Date: Tue, 3 Mar 2020 22:46:46 +0100
From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
To: Dilip Kumar <dilipbalaut@gmail.com>
Cc: Amit Kapila <amit.kapila16@gmail.com>,
	Michael Paquier <michael@paquier.xyz>,
	Peter Eisentraut <peter.eisentraut@2ndquadrant.com>,
	PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Subject: Re: PATCH: logical_work_mem and logical streaming of large
 in-progress transactions
Message-ID: <20200303214646.6adway2z5wnapjem@development>
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Hi,

I started looking at this patch series again, hoping to get it moving
for PG13. There's been a tremendous amount of work done since I last
worked on it, and a lot was discussed on this thread, so it'll take a
while to get familiar with the new code ...

The first thing I realized that WAL-logging of assignments in v12 does
both the "old" logging (using dedicated message) and "new" with
toplevel-XID embedded in the first message. Yes, the patch was wrong,
because it eliminated all calls to ProcArrayApplyXidAssignment() and so
it was trivial to crash the replica due to KnownAssignedXids overflow.
But I don't think re-introducing XLOG_XACT_ASSIGNMENT message is the
right fix.

I actually proposed doing this (having both ways to log assignments) so
that there's no regression risk with (wal_level < logical). But IIRC
Andres objected to it, argumenting that we should not log the same piece
of information in two very different ways at the same time (IIRC it was
discussed on the FOSDEM dev meeting, so I don't have a link to share).
And I do agree with him ...

The question is, why couldn't the replica use the same assignment info
we already write for logical decoding? The main challenge is that now
the assignment can be sent in many different xlog messages, from a bunch
of resource managers (essentially, any xlog message with a xid can have
embedded XID of the toplevel xact). So the handling would either need to
happen in every rmgr, or we need to move it before we call the rmgr.

For exampple, we might do this e.g. in StartupXLOG() I think, per the
attached patch (FWIW this particular fix was written by Masahiko Sawada,
not me). This does the trick for me - I'm no longer able to reproduce
the KnownAssignedXids overflow.

The one difference is that we used to call ProcArrayApplyXidAssignment
for larger groups of XIDs, as sent in the assignment message. Now we
call it for each individual assignment. I don't know if this is an
issue, but I suppose we might introduce some sort of local caching
(accumulate the assignments into a local array, call the function only
when we have enough of them).

Aside from that, I think there's a minor bug in xact.c - the patch adds
a "assigned" field to TransactionStateData, but then it fails to add a
default value into TopTransactionStateData. We probably interpret NULL
as false, but then there's nothing for the pointer. I suspect it might
leave some random garbage there, leading to strange things later.

Another thing I noticed is LogicalDecodingProcessRecord() extracts the
toplevel XID using a macro

   txid = XLogRecGetTopXid(record);

but then it just starts accessing the fields directly again in the
ReorderBufferAssignChild call. I think we should do this instead:

     ReorderBufferAssignChild(ctx->reorder,
                              txid,
			     XLogRecGetXid(record),
                              buf.origptr);


regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services