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Update FinishAggregatingInOrderAlgorithm.h

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Alexander Kuzmenkov 2021-03-09 13:43:06 +03:00 committed by GitHub
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src/Processors/Merges/Algorithms/FinishAggregatingInOrderAlgorithm.h
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@ -21,6 +21,14 @@ using AggregatingTransformParamsPtr = std::shared_ptr<AggregatingTransformParams
* X will never appear later in any of input streams.
* - Aggregate all rows in current blocks of inputs up to the upper_bound of X using
* regular hash table algorithm (Aggregator::mergeBlock).
* The hash table at one step will contain all keys <= X from all blocks.
* There is another, simpler algorithm (AggregatingSortedAlgorithm), that merges
* and aggregates sorted data for one key at a time, using one aggregation state.
* It is a simple k-way merge algorithm and it makes O(n*log(k)) comparisons,
* where * n -- number of rows, k -- number of parts. In comparison, this algorithm
* makes about * O(n + k * log(n)) operations, n -- for hash table, k * log(n)
* -- for finding positions in blocks. It is better than O(n*log(k)), when k is not
* too big and not too small (about 100-1000).
*/
class FinishAggregatingInOrderAlgorithm final : public IMergingAlgorithm
{