This pull request primarily focuses on enhancing the handling of null values in the query engine and transformation logic, and includes several new test cases to validate these changes. The most important changes include new methods for detecting null values in comparisons and calculations, additional imports, and modifications to the transformation logic to handle null values correctly.

Enhancements for handling null values:

• iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/plan/relational/metadata/TableMetadataImpl.java: Added methods isCompareWithNull and isCalculateWithNull to handle null values in comparisons and calculations. [1] [2]
• iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/plan/relational/metadata/TableMetadataImpl.java: Modified getOperatorReturnType to return appropriate types when null values are involved.
• iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/plan/relational/metadata/TableMetadataImpl.java: Added isUnknownType method to identify null values.

Additional imports:

• iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/plan/relational/metadata/TableMetadataImpl.java: Added imports for EQUAL, LESS_THAN, LESS_THAN_OR_EQUAL, and UNKNOWN types. [1] [2]

Transformation logic adjustments:

• iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/transformation/dag/column/unary/scalar/AbstractCastFunctionColumnTransformer.java: Updated transformation methods to handle columns with null values properly. [1] [2] [3]

New test cases:

• integration-test/src/test/java/org/apache/iotdb/relational/it/query/old/IoTDBFilterTableIT.java: Added test cases testCompareWithNull and testCalculateWithNull to verify the new null handling logic.