RE: Parallel INSERT (INTO ... SELECT ...)

From: Greg Nancarrow <gregn4422(at)gmail(dot)com>
> > (1)
> Yes, you're right the wording is not right (and I don't really like
> the wording used before the patch).
>
> Perhaps it could say:
>
> (Note that we do allow CREATE TABLE AS, INSERT INTO...SELECT, SELECT
> INTO, and CREATE MATERIALIZED VIEW to use parallel plans. However, as
> of now, other than in the case of INSERT INTO...SELECT, only the leader
> backend
> writes into a completely new table. In the future, we can extend it to
> allow workers for the
> other commands to write into the table. However, to allow parallel
> updates and deletes, we
> have to solve other problems, especially around combo CIDs.)

That looks good to me, thanks.

> > (4)
> You could, but I'm not sure it would make the code easier to read,
> especially for those who don't know !isParallelWorker() means it's a
> parallel leader.
...
> I really can't see a problem. PrepareParallelMode() is only needed
> prior to execution of a parallel plan, so it's not needed for "other
> call sites" using EnterParallelMode().
My frank first impressions were (and are):

* Why do we have to call a separate function for preparation despite the actual entering follows immediately? We can do necessary preparation in the entering function.

* Those who read the parallel index build and parallel VACUUM code for the first time might be startled at the missing PrepareParallelMode() call: "Oh, EnterParallelMode() is called without preparation unlike the other site I saw the other day. Isn't this a but?"

> Perhaps the name can be changed to disassociate it from generic
> EnterParallelMode() usage. So far, I've only thought of long names
> like: PrepareParallelModePlanExec().
> Ideas?

What PrepareParallelMode() handles is the XID and command ID, which are managed by access/transam/ module and are not executor-specific. It's natural (or at least not unnatural) that EnterParallelMode() prepares them, because EnterParallelMode() is part of access/transam/.

Regards
Takayuki Tsunakawa