Re: BUG #15285: Query used index over field with ICU collation in some cases wrongly return 0 rows

On Wed, 10 Jun 2020 00:29:33 +0200
Jehan-Guillaume de Rorthais <jgdr(at)dalibo(dot)com> wrote:
[...]
> After playing with ICU regression tests, I found functions ucol_strcollIter
> and ucol_nextSortKeyPart are safe. I'll do some performance tests and report
> here.

I did some benchmarks. See attachment for the script and its header to
reproduce.

It sorts 935895 french phrases from 0 to 122 chars with an average of 49.
Performance tests were done on current master HEAD (buggy) and using the patch
in attachment, relying on ucol_strcollIter.

My preliminary test with ucol_getSortKey was catastrophic, as we might
expect. 15-17x slower than the current HEAD. So I removed it from actual tests.
I didn't try with ucol_nextSortKeyPart though.

Using ucol_strcollIter performs ~20% slower than HEAD on UTF8 databases, but
this might be acceptable. Here are the numbers:

DB Encoding HEAD strcollIter ratio
UTF8 2.74 3.27 1.19x
LATIN1 5.34 5.40 1.01x

I plan to add a regression test soon.

> In the meantime, I've been working on various workarounds. The only one I
> found is to use "fr-u-kr-latn-digit-kn" instead of "fr-u-kr-latn-digit".
> Unfortunately, the two collations are not equivalent, but I believe it might
> be useful in many case.
>
> I've been working on a second workaround: creating a type (a char variant for
> our usecase), its operators and opfamily. All operators and function 1 relies
> on ucol_getSortKey. Most of the workaround works good but surprisingly, the
> sort order is only enforced if the field is in the first position:
>
> * this works: "SORT BY f1 COLLATE digitslast"
> * this fails: "SORT BY f2, f1 COLLATE digitslast"

I fixed this. I didn't declare my opclass as default for the type I created.
I'm not sure people would like to see/discuss this user workaround here?

Regards,