Clarify what's the encoding for JSON messages about wal2json

[rcoup]

@gunnarmorling wal2json sets OUTPUT_PLUGIN_TEXTUAL_OUTPUT, so AFAIK it's in the server's encoding.

#40 discusses a couple of other quirks.

[eulerto]

@gunnarmorling From the PostgreSQL manual:

PostgreSQL allows only one character set encoding per database. It is therefore not possible for the JSON types to conform rigidly to the JSON specification unless the database encoding is UTF8. Attempts to directly include characters that cannot be represented in the database encoding will fail; conversely, characters that can be represented in the database encoding but not in UTF8 will be allowed.

[gunnarmorling]

@eulerto, that text above applies to JSONB-typed columns, right? So unless I'm mistaken, the wal2json plug-in could still use any other encoding? As per @rcoup, it's the server's encoding. So I suppose we can query the server for that and use the server encoding when parsing incoming messages.

We'll have to see though how that works with Jackson which we use as a parser; I'm not sure whether it supports any encodings other than UTF-8/16/32.

[eulerto]

@eulerto, that text above applies to JSONB-typed columns, right? So unless I'm mistaken, the wal2json plug-in could still use any other encoding? As per @rcoup, it's the server's encoding. So I suppose we can query the server for that and use the server encoding when parsing incoming messages.

That is not limited to JSONB. It is valid for JSON data types. I said that because we use the same API/rules as JSON data types. Hence, this difference in the encoding between JSON standard and Postgres is also valid for wal2json.

We'll have to see though how that works with Jackson which we use as a parser; I'm not sure whether it supports any encodings other than UTF-8/16/32.

server_encoding is available while establishing a connection.