clintropolis commented on code in PR #12914:
URL: https://github.com/apache/druid/pull/12914#discussion_r953262388
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core/src/main/java/org/apache/druid/segment/column/ValueType.java:
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@@ -63,23 +63,37 @@ public enum ValueType implements TypeDescriptor
* String object type. This type may be used as a grouping key, an input to
certain types of complex sketch
* aggregators, and as an input to expression virtual columns. String types
might potentially be 'multi-valued' when
* stored in segments, and contextually at various layers of query
processing, but this information is not available
- * through this enum alone, and must be accompany this type indicator to
properly handle.
+ * at this level.
+ *
+ * Strings are typically represented as {@link String}, but multi-value
strings might also appear as a
+ * {@link java.util.List<String>}.
*/
STRING,
/**
* Placeholder for arbitrary 'complex' types, which have a corresponding
serializer/deserializer implementation. Note
* that knowing a type is complex alone isn't enough information to work
with it directly, and additional information
- * in the form of a type name that is registered in the complex type
registry must be available to make this type
- * meaningful. This type is not currently supported as a grouping key for
aggregations, and may not be used as an
- * input to expression virtual columns, and might only be supported by the
specific aggregators crafted to handle
- * this complex type.
+ * in the form of a type name which must be registered in the complex type
registry. This type is not currently
+ * supported as a grouping key for aggregations, and might only be supported
by the specific aggregators crafted to
+ * handle this complex type. Filtering on this type with standard filters
will most likely have limited support, and
+ * may match as a "null" value.
+ *
+ * These types are represented by the individual Java type associated with
the complex type name as defined in the
+ * type registry.
*/
COMPLEX,
/**
- * Placeholder for arbitrary arrays of other {@link ValueType}. This type is
not currently supported as a grouping
+ * Placeholder for arbitrary arrays of other {@link ValueType}. This type
has limited support as a grouping
Review Comment:
you can group on specific array types, such as `ARRAY<STRING>`,
`ARRAY<LONG>`, `ARRAY<DOUBLE>`, `ARRAY<FLOAT>`, but not fully generically, like
on `ARRAY<ARRAY<LONG>>`, or `ARRAY<COMPLEX<json>>` (assuming grouping on
`COMPLEX` types was also supported, which it isn't yet... but soon.). I'll
clarify that some specific array grouping is supported, but array grouping in
general is not.
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