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Clarify SQL behavior for multi-value dimensions. (#10276)

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There are some known inconsistencies between SQL and native that
users should be aware of.
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@ -226,6 +226,13 @@ Grouping by a multi-value expression will observe the native Druid multi-value a
the `UNNEST` functionality available in some other SQL dialects. Refer to the documentation on
[multi-value string dimensions](multi-value-dimensions.html) for additional details.
> Because multi-value dimensions are treated by the SQL planner as `VARCHAR`, there are some inconsistencies between how
> they are handled in Druid SQL and in native queries. For example, expressions involving multi-value dimensions may be
> incorrectly optimized by the Druid SQL planner: `multi_val_dim = 'a' AND multi_val_dim = 'b'` will be optimized to
> `false`, even though it is possible for a single row to have both "a" and "b" as values for `multi_val_dim`. The
> SQL behavior of multi-value dimensions will change in a future release to more closely align with their behavior
> in native queries.
### NULL values
The `druid.generic.useDefaultValueForNull` [runtime property](../configuration/index.html#sql-compatible-null-handling)
@ -747,10 +754,13 @@ Druid does not support all SQL features. In particular, the following features a
Additionally, some Druid native query features are not supported by the SQL language. Some unsupported Druid features
include:
- [Union datasources](datasource.html#union)
- [Inline datasources](datasource.html#inline)
- [Union datasources](datasource.html#union).
- [Inline datasources](datasource.html#inline).
- [Spatial filters](../development/geo.html).
- [Query cancellation](querying.html#query-cancellation).
- [Multi-value dimensions](#multi-value-strings) are only partially implemented in Druid SQL. There are known
inconsistencies between their behavior in SQL queries and in native queries due to how they are currently treated by
the SQL planner.
## Client APIs