Date: Fri, 12 Apr 2019 12:27:47 -0700
From: Andres Freund <andres@anarazel.de>
To: Amit Khandekar <amitdkhan.pg@gmail.com>
Cc: tushar <tushar.ahuja@enterprisedb.com>,
	Petr Jelinek <petr.jelinek@2ndquadrant.com>,
	Robert Haas <robertmhaas@gmail.com>,
	Craig Ringer <craig@2ndquadrant.com>,
	Petr Jelinek <petr@2ndquadrant.com>,
	pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: Minimal logical decoding on standbys
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Hi,

On 2019-04-12 23:34:02 +0530, Amit Khandekar wrote:
> I tried to see if I can quickly understand what's going on.
> 
> Here, master wal_level is hot_standby, not logical, though slave
> wal_level is logical.

Oh, that's well diagnosed.  Cool.  Also nicely tested - this'd be ugly
in production.

I assume the problem isn't present if you set the primary to wal_level =
logical?


> Not sure why this is happening. On slave, wal_level is logical, so
> logical records should have tuple data. Not sure what does that have
> to do with wal_level of master. Everything should be there on slave
> after it replays the inserts; and also slave wal_level is logical.

The standby doesn't write its own WAL, only primaries do. I thought we
forbade running with wal_level=logical on a standby, when the primary is
only set to replica.  But that's not what we do, see
CheckRequiredParameterValues().

I've not yet thought this through, but I think we'll have to somehow
error out in this case.  I guess we could just check at the start of
decoding what ControlFile->wal_level is set to, and then raise an error
in decode.c when we pass an XLOG_PARAMETER_CHANGE record that sets
wal_level to something lower?

Could you try to implement that?

Greetings,

Andres Freund