Expose Arrow `Struct` columns as JDBC `STRUCT` metadata in the Flight SQL JDBC driver.

从 flight/flight-sql-jdbc-core/src/main/java/org/apache/arrow/driver/jdbc/utils/SqlTypes.java 开始，该文件将 ArrowType.Struct 映射到 Types.JAVA_OBJECT，并检查相关的 ArrowFlightJdbcStructVectorAccessor.java 实现。更新元数据映射，使 Struct 列报告 JDBC STRUCT，并验证 ResultSetMetaData 对这些列返回 STRUCT 和 "STRUCT"。

Describe the enhancement requested

Please expose Arrow Struct columns as JDBC STRUCT metadata in the Flight SQL JDBC driver.

Today, Arrow Struct is mapped to java.sql.Types.JAVA_OBJECT, which makes
ResultSetMetaData report:

• getColumnType(...) == Types.JAVA_OBJECT
• getColumnTypeName(...) == "JAVA_OBJECT"

This hides the fact that the column is structurally a JDBC STRUCT, even though the driver
already has struct-specific accessor support.

Why this matters

JDBC clients often rely on ResultSetMetaData to understand result schemas before reading
rows. When STRUCT columns are reported as JAVA_OBJECT, clients cannot distinguish:

• struct/object-like columns
• generic opaque objects

This makes schema inspection, BI tooling, ORM adapters, and generic JDBC consumers harder.

Current upstream evidence

In
flight/flight-sql-jdbc-core/src/main/java/org/apache/arrow/driver/jdbc/utils/SqlTypes.java:

• ArrowType.Struct maps to Types.JAVA_OBJECT

In flight/flight-sql-jdbc-core/src/main/java/org/apache/arrow/driver/jdbc/accessor/impl/comple x/ArrowFlightJdbcStructVectorAccessor.java:

• getStruct() is already implemented
• but getObjectClass() returns Map.class
• getObject() returns a Map

So the data path knows about structs, but metadata does not expose them as JDBC STRUCT.

Expected behavior

For Arrow Struct columns:

• ResultSetMetaData.getColumnType(...) should return java.sql.Types.STRUCT
• ResultSetMetaData.getColumnTypeName(...) should return STRUCT

Optionally, getObject() / object-class behavior could also be aligned with JDBC Struct, but
the most important gap is metadata.

Actual behavior

Arrow Struct columns are exposed as:

• Types.JAVA_OBJECT
• "JAVA_OBJECT"

Version

Observed with:

• Arrow Flight SQL JDBC driver 18.2.0

Confirmed in current upstream code and latest published artifact line:

• 19.0.0