ydb/yql/essentials/tests/s-expressions/suites/Optimizers/FlatMapBeforeCombineByKey.yqls

(
#comment
(let config (DataSource 'config))

(let res_sink (DataSink 'result))
(let data (AsList (String 'a) (String 'a) (String 'b) (String 'c)))
(let preMap (lambda '(item) (Just item)))
(let keyExtractor (lambda '(item) item))
(let init (lambda '(key item) item))
(let update (lambda '(key item state) (Concat state item)))
(let finish (lambda '(key state) (AsList '(key state))))
(let map (Filter data (lambda '(x) (< x (String 'c)))))
(let combined (CombineByKey map preMap keyExtractor init update finish))
(let result (Sort combined (Bool 'true) (lambda '(x) (Nth x '0))))
(let world (Write! world res_sink (Key) result '('('type))))
(let world (Commit! world res_sink))
(return world)
)