Re: Parallel Index Scans

On Wed, Feb 15, 2017 at 2:04 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> On Tue, Feb 14, 2017 at 12:48 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>> That sounds way better.
>
> Here's an updated patch. Please review my changes, which include:
>
> * Various comment updates.

1.
+ * BTPARALLEL_IDLE indicates that no backend is currently advancing the scan
+ * to a new page; some process can start doing that.
+ *
+ * BTPARALLEL_DONE implies that the scan is complete (including error exit).

/implies/indicates, to be consistent with other explanations.

2.
+ * of the scan (depending on thes can direction). An invalid block number

/thes can/the scan

I have modified the patch to include above two changes.

3.
+ else if (pageStatus == BTPARALLEL_DONE)
+ {
+ /*
+ * We're done with this set of scankeys, but have not yet advanced
+ * to the next set.
+ */
+ status = false;
+ }

Here second part of the comment (but have not yet advanced ..) seems
to be slightly misleading because this state has nothing to do with
the advancement of scan keys.

I have not changed this because I am not sure what you have in mind.

> * _bt_parallel_seize now unconditionally sets *pageno to P_NONE at the
> beginning, instead of doing it conditionally at the end. This seems
> cleaner to me.
> * I removed various BTScanPosInvalidate calls from _bt_first in places
> where they followed calls to _bt_parallel_done, because I can't see
> how the scan position could be valid at that point; note that
> _bt_first asserts that it is invalid on entry.
> * I added a _bt_parallel_done() call to _bt_first where it apparently
> returned without releasing the scan; search for SK_ROW_MEMBER. Maybe
> there's something I'm missing that makes this unnecessary, but if so
> there should probably be a comment here.
> * I wasn't happy with the strange calling convention where
> _bt_readnextpage usually gets a valid block number but not for
> non-parallel backward scans. I had a stab at fixing that so that the
> block number is always valid, but I'm not entirely sure I've got the
> logic right. Please see what you think.
>

Looks good to me.

> * I repositioned the function prototypes you added to nbtree.h to
> separate the public and non-public interfaces.
>

I have verified all your changes and they look good to me.

> I can't easily test this because your second patch doesn't apply,

I have tried and it works for me on latest code except for one test
output file which could have been excluded. I wonder whether you are
first applying the GUC related patch [1] before applying the optimizer
support related patch. In anycase, to avoid confusion I am attaching
all the three patches with this e-mail.

> so
> I'd appreciate it if you could have a stab at checking whether I've
> broken anything in this revision. Also, it would be good if you could
> rebase the second patch.
>

I have rebased the optimizer/executor support related patch.

[1] - https://www.postgresql.org/message-id/CAA4eK1%2BTnM4pXQbvn7OXqam%2Bk_HZqb0ROZUMxOiL6DWJYCyYow%40mail.gmail.com

--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com