[GitHub] [druid] clintropolis commented on a change in pull request #11254: Update math-expr.md

[GitBox <gi...@apache.org>]

clintropolis commented on a change in pull request #11254:
URL: https://github.com/apache/druid/pull/11254#discussion_r632837576



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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:
       Oh man, no idea why I worded this so strange back in #7588.. i wonder if something like this would be a bit clearer:
   
   > Values that result in arrays will be 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', behaving similar to the `UNNEST` operator available in many SQL dialects.




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