gianm commented on a change in pull request #12253:
URL: https://github.com/apache/druid/pull/12253#discussion_r804721274
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File path: docs/querying/multi-value-dimensions.md
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@@ -375,3 +375,8 @@ This query returns the following result:
Note that, for groupBy queries, you could get similar result with a [having
spec](having.md) but using a filtered
`dimensionSpec` is much more efficient because that gets applied at the lowest
level in the query processing pipeline.
Having specs are applied at the outermost level of groupBy query processing.
+
+## Disable GroupBy on multivalue columns
+
+As grouping on multivalue columns causes implicit unnest, users can avoid this
behaviour by setting
+`groupByEnableMultiValueUnnesting` in the query context to `false`. This will
result the query to error out.
Review comment:
Alternatively we can keep this documented but change the error message
to not mention array-based dimensions. In that case, we can change the error
message to this:
> Encountered multi-value dimension [%s] that cannot be processed with %s
set to false. Consider setting %s to true for unnesting behavior, or using an
expression to create a scalar from the multi-value dimension.
For the docs, a couple style points:
1. The rest of this page uses second person ("you can…") rather than third
("users can…") so we should stick to that.
2. We usually use US spelling in documentation (e.g. behavior instead of
behaviour).
So I'd go with:
> You can disable the implicit unnesting behavior for groupBy by setting
`groupByEnableMultiValueUnnesting: false` in your query context. In this mode,
the groupBy engine will return an error instead of completing the query. This
is a safety feature for situations where you believe that all dimensions are
singly-valued and want the engine to reject any multi-valued dimensions that
were inadvertently included.
Also, all documented groupBy parameters should be included in the
groupbyquery.md document as well, under "GroupBy v2 configurations". So if you
mention this here it should be mentioned in the main doc too.
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