rb, toptxn);
 }

Regards,
Shi yu