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 !