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 <yaowa...@outlook.com> wrote: > > > > 获取Outlook for Android > ________________________________ > From: Heikki Linnakangas <hlinn...@iki.fi> > Sent: Thursday, May 23, 2024 8:47:29 PM > To: Wang Yao <yaowa...@outlook.com>; PostgreSQL Hackers > <pgsql-hack...@postgresql.org> > Cc: inte...@outlook.com <inte...@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) > -- This electronic communication and the information and any files transmitted with it, or attached to it, are confidential and are intended solely for the use of the individual or entity to whom it is addressed and may contain information that is confidential, legally privileged, protected by privacy laws, or otherwise restricted from disclosure to anyone else. If you are not the intended recipient or the person responsible for delivering the e-mail to the intended recipient, you are hereby notified that any use, copying, distributing, dissemination, forwarding, printing, or copying of this e-mail is strictly prohibited. If you received this e-mail in error, please return the e-mail to the sender, delete it from your computer, and destroy any printed copy of it.
