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Rollup measurement query amended (#11479) 

By user request from https://groups.google.com/g/druid-user/c/bFkOtE-1eQg - gives the measure as a floating point instead of an integer.
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docs/ingestion/index.md
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@ -169,16 +169,18 @@ For an example of how to configure rollup, and of how the feature will modify yo
### Maximizing rollup ratio
You can measure the rollup ratio of a datasource by comparing the number of rows in Druid with the number of ingested
events. The higher this number, the more benefit you are gaining from rollup. One way to do this is with a
[Druid SQL](../querying/sql.md) query like:
You can measure the rollup ratio of a datasource by comparing the number of rows in Druid (`COUNT`) with the number of ingested
events. One way to do this is with a
[Druid SQL](../querying/sql.md) query such as the following, where "count" refers to a `count`-type metric generated at ingestion time:
```sql
SELECT SUM("cnt") / COUNT(*) * 1.0 FROM datasource
SELECT SUM("count") / (COUNT(*) * 1.0)
FROM datasource
```
In this query, `cnt` should refer to a "count" type metric specified at ingestion time. See
[Counting the number of ingested events](schema-design.md#counting) on the "Schema design" page for more details about
The higher this number is, the more benefit you are gaining from rollup.
> See [Counting the number of ingested events](schema-design.md#counting) on the "Schema design" page for more details about
how counting works when rollup is enabled.
Tips for maximizing rollup: