When all leading keys are different, mksort will finish the entire sort at the
first sort key and never touch other keys. For the case, mksort falls back to
kind of qsort actually.
I created another data set with distinct values in all sort keys:
create table t2 (c1 int, c2 int, c3 int, c4 int, c5 int, c6 varchar(100));
insert into t2 values (generate_series(1,499999), 0, 0, 0, 0, '');
update t2 set c2 = 999990 - c1, c3 = 999991 - c1, c4 = 999992 - c1, c5
= 999993 - c1;
update t2 set c6 = 'aaaaaaaaaaaaaaaaaaaaaaaaaaabbbbbbbbbbbbbbbbbbbbbbbbbbb'
|| (999994 - c1)::text;
explain analyze select c1 from t2 order by c6, c5, c4, c3, c2, c1;
Results:
MKsort:
12374.427 ms
12528.068 ms
12554.718 ms
qsort:
12251.422 ms
12279.938 ms
12280.254 ms
MKsort is a bit slower than qsort, which can be explained by extra
checks of MKsort.
Yao Wang
On Fri, May 24, 2024 at 8:36 PM Wang Yao <yaowangm@outlook.com> wrote:
>
>
>
> 获取Outlook for Android
> ________________________________
> From: Heikki Linnakangas <hlinnaka@iki.fi>
> Sent: Thursday, May 23, 2024 8:47:29 PM
> To: Wang Yao <yaowangm@outlook.com>; PostgreSQL Hackers <pgsql-hackers@postgresql.org>
> Cc: interma@outlook.com <interma@outlook.com>
> Subject: Re: 回复: An implementation of multi-key sort
>
> On 23/05/2024 15:39, Wang Yao wrote:
> > No obvious perf regression is expected because PG will follow original
> > qsort code path when mksort is disabled. For the case, the only extra
> > cost is the check in tuplesort_sort_memtuples() to enter mksort code path.
>
> And what about the case the mksort is enabled, but it's not effective
> because all leading keys are different?
>
> --
> Heikki Linnakangas
> Neon (https://neon.tech)
>
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