Document window functions

[sploiselle]

We need to complete our page on window functions. Our implementation is similar to Postgres, so we can start by testing everything on that page.

Also need to update the SQL feature table.

[jseldess]

PRs:

• sql: Add initial support for window functions cockroach#8928
• sql/parser: Add initial support for builtin window functions cockroach#9321
• sql: Support ntile(INT) builtin window function cockroach#9335

[jseldess]

Source code:

• https://github.com/cockroachdb/cockroach/blob/8ac54961ab8edc641e41880f8e410032dc1843eb/pkg/sql/window.go#L31
• https://github.com/cockroachdb/cockroach/blob/1c0a035d17d12d37379c54a9e7a648f2171bfab0/pkg/sql/parser/window_builtins.go

[jseldess]

Stub docs put in place with #1948.

[tlvenn]

Hi @jseldess ,

Could you clarify if there is feature parity with what Postgres is offering ? If not, is there a todo list related to this somewhere ?

Thanks !

[jseldess]

@tlvenn, so sorry for my extreme delay here. I believe there is feature parity with Postgres, but I'm not 100% certain. We'll build out more robust docs on window functions soon.

[tlvenn]

no worry, thanks for the feedback @jseldess

[jseldess]

For 2.1, this is part of #2944.

[jseldess]

@rmloveland, in this PR, I just added the auto-generated docs on windows functions:
https://www.cockroachlabs.com/docs/v2.1/functions-and-operators.html#window-functions.

Feel free to cross-reference in your window functions doc updates.

[rmloveland]

This was fixed in #3426 (at least for now - could always be improved, window functions are a big topic).