Re: TRUNCATE on foreign table

2021年4月8日(木) 4:19 Fujii Masao <masao(dot)fujii(at)oss(dot)nttdata(dot)com>:
>
> On 2021/04/06 21:06, Kazutaka Onishi wrote:
> > Thank you for checking v13, and here is v14 patch.
> >
> >> 1) Are we using all of these macros? I see that we are setting them
> >> but we only use TRUNCATE_REL_CONTEXT_ONLY. If not used, can we remove
> >> them?
> >
> > These may be needed for the foreign data handler other than postgres_fdw.
>
> Could you tell me how such FDWs use TRUNCATE_REL_CONTEXT_CASCADING and _NORMAL? I'm still not sure if TRUNCATE_REL_CONTEXT_CASCADING is really required.
>
https://www.postgresql.org/message-id/20200102144644.GM3195%40tamriel.snowman.net

This is the suggestion when I added the flag to inform cascading.

| .... Instead, I'd suggest we have the core code build
| up a list of tables to truncate, for each server, based just on the list
| passed in by the user, and then also pass in if CASCADE was included or
| not, and then let the FDW handle that in whatever way makes sense for
| the foreign server (which, for a PG system, would probably be just
| building up the TRUNCATE command and running it with or without the
| CASCADE option, but it might be different on other systems).
|
Indeed, it is not a strong technical reason at this moment.
(And, I also don't have idea to distinct these differences in my module also.)

> With the patch, both inherited and referencing relations are marked as TRUNCATE_REL_CONTEXT_CASCADING? Is this ok for that use? Or we should distinguish them?
>
In addition, even though my prior implementation distinguished and deliver
the status whether the truncate command is issued with NORMAL or ONLY,
does the remote query by postgres_fdw needs to follow the manner?

Please assume the case when a foreign-table "ft" that maps a remote table
with some child-relations.
If we run TRUNCATE ONLY ft at the local server, postgres_fdw setup
a remote truncate command with "ONLY" qualifier, then remote postgresql
server truncate only parent table of the remote side.
Next, "SELECT * FROM ft" command returns some valid rows from the
child tables in the remote side, even if it is just after TRUNCATE command.
Is it a intuitive behavior for users?

Even though we have discussed about the flags and expected behavior of
foreign truncate, strip of the relids_extra may be the most straight-forward
API design.
So, in other words, the API requires FDW driver to make the entire data
represented by the foreign table empty, by ExecForeignTruncate().
It is probably more consistent to look at DropBehavior for listing-up the
target relations at the local relations only.

How about your thought?

If we stand on the above design, ExecForeignTruncate() don't needs
frels_extra and behavior arguments.

> +#define TRUNCATE_REL_CONTEXT_NORMAL 0x01
> +#define TRUNCATE_REL_CONTEXT_ONLY 0x02
> +#define TRUNCATE_REL_CONTEXT_CASCADING 0x04
>
> With the patch, these are defined as flag bits. But ExecuteTruncate() seems to always set the entry in relids_extra to either of them, not the combination of them. So we can define them as enum?
>
Regardless of my above comment, It's a bug.
When list_member_oid(relids, myrelid) == true, we have to set proper flag on the
relevant frels_extra member, not just ignoring.

Best regards,

Best regards,
--
HeteroDB, Inc / The PG-Strom Project
KaiGai Kohei <kaigai(at)heterodb(dot)com>