Re: Parallel Seq Scan

On Wed, Mar 25, 2015 at 6:27 AM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> Apart from that I have moved the Initialization of dsm segement from
> InitNode phase to ExecFunnel() (on first execution) as per suggestion
> from Robert. The main idea is that as it creates large shared memory
> segment, so do the work when it is really required.

So, suppose we have a plan like this:

Append
-> Funnel
-> Partial Seq Scan
-> Funnel
-> Partial Seq Scan
(repeated many times)

In earlier versions of this patch, that was chewing up lots of DSM
segments. But it seems to me, on further reflection, that it should
never use more than one at a time. The first funnel node should
initialize its workers and then when it finishes, all those workers
should get shut down cleanly and the DSM destroyed before the next
scan is initialized.

Obviously we could do better here: if we put the Funnel on top of the
Append instead of underneath it, we could avoid shutting down and
restarting workers for every child node. But even without that, I'm
hoping it's no longer the case that this uses more than one DSM at a
time. If that's not the case, we should see if we can't fix that.

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