On 2018/10/07 17:43, David Rowley wrote:
> On 6 October 2018 at 18:20, Edmund Horner <ejrh00@gmail.com> wrote:
>> David Rowley said:
>>> I am considering this a bug fix, but I'm proposing this for PG12 only
>>> as I don't think destabilising plans in the back branches is a good
>>> idea. I'll add this to the September commitfest.
>>
>> I played with the new patched today with a set of large partitions.
>> It seems to work, though the effect is subtle. The expected behaviour
>> of index_pages_fetched is a bit hard to fathom when the cache share
>> pushes it between cases A,B and C of the formula, but that's not
>> really down to this patch.
>
> Thanks for looking at this and testing it too.
>
> The primary purpose of this patch was step 1 in delaying the creation
> of RangeTblEntrys for partitions until after partition pruning has
> taken place. The code I had to do this caused problems around the
> total_table_pages calculation due to the lack of RangeTblEntry for the
> partitions at the time it was being calculated. But regardless of
> that, I still believe where we currently calculate this number is
> subtlely broken as it counts all partitions, even ones that will later
> be pruned, thus decreasing the likelihood of an index being used as
> random_page_cost would be applied to a higher number of index pages.
>
> Amit Langote has since posted a patch to delay the RangeTblEntry
> creation until after pruning. His patch happens to also move the
> total_table_pages calculation, but I believe this change should be
> made as an independent commit to anything else. I've kept it in the
> commitfest for that reason.
Yeah, if this patch is a win independent of the other project of delaying
partition RTE creation, which seems to be true, I think we should go ahead
with applying this patch.
>> Basically I think it's ready for a committer to look at. Should I
>> update the CF entry?
>
> That sounds good, please do.
+1
Thanks,
Amit
