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docs/en/operations/settings/settings.md

@@ -2134,6 +2134,21 @@ Default value: `1`.
 
 -   [ORDER BY Clause](../../sql-reference/statements/select/order-by.md#optimize_read_in_order)
 
+## optimize_aggregation_in_order {#optimize_aggregation_in_order}
+
+Enables [GROUP BY](../../sql-reference/statements/select/group-by.md) optimization in [SELECT](../../sql-reference/statements/select/index.md) queries for aggregating data in corresponding order in [MergeTree](../../engines/table-engines/mergetree-family/mergetree.md) tables.
+
+Possible values:
+
+-   0 — `GROUP BY` optimization is disabled.
+-   1 — `GROUP BY` optimization is enabled.
+
+Default value: `0`.
+
+**See Also**
+
+-   [GROUP BY optimization](../../sql-reference/statements/select/group-by.md#aggregation-in-order)
+
 ## mutations_sync {#mutations_sync}
 
 Allows to execute `ALTER TABLE ... UPDATE|DELETE` queries ([mutations](../../sql-reference/statements/alter/index.md#mutations)) synchronously.

docs/en/sql-reference/statements/select/group-by.md

@@ -255,6 +255,10 @@ For every different key value encountered, `GROUP BY` calculates a set of aggreg
 
 Aggregation is one of the most important features of a column-oriented DBMS, and thus it’s implementation is one of the most heavily optimized parts of ClickHouse. By default, aggregation is done in memory using a hash-table. It has 40+ specializations that are chosen automatically depending on “grouping key” data types.
 
+### GROUP BY Optimization Depending on Table Sorting Key {#aggregation-in-order}
+
+If a table is sorted by some key, and `GROUP BY` expression contains at least prefix of sorting key or injective functions, the in-between result of aggreagtion can be finalized and sent to client when a new key is read from table. This behaviour is switched on with the [optimize_aggregation_in_order](../../../operations/settings/settings.md#optimize_aggregation_in_order) setting.
+
 ### GROUP BY in External Memory {#select-group-by-in-external-memory}
 
 You can enable dumping temporary data to the disk to restrict memory usage during `GROUP BY`.