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

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  statement: select * from test order by x limit 3;
>   LOG:  duration: 1766.019 ms  statement: select * from test order by x limit 3;
>   LOG:  duration: 1777.520 ms  statement: select * from test order by x limit 3;
>
> and after the patch:
>
>   LOG:  duration: 449.649 ms  statement: select * from test order by x limit 3;
>   LOG:  duration: 443.450 ms  statement: select * from test order by x limit 3;
>   LOG:  duration: 443.086 ms  statement: 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
> >
> > ---------------------------(end of broadcast)---------------------------
> > TIP 6: explain analyze is your friend
>
> --
>   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. +
>
> ---------------------------(end of broadcast)---------------------------
> TIP 9: In versions below 8.0, the planner will ignore your desire to
>        choose an index scan if your joining column's datatypes do not
>        match

--
  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. +