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/// count the number of items for each publisher
counted = AGGREGATION({AGGREGATIONS = [['FUNCTION' = 'Count']], GROUP_BY = ['publisher', 'item']}, windowed)
/// aggregate the 100 most frequent items for each publisher to an ordered list
TopKItemsByPublisher ::= AGGREGATION({AGGREGATIONS = [
	[
		'FUNCTION' = 'TopK',
		'TOP_K' = '100',                        /// number of items
		'SCORING_ATTRIBUTES' = 'Count',         /// the attribute name that defines the order
		'INPUT_ATTRIBUTES' = 'item',            /// do not use the whole input tuple, just use the 'item' attribute for creating the output top-k set
		'MIN_SCORE' = '0',                      /// remove items that reaches a score of 0 (due to the previous aggregation these are all items that has no valid tuple)
		'UNIQUE_ATTR'='item',                   /// use 'item' as a unique attribute. that means, a new tuple with an known items id replaces the previous value. (this is some kind of element window in this operator)
		'descending' = true,						/// default is true. If you want to have the smallest elements, use 'false', if you want to have the biggest elements, use 'true'
		'ALWAYS_OUTPUT' = true					/// If set to false (default), 'null' is put out instead of the result if the result is equal to the previous result.
  ]], GROUP_BY = ['publisher']}, counted)