Re: Something is wrong with wal_compression

Hi,

On 2023-01-27 19:49:17 -0800, Andres Freund wrote:
> It's quite commonly used as part of trigger based replication tools (IIRC
> that's its origin), monitoring, as part of client side logging, as part of
> snapshot management.

Forgot one: Queues.

The way it's used for trigger based replication, queues and also some
materialized aggregation tooling, is that there's a trigger that inserts into
a "log" table. And that log table has a column into which txid_current() will
be inserted. Together with txid_current_snapshot() etc that's used to get a
(at least semi) "transactional" order out of such log tables.

I believe that's originally been invented by londiste / skytool, later slony
migrated to it. The necessary C code was added as contrib/txid in 1f92630fc4e
2007-10-07 and then moved into core a few days later in 18e3fcc31e7.


For those cases making txid_current() flush would approximately double the WAL
flush rate.

Greetings,

Andres Freund