paul-rogers commented on a change in pull request #2018: DRILL-7633: Fixes for
union and repeated list accessors
URL: https://github.com/apache/drill/pull/2018#discussion_r396947139
##
File path:
exec/vector/src/main/java/org/apache/drill/exec/record/metadata/VariantColumnMetadata.java
##
@@ -96,11 +134,13 @@ public StructureType structureType() {
public boolean isVariant() { return true; }
@Override
- public boolean isArray() { return type() == MinorType.LIST; }
+ public boolean isArray() {
+return super.isArray() || type() == MinorType.LIST;
+ }
@Override
public ColumnMetadata cloneEmpty() {
-return new VariantColumnMetadata(name, type, variantSchema.cloneEmpty());
+return new VariantColumnMetadata(name, type, mode, new VariantSchema());
Review comment:
@arina-ielchiieva, thanks for the reminder; I did miss your response.
I think I see where we had a misunderstanding. I thought the `typeString()`
code already worked because I saw this in `AbstractColumnMetadata`:
```
public String typeString() {
return majorType().toString();
}
```
`PrimitiveColumnMetadata` uses a different set of type names.
It seems this area has gotten a bit muddled: the `AbstractColumnMetadata`
version seems to never be called, except for `VariantColumnMetadata`. (This
confused the heck out of me on an earlier attempt to clean up this area, but I
didn't clue into the problem then.)
So, now that I understand what you are asking for, I did this:
* Removed the `AbstractColumnMetadata` version.
* Added a `VariantColumnMetadata` that produces either `UNION` or
`ARRAY`.
Until I see a good reason otherwise, I think we should treat `UNION` (what I
call a "variant") as an opaque type. I guess I don't see the contents of a
variant as something we want to specify, either in the metastore or in a
provided schema. For example, can the metastore compute an NDV across an `INT`,
`VARCHAR` and `MAP`? Probably not. Nor do the min/max values or other stats
make sense for a variant. As a result, a variant is an opaque type for the
metastore.
Similarly, in a provided schema, I can't convince myself that the user wants
not only to say, "the type of this field can vary", but also that "the type can
be an `INT`, `DOUBLE` or `VARCHAR`, but not a `BIGINT`.) Just does not seem
super-useful.
An additional concern, which I think I mentioned somewhere else, is that as
soon as we start serializing complex types, the SQL-like text format becomes
far too cumbersome. We'd be much better of with a JSON format that can capture
the complex tree structure. Compare:
```
(A UNION)>)
```
With something like:
```
{ schema: [
{name: "a",
type: {
name: "UNION",
nullable: true,
subtypes: [
{name: INT, nullable: false},
{name: MAP, nullable: false},
members: [ ...
```
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