window: the right border can't work?

[vagetablechicken]

We have this example:

            Example:

            |c1|c2|
            |--|--|
            |0 | 1|
            |1 | 1|
            |2 | 2|
            |3 | 2|
            |4 | 2|

SELECT at(c1, 1) over w as co from t1 window w as(partition by c2 order by c1 rows between unbounded preceding and current row);
                -- output
                -- | co |
                -- |----|
                -- |NULL|
                -- |0   |
                -- |NULL|
                -- |2   |
                -- |3   |

But run SELECT at(c1, 1) over w as co from t1 window w as(partition by c2 order by c1 rows between unbounded preceding and 1000 preceding); is the same result. Why?

[aceforeverd]

lag/lead or the old at function, is always relative to current row, window frame bound do not take any real meaning to the SQL engine.

Both SQL is equivalent to select at(c1, 1) over (partition by c2 order by c1) from t1.

at over ( .... between A and B) is allowed just for backward compatibility. Also to mention, it is not allowed in Spark SQL

[aceforeverd]

more discussion here #1554

[vagetablechicken]

Added more info in udf docs, close this issue