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Docs - roll-up tip (#11677)
* Update rollup.md

Added SE tip around roll-up.

* Update docs/ingestion/rollup.md

Co-authored-by: Charles Smith <techdocsmith@gmail.com>

Co-authored-by: Charles Smith <techdocsmith@gmail.com>
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petermarshallio and techdocsmith committed Dec 7, 2021
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@@ -30,6 +30,14 @@ At ingestion time, you control rollup with the `rollup` setting in the [`granula

When you disable rollup, Druid loads each row as-is without doing any form of pre-aggregation. This mode is similar to databases that do not support a rollup feature. Set `rollup` to `false` if you want Druid to store each record as-is, without any rollup summarization.

Use roll-up when creating a table datasource if both:
- You want optimal performance or you have strict space constraints.
- You don't need raw values from [high-cardinality dimensions](schema-design.md#sketches).
Conversely, disable roll-up if either:
- You need results for individual rows.
- You need to execute `GROUP BY` or `WHERE` queries on _any_ column.
If you have conflicting needs for different use cases, you can create multiple tables with different roll-up configurations on each table.

## Maximizing rollup ratio

To measure the rollup ratio of a datasource compare the number of rows in Druid (`COUNT`) with the number of ingested events. For example, run a [Druid SQL](../querying/sql.md) query where "count" refers to a `count`-type metric generated at ingestion time as follows:

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