Re: optimisation? collation "C" sorting for GroupAggregate for all deterministic collations

On Sun, Mar 22, 2020 at 5:33 AM Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com> wrote:
>
> Hi
>
> ne 22. 3. 2020 v 10:12 odesílatel Maxim Ivanov <hi(at)yamlcoder(dot)me> napsal:
>>
>> Hi All,
>>
>> It is known, that collation "C" significantly speeds up string comparisons and as a result sorting. I was wondering, whether it is possible to use it regardless of collation set on a column in sorts not visible to users?
>>
>> Example I have in mind is sorting performed for GroupAggregate. Purpose of that sort is to bring equal values next to each other, so as long as:
>> 1) user didn't request ORDER BY in addition to GROUP BY
>> 2) source column has any deterministic collation (as per docs all builtin collations are deterministic)
>>
>> it seems to be possible to do sorting with any deterministic collation, regardless of what user specifid for the column being sorted. "C" collation is deterministic and fastest.
>>
>> In other words, couldn't PostgreSQL convert this:
>>
>> -> GroupAggregate (cost=15726557.87..22944558.69 rows=7200001 width=176) (actual time=490103.209..771536.389 rows=36000000 loops=1)
>> Group Key: ec_180days.msn, ec_180days.to_date_time
>> -> Sort (cost=15726557.87..15906557.89 rows=72000008 width=113) (actual time=490094.849..524854.662 rows=72000000 loops=1)
>> Sort Key: ec_180days.msn, ec_180days.to_date_time
>> Sort Method: external merge Disk: 7679136kB
>>
>> To this:
>>
>> -> GroupAggregate (cost=14988274.87..22206275.69 rows=7200001 width=155) (actual time=140497.729..421510.001 rows=36000000 loops=1)
>> Group Key: ec_180days.msn, ec_180days.to_date_time
>> -> Sort (cost=14988274.87..15168274.89 rows=72000008 width=92) (actual time=140489.807..174228.722 rows=72000000 loops=1)
>> Sort Key: ec_180days.msn COLLATE "C", ec_180days.to_date_time
>> Sort Method: external merge Disk: 7679136kB
>>
>>
>> which is 3 times faster in my tests.
>
>
> I had a same idea. It is possible only if default collation is deterministic. Probably it will be less important if abbreviate sort will be enabled, but it is disabled now.
>
> p.s. can be interesting repeat your tests with ICU locale where abbreviate sort is enabled.

Perhaps this is what you mean by "deterministic", but isn't it
possible for some collations to treat multiple byte sequences as equal
values? And those multiple byte sequences wouldn't necessarily occur
sequentially in C collation, so it wouldn't be possible to work around
that by having the grouping node use one collation but the sorting
node use the C one.

If my memory is incorrect, then this sounds like an intriguing idea.

James