builtins: Implement unaccent function

[mknycha]

Unaccent function should work the same as in PostgreSQL.

Release note (sql change): Implement the string builtin unaccent.

Described in #41299

[cockroach-teamcity]

All committers have signed the CLA.

[blathers-crl]

Thank you for contributing to CockroachDB. Please ensure you have followed the guidelines for creating a PR.

My owl senses detect your PR is good for review. Please keep an eye out for any test failures in CI.

I have added a few people who may be able to assist in reviewing:

• @rohany (author of sql: Support unaccent() #41299)
• @jordanlewis (commented on sql: Support unaccent() #41299)
• @otan (commented on sql: Support unaccent() #41299)

_{🦉 Hoot! I am a Blathers, a bot for CockroachDB. My owner is otan.}

[cockroach-teamcity]

This change is 

[mknycha]

There is actually U+00AD in the middle, it's visible in git diff.

[otan]

we can combine these into one test case that's easier to mess with if you do:

SELECT unaccent(str) FROM ( VALUES 
   ('no_special_CHARACTERS1!'),
   ('Żółć')
) tbl(str)

[mknycha]

Ok, done

[mknycha]

I forgot to add a label "first PR", sorry

[otan]

hello @mknycha, thanks for doing this!
I was away last week, so apologies for the delay.
I have a couple of small comments.

[otan]

nit: 2020.

I think this may be useful as a separate package, i.e. in pkg/util/unaccent/unaccent.go

[mknycha]

Ok, done

[otan]

we can combine these into one test case that's easier to mess with if you do:

SELECT unaccent(str) FROM ( VALUES 
   ('no_special_CHARACTERS1!'),
   ('Żółć')
) tbl(str)

[otan]

i'm curious - should this be map[rune]string?

[otan]

(if not, I'm not sure how strings.Split(s, "") above works with this dictionary.

[mknycha]

You're right, I have just realized that strings.Split may not work correctly. I guess that map[rune]string is indeed a way to go. Some "characters" in the dictionary consist of two code points, so I need to modify this dictionary a bit.

[otan]

nit: this is more efficient if we use a strings.Builder to write out the whole string.
I feel as if we should be iterating rune by rune, something like

var b strings.Builder
for _, ch := range s {
   if repl, ok := unaccent.Characters[ch]; ok {
      b.WriteString(repl)
   } else {
      b.WriteRune(ch)
   }
}

return tree.NewDString(b.String()), nil

[mknycha]

Ok, done

[blathers-crl]

Thank you for updating your pull request.

My owl senses detect your PR is good for review. Please keep an eye out for any test failures in CI.

_{🦉 Hoot! I am a Blathers, a bot for CockroachDB. My owner is otan.}

[otan]

looks like some linting issues -- i'll merge it after those are fixed!

[otan]

bors r+

thank you for the contribution!

[craig]

Build succeeded:

• GitHub CI (Cockroach)