Last month I wrote:
> It seems clear that our qsort.c is doing a pretty awful job of picking
> qsort pivots, while glibc is mostly managing not to make that mistake.
I re-ran Gary's test script using the just-committed improvements to
qsort.c, and got pretty nice numbers (attached --- compare to
h
; > [EMAIL PROTECTED]] On Behalf Of Tom Lane> > Sent: Wednesday, February 15, 2006 5:22 PM
> > To: Ron> > Cc: pgsql-performance@postgresql.org; pgsql-hackers@postgresql.org> > Subject: Re: [HACKERS] qsort again (was Re: [PERFORM] Strange Create
> Index> > behav
gt; > Subject: Re: [HACKERS] qsort again (was Re: [PERFORM] Strange Create
> Index
> > behaviour)
> >
> > Ron <[EMAIL PROTECTED]> writes:
> > > How are we choosing our pivots?
> >
> > See qsort.c: it looks like median of nine equally spaced input
"Qingqing Zhou" <[EMAIL PROTECTED]> writes:
> "Tom Lane" <[EMAIL PROTECTED]> wrote
>> No, I mean I ran the bit of SQL script I gave 100 separate times.
> I must misunderstand something here -- I can't figure out that why the cost
> of the same procedure keep climbing?
No, the run cost varies rand
"Qingqing Zhou" <[EMAIL PROTECTED]> wrote
>
> I must misunderstand something here -- I can't figure out that why the
cost
> of the same procedure keep climbing?
>
Ooops, I mis-intepret the sentence -- you sorted the results ...
Regards,
Qingqing
---(end of broadcast)-
"Tom Lane" <[EMAIL PROTECTED]> wrote
> "Qingqing Zhou" <[EMAIL PROTECTED]> writes:
> > By "did this 100 times" do you mean generate a sequence of at most
> > 20*100 numbers, and for every 20 numbers, the first half are all
> > zeros and the other half are uniform random numbers?
>
> No, I
"Qingqing Zhou" <[EMAIL PROTECTED]> writes:
> By "did this 100 times" do you mean generate a sequence of at most
> 20*100 numbers, and for every 20 numbers, the first half are all
> zeros and the other half are uniform random numbers?
No, I mean I ran the bit of SQL script I gave 100 separ
"Tom Lane" <[EMAIL PROTECTED]> wrote
>
> I did this 100 times and sorted the reported runtimes.
>
> I'd say this puts a considerable damper on my enthusiasm for using our
> qsort all the time, as was recently debated in this thread:
> http://archives.postgresql.org/pgsql-hackers/2005-12/msg00610.p
Ouch! That confirms my problem. I generated the random test case because
it was easier than including the dump of my tables, but you can
appreciate that tables 20 times the size are basically crippled when it
comes to creating an index on them.
I have to say that I restored a few gigabyte dum
> -Original Message-
> From: [EMAIL PROTECTED] [mailto:pgsql-hackers-
> [EMAIL PROTECTED] On Behalf Of Tom Lane
> Sent: Wednesday, February 15, 2006 5:22 PM
> To: Ron
> Cc: pgsql-performance@postgresql.org; pgsql-hackers@postgresql.org
> Subject: Re: [HACKERS
Ron <[EMAIL PROTECTED]> writes:
> How are we choosing our pivots?
See qsort.c: it looks like median of nine equally spaced inputs (ie,
the 1/8th points of the initial input array, plus the end points),
implemented as two rounds of median-of-three choices. With half of the
data inputs zero, it's n
I wrote:
> Gary Doades <[EMAIL PROTECTED]> writes:
>> Ouch! That confirms my problem. I generated the random test case because
>> it was easier than including the dump of my tables, but you can
>> appreciate that tables 20 times the size are basically crippled when it
>> comes to creating an ind
Gary Doades <[EMAIL PROTECTED]> writes:
> Ouch! That confirms my problem. I generated the random test case because
> it was easier than including the dump of my tables, but you can
> appreciate that tables 20 times the size are basically crippled when it
> comes to creating an index on them.
Ac
Gary Doades <[EMAIL PROTECTED]> writes:
> Is this likely to hit me in a random fashion during normal operation,
> joins, sorts, order by for example?
Yup, anytime you're passing data with that kind of distribution
through a sort.
> So the options are:
> 1) Fix the included qsort.c code and use t
Tom Lane wrote:
For some reason I hadn't immediately twigged to the fact that your test
script is just N repetitions of the exact same structure with random data.
So it's not so surprising that you get random variations in behavior
with different test data sets.
> It seems clear that our qsort
Gary Doades <[EMAIL PROTECTED]> writes:
> If I run the script again, it is not always the first case that is slow,
> it varies from run to run, which is why I repeated it quite a few times
> for the test.
For some reason I hadn't immediately twigged to the fact that your test
script is just N re
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