Re: BUG #14771: "Logical decoding" does not cover the impact of "TRUNCATE TABLE" command

Hi,

On 2017-08-07 12:20:30 +0000, hillel(dot)eilat(at)attunity(dot)com wrote:
> From "Logical Decoding" perspective - "TRUNCATE TABLE" is logically
> equivalent to "DELETE FROM TABLE".

It's absolutely not. A DELETE FROM TABLE has to include all the deleted
rows (think of concurrency and servers that don't match), whereas a
truncate doesn't include that. So changing TRUNCATE wouldn't be
appropriate.

> Both affect the contents of the PostgreSQL database identically.
> However - "DELETE FROM TABLE" will be reflected at the target database
> properly - as expected - while "TRUNCATE" will NOT!!.
> "Logical Decoding" aims to cover ALL changes made onto data in a coherent
> fashion.
> "TRUNCATE" stands for a case where change in data contents at PostgreSQL are
> not handled by "Logical Decoding", hence synchronization is not achieved.
> Did I miss something?
> Is it the only case?

Others remarked on this.

> How can one cope with this deficiency?

The best solution imo is to have a TRUNCATE trigger that leads to the
truncation being logged. Either by having a 'ddl commands' table, or by
using wal messages.

Greetings,

Andres Freund