Re: LIMIT/SORT optimization - Mailing list pgsql-patches

Oh, sorry, forgot to do a random table test.  The test used:

    DROP TABLE test;
    CREATE TABLE test (x INTEGER);
    INSERT INTO test SELECT random()*1000000 FROM generate_series(1, 1000000);

As expected the unpatched version is consistent for all LIMIT values
(first query was slow due to load after INSERT):

    14567.074 ms select * from (select * from test order by x limit 3) as x limit 1;
    4031.029 ms select * from (select * from test order by x limit 3) as x limit 1;
    3612.417 ms select * from (select * from test order by x limit 3) as x limit 1;
    3505.966 ms select * from (select * from test order by x limit 10) as x limit 1;
    3707.830 ms select * from (select * from test order by x limit 10) as x limit 1;
    3619.410 ms select * from (select * from test order by x limit 10) as x limit 1;
    5548.770 ms select * from (select * from test order by x limit 100) as x limit 1;
    3839.660 ms select * from (select * from test order by x limit 100) as x limit 1;
    4098.445 ms select * from (select * from test order by x limit 100) as x limit 1;
    3677.659 ms select * from (select * from test order by x limit 1000) as x limit 1;
    3956.980 ms select * from (select * from test order by x limit 1000) as x limit 1;
    3824.934 ms select * from (select * from test order by x limit 1000) as x limit 1;
    4641.589 ms select * from (select * from test order by x limit 10000) as x limit 1;
    4057.902 ms select * from (select * from test order by x limit 10000) as x limit 1;
    4682.779 ms select * from (select * from test order by x limit 10000) as x limit 1;
    4032.351 ms select * from (select * from test order by x limit 100000) as x limit 1;
    4572.528 ms select * from (select * from test order by x limit 100000) as x limit 1;
    4985.500 ms select * from (select * from test order by x limit 100000) as x limit 1;
    4942.422 ms select * from (select * from test order by x limit 1000000) as x limit 1;
    4669.230 ms select * from (select * from test order by x limit 1000000) as x limit 1;
    4639.258 ms select * from (select * from test order by x limit 1000000) as x limit 1;

and with the patch:

    1731.234 ms select * from (select * from test order by x limit 3) as x limit 1;
    570.315 ms select * from (select * from test order by x limit 3) as x limit 1;
    430.119 ms select * from (select * from test order by x limit 3) as x limit 1;
    431.580 ms select * from (select * from test order by x limit 10) as x limit 1;
    431.253 ms select * from (select * from test order by x limit 10) as x limit 1;
    432.112 ms select * from (select * from test order by x limit 10) as x limit 1;
    433.536 ms select * from (select * from test order by x limit 100) as x limit 1;
    433.115 ms select * from (select * from test order by x limit 100) as x limit 1;
    432.478 ms select * from (select * from test order by x limit 100) as x limit 1;
    442.886 ms select * from (select * from test order by x limit 1000) as x limit 1;
    442.133 ms select * from (select * from test order by x limit 1000) as x limit 1;
    444.905 ms select * from (select * from test order by x limit 1000) as x limit 1;
    522.782 ms select * from (select * from test order by x limit 10000) as x limit 1;
    521.481 ms select * from (select * from test order by x limit 10000) as x limit 1;
    521.526 ms select * from (select * from test order by x limit 10000) as x limit 1;
    3317.216 ms select * from (select * from test order by x limit 100000) as x limit 1;
    3365.467 ms select * from (select * from test order by x limit 100000) as x limit 1;
    3355.447 ms select * from (select * from test order by x limit 100000) as x limit 1;
    3307.745 ms select * from (select * from test order by x limit 1000000) as x limit 1;
    3315.602 ms select * from (select * from test order by x limit 1000000) as x limit 1;
    3585.736 ms select * from (select * from test order by x limit 1000000) as x limit 1;

---------------------------------------------------------------------------

Bruce Momjian wrote:
>
> I reran the test using:
>
>     test=> CREATE TABLE test (x INTEGER);
>     test=> INSERT INTO test SELECT * FROM generate_series(1, 1000000);
>     test=> SET log_min_duration_statement = 0;
>
> and got on an unpatched system:
>
>     1751.320 ms  select * from (select * from test order by x limit 3) as x limit 1;
>     1725.092 ms  select * from (select * from test order by x limit 3) as x limit 1;
>     1709.463 ms  select * from (select * from test order by x limit 3) as x limit 1;
>     1702.917 ms  select * from (select * from test order by x limit 10) as x limit 1;
>     1705.793 ms  select * from (select * from test order by x limit 10) as x limit 1;
>     1704.046 ms  select * from (select * from test order by x limit 10) as x limit 1;
>     1699.730 ms  select * from (select * from test order by x limit 100) as x limit 1;
>     1712.628 ms  select * from (select * from test order by x limit 100) as x limit 1;
>     1699.454 ms  select * from (select * from test order by x limit 100) as x limit 1;
>     1720.207 ms  select * from (select * from test order by x limit 1000) as x limit 1;
>     1725.519 ms  select * from (select * from test order by x limit 1000) as x limit 1;
>     1728.933 ms  select * from (select * from test order by x limit 1000) as x limit 1;
>     1699.609 ms  select * from (select * from test order by x limit 10000) as x limit 1;
>     1698.386 ms  select * from (select * from test order by x limit 10000) as x limit 1;
>     1698.985 ms  select * from (select * from test order by x limit 10000) as x limit 1;
>     1700.740 ms  select * from (select * from test order by x limit 100000) as x limit 1;
>     1700.989 ms  select * from (select * from test order by x limit 100000) as x limit 1;
>     1695.771 ms  select * from (select * from test order by x limit 100000) as x limit 1;
>
> which is expected because the sort work is constant.  With the patch I
> see:
>
>     433.892 ms  select * from (select * from test order by x limit 3) as x limit 1;
>     496.016 ms  select * from (select * from test order by x limit 3) as x limit 1;
>     434.604 ms  select * from (select * from test order by x limit 3) as x limit 1;
>     433.265 ms  select * from (select * from test order by x limit 10) as x limit 1;
>     432.058 ms  select * from (select * from test order by x limit 10) as x limit 1;
>     431.329 ms  select * from (select * from test order by x limit 10) as x limit 1;
>     429.722 ms  select * from (select * from test order by x limit 100) as x limit 1;
>     434.754 ms  select * from (select * from test order by x limit 100) as x limit 1;
>     429.758 ms  select * from (select * from test order by x limit 100) as x limit 1;
>     432.060 ms  select * from (select * from test order by x limit 1000) as x limit 1;
>     432.523 ms  select * from (select * from test order by x limit 1000) as x limit 1;
>     433.917 ms  select * from (select * from test order by x limit 1000) as x limit 1;
>     449.885 ms  select * from (select * from test order by x limit 10000) as x limit 1;
>     450.182 ms  select * from (select * from test order by x limit 10000) as x limit 1;
>     450.536 ms  select * from (select * from test order by x limit 10000) as x limit 1;
>     1771.807 ms  select * from (select * from test order by x limit 100000) as x limit 1;
>     1746.628 ms  select * from (select * from test order by x limit 100000) as x limit 1;
>     1795.600 ms  select * from (select * from test order by x limit 100000) as x limit 1;
>
> The patch is faster until we hit 100k or 10% of the table, at which
> point it is the same speed.  What is interesting is 1M is also the same
> speed:
>
>     1756.401 ms  select * from (select * from test order by x limit 1000000) as x limit 1;
>     1744.104 ms  select * from (select * from test order by x limit 1000000) as x limit 1;
>     1734.198 ms  select * from (select * from test order by x limit 1000000) as x limit 1;
>
> This is with the default work_mem of '1M'.  I used LIMIT 1 so the times
> were not affected by the size of the data transfer to the client.
>
>
> ---------------------------------------------------------------------------
>
> Bruce Momjian wrote:
> >
> > I did some performance testing of the patch, and the results were good.
> > I did this:
> >
> >     test=> CREATE TABLE test (x INTEGER);
> >     test=> INSERT INTO test SELECT * FROM generate_series(1, 1000000);
> >     test=> SET log_min_duration_statement = 0;
> >     test=> SELECT * FROM test ORDER BY x LIMIT 3;
> >
> > and the results where, before the patch, for three runs:
> >
> >   LOG:  duration: 1753.518 ms select * from test order by x limit 3;
> >   LOG:  duration: 1766.019 ms select * from test order by x limit 3;
> >   LOG:  duration: 1777.520 ms select * from test order by x limit 3;
> >
> > and after the patch:
> >
> >   LOG:  duration: 449.649 ms select * from test order by x limit 3;
> >   LOG:  duration: 443.450 ms select * from test order by x limit 3;
> >   LOG:  duration: 443.086 ms select * from test order by x limit 3;
> >
> > ---------------------------------------------------------------------------
> >
> > Gregory Stark wrote:
> > >
> > > Updated patch attached:
> > >
> > > 1) Removes #if 0 optimizations
> > >
> > > 2) Changes #if 0 to #if NOT_USED for code that's there for completeness and to
> > >    keep the code self-documenting purposes rather but isn't needed by anything
> > >    currently
> > >
> > > 3) Fixed cost model to represent bounded sorts
> > >
> > >
> >
> > [ Attachment, skipping... ]
> >
> > >
> > >
> > > "Gregory Stark" <stark@enterprisedb.com> writes:
> > >
> > > > "Heikki Linnakangas" <heikki@enterprisedb.com> writes:
> > > >
> > > >> There's a few blocks of code surrounded with "#if 0 - #endif". Are those just
> > > >> leftovers that should be removed, or are things that still need to finished and
> > > >> enabled?
> > > >
> > > > Uhm, I don't remember, will go look, thanks.
> > >
> > > Ok, they were left over code from an optimization that I decided wasn't very
> > > important to pursue. The code that was ifdef'd out detected when disk sorts
> > > could abort a disk sort merge because it had already generated enough tuples
> > > for to satisfy the limit.
> > >
> > > But I never wrote the code to actually abort the run and it looks a bit
> > > tricky. In any case the disk sort use case is extremely narrow, you would need
> > > something like "LIMIT 50000" or more to do it and it would have to be a an
> > > input table huge enough to cause multiple rounds of merges.
> > >
> > >
> > > I think I've figured out how to adjust the cost model. It turns out that it
> > > doesn't usually matter whether the cost model is correct since any case where
> > > the optimization kicks in is a case you're reading a small portion of the
> > > input so it's a case where an index would be *much* better if available. So
> > > the only times the optimization is used is when there's no index available.
> > > Nonetheless it's nice to get the estimates right so that higher levels in the
> > > plan get reasonable values.
> > >
> > > I think I figured out how to do the cost model. At least the results are
> > > reasonable. I'm not sure if I've done it the "right" way though.
> > >
> > >
> > > --
> > >   Gregory Stark
> > >   EnterpriseDB          http://www.enterprisedb.com
> > >
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> >
> > --
> >   Bruce Momjian  <bruce@momjian.us>          http://momjian.us
> >   EnterpriseDB                               http://www.enterprisedb.com
> >
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>
> --
>   Bruce Momjian  <bruce@momjian.us>          http://momjian.us
>   EnterpriseDB                               http://www.enterprisedb.com
>
>   + If your life is a hard drive, Christ can be your backup. +
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--
  Bruce Momjian  <bruce@momjian.us>          http://momjian.us
  EnterpriseDB                               http://www.enterprisedb.com

  + If your life is a hard drive, Christ can be your backup. +