Add `arrayUnion` function

[rschu1ze]

Basically arrayDistinct(arrayConcat(arr1, arr2, arr3, ...))

Related function: arrayIntersect

[tinfoil-knight]

Hey @rschu1ze, I'd like to try fixing this issue. Can you assign it to me?

[rschu1ze]

Cool, please go ahead, thanks.

[tinfoil-knight]

@rschu1ze Just wanted to confirm this before I start working to not miss anything.

Initial Plan:

• Add an arrayUnion.cpp file under src/Functions/array & implement the required function
• Use the tests/queries/0_stateless/add-test script to generate a .sql & .reference file
• Add tests in the generated .sql file & then update the reference file with: clickhouse-client --multiquery < 0xxxx_arrayUnion.sql > 0xxxx_arrayUnion.reference
• Update docs/en/sql-reference/functions/array-functions.md
• Update utils/check-style/aspell-ignore/en/aspell-dict.txt

[rschu1ze]

@tinfoil-knight The new function arrayUnion is conceptually similar to the existing function arrayIntersect. We should avoid to re-implement logic. Therefore, I'd suggest as first step to check src/Functions/array/arrayIntersect.cpp (possibly step through it using a debugger) which has a well-optimized implementation. As far as I see, most of the existing code also applies to arrayUnion and we only need to templatize FunctionArrayIntersect::execute (e.g. add a new template parameter bool intersect to distinguish intersect and union cases) and add a path under an if constexpr to it.

The other steps look good!

[Blargian]

@rschu1ze I'll work on this one

[rschu1ze]

@Blargian That's nice, I assigned you. Please ping me if you like me to have a look.

[petern48]

@Blargian Are you still working on this? If not, I'd like to give it a shot.

[Blargian]

@petern48 please go ahead! I got distracted with another project.