DX-66740: Added creating MapWriter in ComplexWriter.

[xxlaykxx]

Added new method rootAsMap() to ComplexWriter and implement it in ComplexWriterImpl for supporting @output map type. Need this to unlock ARRAY_FREQUENCY function.

Previously in dremio side:

When i trying to return map like @output ComplexWrite
with this code:

org.apache.arrow.vector.complex.writer.BaseWriter.MapWriter mapWriter = out.rootAsList().map(false);

mapWriter.startMap();
for (java.util.Map.Entry<java.lang.Integer, java.lang.Integer> element : map.entrySet()) {
    mapWriter.startEntry();
    mapWriter.key().integer().writeInt((Integer) element.getKey());
    mapWriter.value().integer().writeInt((Integer) element.getValue());
    mapWriter.endEntry();
}
mapWriter.endMap();

It use UnionMapWriter and generate schema like:
EXPR$0: Map(false)<$data$: Union(Sparse, [1, 39])<struct: Struct<key: Int(32, true) not null, value: Int(32, true) not null> not null, map: Map(false)<entries: Struct<key: Int(32, true) not null, value: Int(32, true)> not null>>>
But in OutputDerivation impl class where i should create output Complete type

List<Field> children = Arrays.asList( CompleteType.INT.toField("key", false), CompleteType.INT.toField("value", false));
return new CompleteType(CompleteType.MAP.getType(), CompleteType.struct(children).toField(MapVector.DATA_VECTOR_NAME, false));

(This is only one valid case, because MapVector.initializeChildrenFromFields())
return
EXPR$0::map<key::int32, value::int32> I found a place where it start using union - PromotableWriter.promoteToUnion.
And in the end i have

SCHEMA_CHANGE ERROR: Schema changed during projection. Schema was 
schema(EXPR$0::map<key::int32, value::int32>)
 but then changed to 
schema(EXPR$0::map<struct::struct<key::int32, value::int32>, map::map<key::int32, value::int32>>)

[github-actions]

Thanks for opening a pull request!

If this is not a minor PR. Could you open an issue for this pull request on GitHub? https://github.com/apache/arrow/issues/new/choose

Opening GitHub issues ahead of time contributes to the Openness of the Apache Arrow project.

Then could you also rename the pull request title in the following format?

GH-${GITHUB_ISSUE_ID}: [${COMPONENT}] ${SUMMARY}

or

MINOR: [${COMPONENT}] ${SUMMARY}

In the case of PARQUET issues on JIRA the title also supports:

PARQUET-${JIRA_ISSUE_ID}: [${COMPONENT}] ${SUMMARY}

See also:

• Other pull requests
• Contribution Guidelines - How to contribute patches