Re: Parallel copy

On Mon, May 18, 2020 at 12:48 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> In the above case, even though we are executing a single command from
> the user perspective, but the currentCommandId will be four after the
> command. One increment will be for the copy command and the other
> three increments are for locking tuple in PK table
> (tab_fk_referenced_chk) for three tuples in FK table
> (tab_fk_referencing_chk). Now, for parallel workers, it is
> (theoretically) possible that the three tuples are processed by three
> different workers which don't get synced as of now. The question was
> do we see any kind of problem with this and if so can we just sync it
> up at the end of parallelism.

I strongly disagree with the idea of "just sync(ing) it up at the end
of parallelism". That seems like a completely unprincipled approach to
the problem. Either the command counter increment is important or it's
not. If it's not important, maybe we can arrange to skip it in the
first place. If it is important, then it's probably not OK for each
backend to be doing it separately.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company