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Re: LIMIT/SORT optimization

From: Bruce Momjian <bruce(at)momjian(dot)us>
To: Bruce Momjian <bruce(at)momjian(dot)us>
Cc: Gregory Stark <stark(at)enterprisedb(dot)com>, Heikki Linnakangas <heikki(at)enterprisedb(dot)com>, Gregory Stark <gsstark(at)mit(dot)edu>, Simon Riggs <simon(at)2ndquadrant(dot)com>, pgsql-patches <pgsql-patches(at)postgresql(dot)org>
Subject: Re: LIMIT/SORT optimization
Date: 2007-04-08 02:16:34
Message-ID: 200704080216.l382GYH27157@momjian.us (view raw or flat)
Thread:
Lists: 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(at)enterprisedb(dot)com> writes:
> > > 
> > > > "Heikki Linnakangas" <heikki(at)enterprisedb(dot)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(at)momjian(dot)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(at)momjian(dot)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 4: Have you searched our list archives?
> 
>                http://archives.postgresql.org

-- 
  Bruce Momjian  <bruce(at)momjian(dot)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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