Re: ERROR: no known snapshots

On 2018-08-25 07:04:14 +0000, Julien Riou wrote:
> Hello,
>
> We have few highly available clusters with patroni managing PostgreSQL
> 9.6 (pgdg on debian stretch). We use logical decoding with wal2json to
> stream data to a remote platform. On a majority of clusters, it works
> like a charm. But for one, we have the following error right after
> decoding start:
>
> [1353-1] LOG: starting logical decoding for slot "*****"
> [1353-2] DETAIL: streaming transactions committing after 321/292C06F0, reading WAL from 321/28BA9898
> [1354-1] LOG: logical decoding found consistent point at 321/28BA9898
> [1354-2] DETAIL: There are no running transactions.
> [1355-1] ERROR: no known snapshots
> [1355-2] CONTEXT: slot "*****", output plugin "wal2json", in the change callback, associated LSN 321/2A985A30
>
> The only reference of the "no known snapshots" error is in PostgreSQL source
> code:
> https://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f=src/backend/access/heap/tuptoaster.c;h=ade4bd142dfbd7ef3340495b06d1041e4032a17f;hb=ca3489e768990e32096f0f24d46d5f0b8b70cff4#l2320
>
> Do you know what can cause this error?

Any chance that you're specifying the "include_unchanged_toast" option?
I briefly looked over wal2json (which isn't part of PG) and as far as I
can tell that option simply can't work correctly, and would cause errors
like the above.

Greetings,

Andres Freund