Re: Getting sorted data from foreign server

On Tue, Oct 27, 2015 at 5:26 AM, Ashutosh Bapat <ashutosh.bapat@enterprisedb.com> wrote:
>
>
>
> On Fri, Oct 23, 2015 at 2:43 AM, Robert Haas <robertmhaas@gmail.com> wrote:
>>
>> On Wed, Oct 21, 2015 at 5:23 AM, Ashutosh Bapat
>> <ashutosh.bapat@enterprisedb.com> wrote:
>> > Increasing the sorting cost factor (when use_remote_estimates = false) from
>> > 1.1 to 1.2 makes the difference disappear.
>> >
>> > Since the startup costs for postgres_fdw are large portion of total cost,
>> > extra 10% of rest of the cost is comparable to 1% fuzzy limit. IMO, we
>> > shouldn't bother too much about it as the path costs are not much different.
>>
>> My feeling is that cranking the sorting cost factor up to 20-25% would
>> be a good idea, just so we have less unnecessary plan churn.  I dunno
>> if sorting always costs that much, but if a 10% cost overhead is
>> really 1% because it only applies to a fraction of the cost, I don't
>> think that's good.  The whole point was to pick something large enough
>> that we wouldn't take the sorted path unless we will benefit from the
>> sort, and clearly that's not what happened here.
>>
>
> PFA patch with the default multiplication factor for sort bumped up to 1.2.
>  

+/* If no remote estimates, assume a sort costs 10% extra */
+#define DEFAULT_FDW_SORT_MULTIPLIER 1.2

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