Examples can be found in the unit tests (query_optimizer/tests/execution_generator/Select.test)
SELECT sum(avg(int_col) OVER w) FROM test
WINDOW w AS
(PARTITION BY char_col
ORDER BY long_col
ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW);
Each window query will have an OVER keyword which can immediately reference a WINDOW clause or immediately define the WINDOW clause.
At the time of writing, the only supported aggregation is AVG.
This is implemented in a single thread mode. There has not been an attempt to make it support multithreaded environments.