Re: pgsql: Add a new GUC and a reloption to enable inserts in parallel-mode

On Wed, Mar 17, 2021 at 10:14 PM Amit Kapila <akapila(at)postgresql(dot)org> wrote:
> Add a new GUC and a reloption to enable inserts in parallel-mode.
>
> Commit 05c8482f7f added the implementation of parallel SELECT for
> "INSERT INTO ... SELECT ..." which may incur non-negligible overhead in
> the additional parallel-safety checks that it performs, even when, in the
> end, those checks determine that parallelism can't be used. This is
> normally only ever a problem in the case of when the target table has a
> large number of partitions.
>
> A new GUC option "enable_parallel_insert" is added, to allow insert in
> parallel-mode. The default is on.
>
> In addition to the GUC option, the user may want a mechanism to allow
> inserts in parallel-mode with finer granularity at table level. The new
> table option "parallel_insert_enabled" allows this. The default is true.

I find this fairly ugly. If you can't make the cost of checking
whether parallelism is safe low enough that you don't need a setting
for this, then I think perhaps you shouldn't have the feature at all.
In other words, I propose that you revert both this and 05c8482f7f and
come back when you have a better design that doesn't introduce so much
overhead.

--
Robert Haas
EDB: http://www.enterprisedb.com