Hi David
Thanks for your revision.
Quoting David Rowley <david.rowley@2ndquadrant.com>:
> On Wed, 26 Dec 2018 at 00:54, Thiemo Kellner
> <thiemo@gelassene-pferde.biz> wrote:
>> Explain analyze verbose showed for:
>> A (cost=264.72..626.97 rows=31 width=90) (actual time=1.117..1.117
>> rows=0 loops=1)
>> B (cost=264.72..974.25 rows=31 width=58) (actual time=1.508..1.508
>> rows=0 loops=1)
>> C (cost=0.42..611.19 rows=31 width=52) (actual time=2.217..2.217
>> rows=0 loops=1)
>>
>> I am very surprised that the cost of A is (much) higher than that of C
>> which I suspected to be the most inefficient. I was that much fixed on
>> the costs that I initially ignored the actual time where my
>> assumptions on efficiency are reflected. Funny though is that the
>> subjective impression when waiting for the update queries to complete
>> was that C was fastest by far, followed by B and only at the end was
>> update A.
>
> While the times mentioned in "actual time" are for execution only and
> don't account for the time taken to plan the query, the results you
> put in [1] disagree entirely with your claim that 'C' was faster. 'A'
> comes out fastest with the explain analyzes you've listed.
>
> A:
> Planning TIME: 0.423 ms
> Execution TIME: 1.170 ms
>
> C:
> Planning TIME: 0.631 ms
> Execution TIME: 2.281 ms
>
> Have you confused each of the results, perhaps because they're in a
> different order as to your cases above?
I am pretty sure I did not confuse. I am not worried about planning
times as I assume that PostgreSQL has a time limit restricting the
time used to find the best execution path in the order of seconds such
that for a heavy load query it would get neglectable.
> I'd certainly expect 'A' to be the fastest of the bunch since it's
> both less effort for the planner and also the executor. I didn't look
> at why the cost is estimated to be slightly higher, but the planner
> wouldn't consider rewriting the queries to one of the other cases
> anyway, so it's likely not that critical that the costings are
> slightly out from reality.
I am glad, that my feeling what should be the best query meets
reality. However, I am left a bit concerned about the usefulness of
the costs of the execution plan. I feel the costs rather contradict
the actual execution times in my test case. To me this would render
the cost useless for comparison of queries.
>> where U.KEY_U in ({correlated subquery 3})
>
> This is not correlated in [1].
>
> [1] https://pastebin.com/W2HsTBwi
Right you are, my fault. Thanks for your attention there as well. :-)
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