Re: Minimal logical decoding on standbys

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