Github user paul-rogers commented on a diff in the pull request:
https://github.com/apache/drill/pull/1112#discussion_r168682470
--- Diff:
exec/vector/src/main/java/org/apache/drill/exec/record/metadata/ColumnMetadata.java
---
@@ -15,36 +15,115 @@
* See the License for the specific language governing permissions and
* limitations under the License.
*/
-package org.apache.drill.exec.record;
+package org.apache.drill.exec.record.metadata;
import org.apache.drill.common.types.TypeProtos.DataMode;
import org.apache.drill.common.types.TypeProtos.MajorType;
import org.apache.drill.common.types.TypeProtos.MinorType;
+import org.apache.drill.exec.record.MaterializedField;
/**
* Metadata description of a column including names, types and structure
* information.
*/
public interface ColumnMetadata {
+
+ /**
+ * Rough characterization of Drill types into metadata categories.
+ * Various aspects of Drill's type system are very, very messy.
+ * However, Drill is defined by its code, not some abstract design,
+ * so the metadata system here does the best job it can to simplify
+ * the messy type system while staying close to the underlying
+ * implementation.
+ */
+
enum StructureType {
- PRIMITIVE, LIST, TUPLE
+
+ /**
+ * Primitive column (all types except List, Map and Union.)
+ * Includes (one-dimensional) arrays of those types.
+ */
+
+ PRIMITIVE,
+
+ /**
+ * Map or repeated map. Also describes the row as a whole.
+ */
+
+ TUPLE,
+
+ /**
+ * Union or (non-repeated) list. (A non-repeated list is,
+ * essentially, a repeated union.)
+ */
+
+ VARIANT,
+
+ /**
+ * A repeated list. A repeated list is not simply the repeated
+ * form of a list, it is something else entirely. It acts as
+ * a dimensional wrapper around any other type (except list)
+ * and adds a non-nullable extra dimension. Hence, this type is
+ * for 2D+ arrays.
+ * <p>
+ * In theory, a 2D list of, say, INT would be an INT column, but
+ * repeated in to dimensions. Alas, that is not how it is. Also,
+ * if we have a separate category for 2D lists, we should have
+ * a separate category for 1D lists. But, again, that is not how
+ * the code has evolved.
+ */
+
+ MULTI_ARRAY
}
- public static final int DEFAULT_ARRAY_SIZE = 10;
+ int DEFAULT_ARRAY_SIZE = 10;
--- End diff --
This is an interface. The default (and only legal) form of fields is
`static final`. Thanks to Tim for reminding me of this.
---
