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Re: Parallel Seq Scan - Mailing list pgsql-hackers

From Amit Kapila
Subject Re: Parallel Seq Scan
Date
Msg-id CAA4eK1JUwzzzKJDtZkOjkwMqGnYiME21T5scaK_i3iCb04O45A@mail.gmail.com
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In response to Re: Parallel Seq Scan  (Haribabu Kommi <kommi.haribabu@gmail.com>)
Responses Re: Parallel Seq Scan  (Haribabu Kommi <kommi.haribabu@gmail.com>)
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On Fri, Sep 18, 2015 at 1:33 PM, Haribabu Kommi <kommi.haribabu@gmail.com> wrote:
On Thu, Sep 3, 2015 at 8:21 PM, Amit Kapila <amit.kapila16@gmail.com> wrote:
>
> Attached, find the rebased version of patch.

Here are the performance test results:

Query          selectivity    HashAgg                         HashAgg
                    (million)     + seqscan(ms)                +
parallel seq scan(ms)
                                                                    2
workers   4 workers      8 workers
$1 <= '001'      0.1           16717.00             7086.00
4459.00         2912.00
$1 <= '004'      0.4           17962.00             7410.00
4651.00         2977.00
$1 <= '008'      0.8           18870.00             7849.00
4868.00         3092.00
$1 <= '016'      1.5           21368.00             8645.00
6800.00         3486.00
$1 <= '030'      2.7           24622.00           14796.00    13108.00
        9981.00
$1 <= '060'      5.4           31690.00           29839.00    26544.00
      23814.00
$1 <= '080'      7.2           37147.00           40485.00    35763.00
      32679.00


I think here probably when the selectivity is more than 5, then it should
not have selected Funnel plan.  Have you by any chance changed
cpu_tuple_comm_cost?  If not, then you can try by setting value of
parallel_setup_cost (may be 10) and then see if it selects the Funnel
Plan.  Is it possible for you to check the cost difference of Sequence
and Funnel plan, hopefully explain or explain analyze should be sufficient?



And also attached perf results for selectivity of 0.1 million and 5.4
million cases for analysis.


I have checked perf reports and it seems that when selectivity is more, it
seems to be spending time in some kernel calls which could be due
communication of tuples.

With Regards,
Amit Kapila.

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