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.
