ydb/yql/essentials/tests/s-expressions/suites/InMem/PartitionByKey.yqls

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

(let res_sink (DataSink 'result))
(let list (AsList
'((Uint32 '1) (String 'a))
'((Uint32 '2) (String 'b))
'((Uint32 '1) (String 'c))))

(let keyExtractor (lambda '(x) (Nth x '0)))
(let handler (lambda '(groups) (block '(
   (return (Map groups (lambda '(pair) (block '(
       (let key (Nth pair '0))
       (let stream (Nth pair '1))
       (let state (Concat (String '"key:") (ToString key)))
       (let state (Condense stream state (lambda '(row state) (Bool 'False)) (lambda '(row state) (Concat (Concat state (String '" ")) (Nth row '1)))))
       (return (Collect state))
   )))))
))))
(let data (PartitionByKey list keyExtractor (Void) (Void) handler))

(let world (Write! world res_sink (Key) data '()))
(let world (Commit! world res_sink))
(return world)
)