Change data not captured about wal2json

[ericvergnaud]

Worth noting, when I use SQL instructions:

SELECT 'init' FROM pg_create_logical_replication_slot('test_slot2', 'wal2json');
.../... (do some data changes)
SELECT data FROM pg_logical_slot_get_changes('test_slot2', NULL, NULL, 'format-version', '2');
SELECT 'stop' FROM pg_drop_replication_slot('test_slot2');

I get the expected output.

[ericvergnaud]

Turns out the issue originates from the readme, these lines:

$ pg_recvlogical -d postgres --slot test_slot --create-slot -P wal2json
$ pg_recvlogical -d postgres --slot test_slot --start -o pretty-print=1 -f -

only receive changes to the db named 'postgres'.
They should read:

$ pg_recvlogical -d table1_with_pk --slot test_slot --create-slot -P wal2json
etc...

(at least that's how I got the expected output with postgres 12)

I'll happily submit a PR if the above finding is correct.

btw, improved clarity would be welcome re max_wal_senders and max_replication_slots. Should they be set to 1 or 10 ?

[eulerto]

wal2json doesn't require to be loaded by shared_preload_libraries parameter. Logical replication is per database. You need to make sure to use the correct database name in the pg_recvlogical command line.
Read the documentation about max_wal_senders and max_replication_slots. BTW, I only added the max_wal_senders instruction because old versions ( < 10 ) defaults to zero. wal2json supports 9.4+.