Re: TRUNCATE on foreign table

On Thu, Apr 8, 2021 at 6:44 PM Fujii Masao <masao(dot)fujii(at)oss(dot)nttdata(dot)com> wrote:
> The followings are the open items and discussion points that I'm thinking of.
>
> 1. Currently the extra information (TRUNCATE_REL_CONTEXT_NORMAL, TRUNCATE_REL_CONTEXT_ONLY or TRUNCATE_REL_CONTEXT_CASCADING) about how a foreign table was specified as the target to truncate in TRUNCATE command is collected and passed to FDW. Does this really need to be passed to FDW? Seems Stephen, Michael and I think that's necessary. But Kaigai-san does not. I also think that TRUNCATE_REL_CONTEXT_CASCADING can be removed because there seems no use case for that maybe.

I think we should remove the unused enums/macros, instead we could
mention a note of the extensibility of those enums/macros in the
comments section around the enum/macro definitions.

> 2. Currently when the same foreign table is specified multiple times in the command, the extra information only for the foreign table found first is collected. For example, when "TRUNCATE ft, ONLY ft" is executed, TRUNCATE_REL_CONTEXT_NORMAL is collected and _ONLY is ignored because "ft" is found first. Is this OK? Or we should collect all, e.g., both _NORMAL and _ONLY should be collected in that example? I think that the current approach (i.e., collect the extra info about table found first if the same table is specified multiple times) is good because even local tables are also treated the same way. But Kaigai-san does not.

IMO, the foreign truncate command should be constructed by collecting
all the information i.e. "TRUNCATE ft, ONLY ft" and let the remote
server execute how it wants to execute. That will be consistent and no
extra logic is required to track the already seen foreign tables while
foreign table collection/foreign truncate command is being prepared on
the local server.

I was thinking that the postgres throws error or warning for commands
such as truncate, vaccum, analyze when the same tables are specified,
but seems like that's not what it does.

> 3. Currently postgres_fdw specifies ONLY clause in TRUNCATE command that it constructs. That is, if the foreign table is specified with ONLY, postgres_fdw also issues the TRUNCATE command for the corresponding remote table with ONLY to the remote server. Then only root table is truncated in remote server side, and the tables inheriting that are not truncated. Is this behavior desirable? Seems Michael and I think this behavior is OK. But Kaigai-san does not.

I'm okay with the behaviour as it is consistent with what ONLY does to
local tables. Documenting this behaviour(if not done already) is a
better way I think.

> 4. Tab-completion for TRUNCATE should be updated so that also foreign tables are displayed.

It will be good to have.

With Regards,
Bharath Rupireddy.
EnterpriseDB: http://www.enterprisedb.com