Subject: Re: row filtering for logical replication
To: Dilip Kumar <dilipbalaut@gmail.com>, Amit Kapila <amit.kapila16@gmail.com>
Cc: Alvaro Herrera <alvherre@alvh.no-ip.org>,
 Greg Nancarrow <gregn4422@gmail.com>, Euler Taveira <euler@eulerto.com>,
 Peter Smith <smithpb2250@gmail.com>, Rahila Syed <rahilasyed90@gmail.com>,
 Peter Eisentraut <peter.eisentraut@enterprisedb.com>,
 =?UTF-8?B?w5ZuZGVyIEthbGFjxLE=?= <onderkalaci@gmail.com>,
 japin <japinli@hotmail.com>, Michael Paquier <michael@paquier.xyz>,
 David Steele <david@pgmasters.net>, Craig Ringer <craig@2ndquadrant.com>,
 Tomas Vondra <tomas.vondra@2ndquadrant.com>,
 Amit Langote <amitlangote09@gmail.com>,
 PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
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From: Tomas Vondra <tomas.vondra@enterprisedb.com>
Message-ID: <fb7a3099-1232-17d4-dd45-a2d42d780b1e@enterprisedb.com>
Date: Mon, 19 Jul 2021 15:31:56 +0200
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On 7/19/21 1:00 PM, Dilip Kumar wrote:
> On Mon, Jul 19, 2021 at 3:12 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
>> a. Just log it and move to the next row
>> b. send to stats collector some info about this which can be displayed
>> in a view and then move ahead
>> c. just skip it like any other row that doesn't match the filter clause.
>>
>> I am not sure if there is any use of sending a row if one of the
>> old/new rows doesn't match the filter. Because if the old row doesn't
>> match but the new one matches the criteria, we will anyway just throw
>> such a row on the subscriber instead of applying it.
> 
> But at some time that will be true even if we skip the row based on
> (a) or (c) right.  Suppose the OLD row was not satisfying the
> condition but the NEW row is satisfying the condition, now even if we
> skip this operation then in the next operation on the same row even if
> both OLD and NEW rows are satisfying the filter the operation will
> just be dropped by the subscriber right? because we did not send the
> previous row when it first updated to value which were satisfying the
> condition.  So basically, any row is inserted which did not satisfy
> the condition first then post that no matter how many updates we do to
> that row either it will be skipped by the publisher because the OLD
> row was not satisfying the condition or it will be skipped by the
> subscriber as there was no matching row.
> 

I have a feeling it's getting overly complicated, to the extent that
it'll be hard to explain to users and reason about. I don't think
there's a "perfect" solution for cases when the filter expression gives
different answers for old/new row - it'll always be surprising for some
users :-(

So maybe the best thing is to stick to the simple approach already used
e.g. by pglogical, which simply user the new row when available (insert,
update) and old one for deletes.

I think that behaves more or less sensibly and it's easy to explain.

All the other things (e.g. turning UPDATE to INSERT, advanced conflict
resolution etc.) will require a lot of other stuff, and I see them as
improvements of this simple approach.

>>> Maybe a second option is to have replication change any UPDATE into
>>> either an INSERT or a DELETE, if the old or the new row do not pass the
>>> filter, respectively.  That way, the databases would remain consistent.
> 
> Yeah, I think this is the best way to keep the data consistent.
> 

It'd also require REPLICA IDENTITY FULL, which seems like it'd add a
rather significant overhead.


regards

-- 
Tomas Vondra
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