Document Window functions

[alamb]

Is your feature request related to a problem or challenge?

@mustafasrepo and others have contributed to a sophisticated window function implementation in DataFusion like

select x, lag(x, 1) OVER (partition by y, order by x) FROM foo

However, the user guide does not appear to mention this

Describe the solution you'd like

1. Note in the user guide that DataFusion supports window functions
2. Add a new page for WindowFunctions (similar to the aggregate function list)

Note the docs (and instructions) are in https://github.com/apache/arrow-datafusion/tree/main/docs

The list of window functions is

1. aggregate functions
2. The list here https://docs.rs/datafusion-expr/24.0.0/datafusion_expr/window_function/enum.BuiltInWindowFunction.html

Describe alternatives you've considered

No response

Additional context

No response

[toppyy]

started work on this!

[toppyy]

Made a draft PR.

The issue states that the PR should "Note in the user guide that DataFusion supports window functions". Any ideas where exactly to mention this? I had a look and I didn't find a place that felt suitable.

Also, I was on the fence about whether to describe the over-clause with each function as it seems repetative. The alternative would be to described the syntax for window functions on top of the page once in a general form (ie. something like function(expr) over([partition by expr2] [order by expr3 [ asc | desc ][, …]])). Any thoughts?

[alamb]

Thanks @toppyy !

The issue states that the PR should "Note in the user guide that DataFusion supports window functions". Any ideas where exactly to mention this? I had a look and I didn't find a place that felt suitable.

Perhaps we can add a new Window Functions page, following the model of Aggregate Functions? (UPDATE: I see this is exactly what you did in #6402 🤦 sorry for not checking that first)

Also, I was on the fence about whether to describe the over-clause with each function as it seems repetative. The alternative would be to described the syntax for window functions on top of the page once in a general form (ie. something like function(expr) over([partition by expr2] [order by expr3 [ asc | desc ][, …]])). Any thoughts?

I think describing the OVER clause once in general rather than for each function makes much more sense.

Perhaps we can take inspiration (and/or some of the text) from the postgres description: https://www.postgresql.org/docs/current/tutorial-window.html