sthetland commented on a change in pull request #11254:
URL: https://github.com/apache/druid/pull/11254#discussion_r644180490
##########
File path: docs/misc/math-expr.md
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@@ -50,7 +50,7 @@ Expressions can contain variables. Variable names may contain
letters, digits, '
For logical operators, a number is true if and only if it is positive (0 or
negative value means false). For string type, it's the evaluation result of
'Boolean.valueOf(string)'.
-[Multi-value string dimensions](../querying/multi-value-dimensions.md) are
supported and may be treated as either scalar or array typed values. When
treated as a scalar type, an expression will automatically be transformed to
apply the scalar operation across all values of the multi-valued type, to mimic
Druid's native behavior. Values that result in arrays will be coerced back into
the native Druid string type for aggregation. Druid aggregations on multi-value
string dimensions on the individual values, _not_ the 'array', behaving similar
to the `UNNEST` operator available in many SQL dialects. However, by using the
`array_to_string` function, aggregations may be done on a stringified version
of the complete array, allowing the complete row to be preserved. Using
`string_to_array` in an expression post-aggregator, allows transforming the
stringified dimension back into the true native array type.
+[Multi-value string dimensions](../querying/multi-value-dimensions.md) are
supported and may be treated as either scalar or array typed values. When
treated as a scalar type, an expression will automatically be transformed to
apply the scalar operation across all values of the multi-valued type, to mimic
Druid's native behavior. Values that result in arrays will be coerced back into
the native Druid string type for aggregation. Druid aggregations on multi-value
string dimensions act on the individual values, _not_ the 'array', behaving
similar to the `UNNEST` operator available in many SQL dialects. However, by
using the `array_to_string` function, aggregations may be done on a stringified
version of the complete array, allowing the complete row to be preserved. Using
`string_to_array` in an expression post-aggregator, allows transforming the
stringified dimension back into the true native array type.
Review comment:
Incorporating your suggestion @clintropolis and adding my ¢2:
```suggestion
[Multi-value string dimensions](../querying/multi-value-dimensions.md) are
supported and may be treated as either scalar or array typed values, as
follows:
* When treated as a scalar type, the expression is automatically transformed
so that the scalar operation is applied across all values of the multi-valued
type, mimicking Druid's native behavior.
* Values that result in arrays are coerced back into the native Druid string
type for grouping and aggregation. Grouping on multi-value string dimensions in
Druid will group by the individual values, not the 'array', resulting in
behavior similar to the UNNEST operator available in many SQL dialects.
Alternatively, you can use the `array_to_string` function to perform the
aggregation on a _stringified_ version of the complete array, preserving the
complete row. Using `string_to_array` in an expression post-aggregator
transforms the stringified dimension back into the true native array type.
```
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