Re: pgsql: Fix parallel index and index-only scans to fall back to serial.

On 14/07/18 00:56, Robert Haas wrote:
> On Fri, Jul 13, 2018 at 2:22 PM, Heikki Linnakangas <hlinnaka@iki.fi> wrote:
>> I just bumped into this comment, from commit 09529a70bb5, and I can't make
>> sense of it:
>>
>>> +               /*
>>> +                * We reach here if the index only scan is not parallel,
>>> or if we're
>>> +                * executing a index only scan that was intended to be
>>> parallel
>>> +                * serially.
>>> +                */
>>
>>
>> What was that intended to say?
> 
> There are two ways that you can reach that code.  One is that you have
> the thing that shows up in EXPLAIN output as "Index-Only Scan".  The
> other is that you have the thing that shows up in EXPLAIN output as
> "Parallel Index-Only Scan", but you didn't get any workers, so now
> you're falling back to running what was intended to be a parallel plan
> without parallelism i.e. serially.  The comment is intended to alert
> you to the fact that an intended-as-parallel scan can end up here in
> corner cases where the plan doesn't end up being parallel.  We've had
> some difficulty in consistently getting that case correct.

Ah, gotcha. I changed that (and the other copies) per David's suggestion.

- Heikki