该问题是由于SchemaPlus.getSubSchema方法返回null而导致的。出现该问题的原因是Calcite在执行查询时需要从一个或多个模式中检索表。如果SchemaPlus中没有子模式，则会触发该异常。

解决方法是在SchemaPlus中添加一个子模式，以确保在执行查询时不会出现null返回值。以下是一个简单的示例：

public class MySchema extends AbstractSchema {

private final Map tables = new HashMap<>();

public MySchema() {
  super();
}

@Override
protected Map getTableMap() {
  return tables;
}

public void addTable(Table table) {
  tables.put(table.getName(), table);
}

}

// ...

MySchema mySchema = new MySchema(); Table myTable = new CsvTable.Factory().create(mySchema, mapOf("file", "my_table.csv")); mySchema.addTable(myTable);

FrameworkConfig config = Frameworks.newConfigBuilder() .defaultSchema(mySchema).build(); SchemaPlus rootSchema = Frameworks.createRootSchema(true); rootSchema.add("my_schema", mySchema);

WithStatement with = parseQuery("SELECT * FROM my_schema.my_table"); Planner planner = Frameworks.getPlanner(config); RelOptCluster cluster = RelOptCluster.create(planner.getContext(), new RexBuilder()); SqlValidator validator = planner.getValidator(); SqlNode validated = validator.validate(with.getQuery()); SqlToRelConverter converter = planner.getSqlToRelConverter(); RelRoot relRoot = converter.convertQuery(validated, true, false); // ...