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add note regarding -MapState

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Yakov Olkhovskiy 2023-06-14 10:42:20 -04:00 committed by GitHub
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docs/en/sql-reference/aggregate-functions/combinators.md
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@ -97,6 +97,10 @@ Result:
If you apply this combinator, the aggregate function does not return the resulting value (such as the number of unique values for the [uniq](../../sql-reference/aggregate-functions/reference/uniq.md#agg_function-uniq) function), but an intermediate state of the aggregation (for `uniq`, this is the hash table for calculating the number of unique values). This is an `AggregateFunction(...)` that can be used for further processing or stored in a table to finish aggregating later.
:::note
Please notice, that -MapState is not an invariant for the same data due to the fact that order of data in intermediate state can change, though it doesn't impact ingestion of this data.
:::
To work with these states, use:
- [AggregatingMergeTree](../../engines/table-engines/mergetree-family/aggregatingmergetree.md) table engine.