Re: TRUNCATE on foreign table

Fujii-san,

> XLOG_HEAP_TRUNCATE record is written even for the truncation of
> a foreign table. Why is this necessary?
>
Foreign-tables are often used to access local data structure, like
columnar data files
on filesystem, not only remote accesses like postgres_fdw.
In case when we want to implement logical replication on this kind of
foreign-tables,
truncate-command must be delivered to subscriber node - to truncate
its local data.

In case of remote-access FDW drivers, truncate-command on the subscriber-side is
probably waste of cycles, however, only FDW driver and DBA who configured the
foreign-table know whether it is necessary, or not.

How about your opinions?

Best regards,

2021年3月25日(木) 3:47 Fujii Masao <masao.fujii@oss.nttdata.com>:
>
>
>
> On 2021/03/13 18:57, Kazutaka Onishi wrote:
> > I have fixed the patch to pass check-world test. :D
>
> Thanks for updating the patch! Here are some review comments from me.
>
>
>        By default all foreign tables using <filename>postgres_fdw</filename> are assumed
>        to be updatable.  This may be overridden using the following option:
>
> In postgres-fdw.sgml, "and truncatable" should be appended into
> the above first description? Also "option" in the second description
> should be a plural form "options"?
>
>
>       <command>TRUNCATE</command> is not currently supported for foreign tables.
>       This implies that if a specified table has any descendant tables that are
>       foreign, the command will fail.
>
> truncate.sgml should be updated because, for example, it contains
> the above descriptions.
>
>
> +     <literal>frels_extra</literal> is same length with
> +     <literal>frels_list</literal>, that delivers extra information of
> +     the context where the foreign-tables are truncated.
> +    </para>
>
> Don't we need to document the detail information about frels_extra?
> Otherwise the developers of FDW would fail to understand how to
> handle the frels_extra when trying to make their FDWs support TRUNCATE.
>
>
> +               relids_extra = lappend_int(relids_extra, (recurse ? 0 : 1));
> +                               relids_extra = lappend_int(relids_extra, -1);
>
> postgres_fdw determines whether to specify ONLY or not by checking
> whether the passed extra value is zero or not. That is, for example,
> using only 0 and 1 for extra values is enough for the purpose. But
> ExecuteTruncate() sets three values 0, -1 and 1 as extra ones. Why are
> these three values necessary?
>
>
> With the patch, if both local and foreign tables are specified as
> the target tables to truncate, TRUNCATE command tries to truncate
> foreign tables after truncating local ones. That is, if "truncatable"
> option is set to false or enough permission to truncate is not granted
> yet in the foreign server, an error will be thrown after the local tables
> are truncated. I don't think this is good order of processings. IMO,
> instead, we should check whether foreign tables can be truncated
> before any actual truncation operations. For example, we can easily
> do that by truncate foreign tables before local ones. Thought?
>
>
> XLOG_HEAP_TRUNCATE record is written even for the truncation of
> a foreign table. Why is this necessary?
>
> Regards,
>
> --
> Fujii Masao
> Advanced Computing Technology Center
> Research and Development Headquarters
> NTT DATA CORPORATION

--
HeteroDB, Inc / The PG-Strom Project
KaiGai Kohei <kaigai@heterodb.com>