On Tue, Jan 9, 2018 at 12:48 AM, Tom Lane wrote:
> Robert Haas writes:
>> On Sun, Jan 7, 2018 at 11:40 PM, Amit Kapila wrote:
>>> One theory that can explain above failure is that the costs of
>>> scanning some of the sub-paths is very close due to which sometimes
>>> the results can vary. If t
I wrote:
> BTW, I had not looked closely at list_qsort before, but now that I have
> it seems seriously bizarre. Why is it allocating a new List header
> rather than reusing the old one? Because it stomps on the next-links
> of the list cells, the old header is effectively corrupted and can't
> p
I wrote:
>> In the regression test case at hand, the startup costs are all zero so
>> this change wouldn't improve the test case's stability. So I'm thinking
>> that in addition, it would be a good idea for these functions to break
>> exact compare_path_costs ties in some arbitrary but determinist
On Mon, Jan 8, 2018 at 1:26 PM, Jim Finnerty wrote:
> I see. Maybe the EXPLAIN output can be made more stable when "EXPLAIN (costs
> FALSE)", though?
I think that would be fixing the problem from the wrong end. We want
to actually get a stable choice of plan, not display something other
than th
I wrote:
> In the regression test case at hand, the startup costs are all zero so
> this change wouldn't improve the test case's stability. So I'm thinking
> that in addition, it would be a good idea for these functions to break
> exact compare_path_costs ties in some arbitrary but deterministic w
Robert Haas writes:
> On Sun, Jan 7, 2018 at 11:40 PM, Amit Kapila wrote:
>> One theory that can explain above failure is that the costs of
>> scanning some of the sub-paths is very close due to which sometimes
>> the results can vary. If that is the case, then probably using
>> fuzz_factor in c
Robert Haas writes:
> [ example where current parallel-append dispatch algorithm loses ]
I should clarify my thinking a bit more: I was imagining that we might
switch to a system where, as each worker comes free, it makes a dynamic
choice of which subplan to take next. That means that not only d
On Mon, Jan 8, 2018 at 11:57 AM, Tom Lane wrote:
>> Ignoring some details around partial vs. non-partial plans, that's
>> pretty much what we ARE doing, but to make it efficient, we sort the
>> paths at plan time so that those choices are easy to make at runtime.
>> If we didn't do that, we could
Robert Haas writes:
> On Mon, Jan 8, 2018 at 11:42 AM, Tom Lane wrote:
>> More generally, I wonder if
>> it wouldn't be better to implement this behavior at runtime rather
>> than plan time.
> Ignoring some details around partial vs. non-partial plans, that's
> pretty much what we ARE doing, but
On Mon, Jan 8, 2018 at 11:42 AM, Tom Lane wrote:
> Jim Finnerty writes:
>> Ordering the plan output elements by estimated cost will cause spurious plan
>> changes to be reported after table cardinalities change. Can we choose an
>> explain output order that is stable to changes in cardinality, p
Jim Finnerty writes:
> Ordering the plan output elements by estimated cost will cause spurious plan
> changes to be reported after table cardinalities change. Can we choose an
> explain output order that is stable to changes in cardinality, please?
I found the code that's doing this, in create_a
On Mon, Jan 8, 2018 at 11:28 AM, Jim Finnerty wrote:
> Ordering the plan output elements by estimated cost will cause spurious plan
> changes to be reported after table cardinalities change. Can we choose an
> explain output order that is stable to changes in cardinality, please?
No. The order
On Sun, Jan 7, 2018 at 11:40 PM, Amit Kapila wrote:
> One theory that can explain above failure is that the costs of
> scanning some of the sub-paths is very close due to which sometimes
> the results can vary. If that is the case, then probably using
> fuzz_factor in costs comparison (as is done
On Mon, Jan 8, 2018 at 2:11 PM, Amit Khandekar wrote:
> On 8 January 2018 at 13:35, Amit Kapila wrote:
>> On Mon, Jan 8, 2018 at 11:26 AM, Tom Lane wrote:
>>> Amit Khandekar writes:
The fact that b_star gets moved from 5th position to the first
position in the scans, indicates that i
On 8 January 2018 at 13:35, Amit Kapila wrote:
> On Mon, Jan 8, 2018 at 11:26 AM, Tom Lane wrote:
>> Amit Khandekar writes:
>>> The fact that b_star gets moved from 5th position to the first
>>> position in the scans, indicates that it's cost shoots up from 1.04 to
>>> a value greater than 1.16
On Mon, Jan 8, 2018 at 11:26 AM, Tom Lane wrote:
> Amit Khandekar writes:
>> The fact that b_star gets moved from 5th position to the first
>> position in the scans, indicates that it's cost shoots up from 1.04 to
>> a value greater than 1.16. It does not look like a case where two
>> costs are
Amit Khandekar writes:
> The fact that b_star gets moved from 5th position to the first
> position in the scans, indicates that it's cost shoots up from 1.04 to
> a value greater than 1.16. It does not look like a case where two
> costs are almost same due to which their positions swap sometimes.
On 8 January 2018 at 10:10, Amit Kapila wrote:
> regression=# explain select round(avg(aa)), sum(aa) from a_star;
> QUERY PLAN
> ---
> Finalize Aggregate (cost=2.30..2.31 rows=1 width=40
On Sun, Jan 7, 2018 at 5:44 AM, Tom Lane wrote:
> According to buildfarm member silverfish,
>
> https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=silverfish&dt=2018-01-06%2008%3A53%3A38
>
> it's possible to sometimes get this failure in the regression tests:
>
> ***
> /mnt/buildfarm/buildro
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