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

On Thu, Nov 14, 2019 at 12:10 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Thu, Nov 14, 2019 at 9:37 AM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
> >
> > On Wed, Nov 13, 2019 at 5:55 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> > >
> > > On Thu, Oct 3, 2019 at 1:18 PM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
> > > >
> > >
> > > As mentioned by me a few days back that the first patch in this series
> > > is ready to go [1] (I am hoping Tomas will pick it up), so I have
> > > started the review of other patches
> > >
> > > Review/Questions on 0002-Immediately-WAL-log-assignments.patch
> > > -------------------------------------------------------------------------------------------------
> > > 1. This patch adds the top_xid in WAL whenever the first time WAL for
> > > a subtransaction XID is written to correctly decode the changes of
> > > in-progress transaction. This patch also removes logging and applying
> > > WAL for XLOG_XACT_ASSIGNMENT which might have some effect. As replay
> > > of that, it prunes KnownAssignedXids to prevent overflow of that
> > > array. See comments in procarray.c (KnownAssignedTransactionIds
> > > sub-module). Can you please explain how after removing the WAL for
> > > XLOG_XACT_ASSIGNMENT, we will handle that or I am missing something
> > > and there is no impact of same?
> >
> > It seems like a problem to me as well. One option could be that
> > since now we are adding the top transaction id in the first WAL of the
> > subtransaction we can directly update the pg_subtrans and avoid adding
> > sub transaction id in the KnownAssignedXids and mark it as
> > lastOverflowedXid.
> >
>
> Hmm, I am not sure if we can do that easily because I think in
> RecordKnownAssignedTransactionIds, we add those based on the gap via
> KnownAssignedXidsAdd and only remove them later while applying WAL for
> XLOG_XACT_ASSIGNMENT. I think if we really want to go in this
> direction then for each WAL record we need to check if it has
> XLR_BLOCK_ID_TOPLEVEL_XID set and then call function
> ProcArrayApplyXidAssignment() with the required information. I think
> this line of attack has WAL overhead both on master whenever
> subtransactions are involved and also on hot-standby for doing the
> work for each subtransaction separately. The WAL apply needs to
> acquire and release PROCArrayLock in exclusive mode for each
> subtransaction whereas now it does it once for
> PGPROC_MAX_CACHED_SUBXIDS number of subtransactions which can conflict
> with queries running on standby.
Right
>
> The other idea could be that we keep the current XLOG_XACT_ASSIGNMENT
> mechanism (WAL logging and apply of same on hot-standby) as it is and
> additionally log top_xid the first time when WAL is written for a
> subtransaction only when wal_level >= WAL_LEVEL_LOGICAL. Then use the
> same for logical decoding. The advantage of this approach is that we
> will incur the overhead of additional transactionid only when required
> especially not with default server configuration.
>
> Thoughts?
The idea seems reasonable to me.

Apart from this, I have another question in
0003-Issue-individual-invalidations-with-wal_level-logical.patch

@@ -543,6 +588,18 @@ RegisterSnapshotInvalidation(Oid dbId, Oid relId)
{
AddSnapshotInvalidationMessage(&transInvalInfo->CurrentCmdInvalidMsgs,
dbId, relId);
+
+ /* Issue an invalidation WAL record (when wal_level=logical) */
+ if (XLogLogicalInfoActive())
+ {
+ SharedInvalidationMessage msg;
+
+ msg.sn.id = SHAREDINVALSNAPSHOT_ID;
+ msg.sn.dbId = dbId;
+ msg.sn.relId = relId;
+
+ LogLogicalInvalidations(1, &msg, false);
+ }
}

I am not sure why do we need to explicitly WAL log the snapshot
invalidation? because this is logged for invalidating the catalog
snapshot and for logical decoding we use HistoricSnapshot, not the
catalog snapshot. I might be missing something?

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