[SPARK-31434][SQL][DOCS] Drop builtin function pages from SQL references

[maropu]

What changes were proposed in this pull request?

This PR intends to drop the built-in function pages from SQL references. We've already had a complete list of built-in functions in the API documents.

See related discussions for more details:
#28170 (comment)

Why are the changes needed?

For better SQL documents.

Does this PR introduce any user-facing change?

How was this patch tested?

Manually checked.

[maropu]

cc: @huaxingao@HyukjinKwon@srowen

[SparkQA]

Test build #121193 has finished for PR 28203 at commit 9b3b71a.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[HyukjinKwon]

+1 as the temporary status.

I will try to get the auto-generation done (SPARK-31429) for Spark 3.0 if nobody takes it .. if we fail, we could do it for Spark 3.1.

[HyukjinKwon]

From my cursory look, seems good to go.

[huaxingao]

The changes look good. The only problem is that I don't have a place any more to document the window functions. We don't want to list the window function names and descriptions, but we still want to document what window functions are, how to use them and examples, right?

[maropu]

The changes look good. The only problem is that I don't have a place any more to document the window functions. We don't want to list the window function names and descriptions, but we still want to document what window functions are, how to use them and examples, right?

We still need to document them? They are also listed in the auto-generated API doc, right?

[huaxingao]

The auto-generated API doc has the function description, but it doesn't cover the detailed usage such as how to mark the function as window function, how to define the window spec, what's inside the window spec, etc. Window function is quite important and complex, so we need more documentation than function description.
I discussed with @gatorsmile earlier today, if we can't document window function here, he wants me to have detailed documentation for all these window functions in SQL syntax section.

[maropu]

If necessary, how about adding them in sql-ref-syntax-qry-window.md just like table-value functions?

[huaxingao]

Sure. I will document these in SQL syntax section instead.

[maropu]

Thanks!

[HyukjinKwon]

@huaxingao, do you have any page you want to keep here otherwise? If not I will just merge this.

[huaxingao]

No... The main purpose of having a builtin function page is to document window function. Now since there is an alternative place to document this, I am OK to remove this page. Please go ahead to merge. Thanks for checking with me. @HyukjinKwon

[HyukjinKwon]

LGTM

[HyukjinKwon]

Merged to master and branch-3.0.

[maropu]

Thanks, all.