Understanding Union Types in Hive: A Detailed Guide

What is UNION Type?

UNION type in Hive is a complex data type that represents a value that can be one of several types. It is similar to the concept of a union in C/C++ and can be seen as an OR operation between types. Each UNION type can contain many types, but only one of them can hold a value at any given time.

Defining a UNION Type

Let's consider an example of how to create a table with a UNION type:

codeCREATE TABLE employee ( 
    id INT, 
    name STRING, 
    salary DOUBLE, 
    additional_info UNIONTYPE<STRING, INT, DOUBLE>); 

In this example, the additional_info column can hold a string, integer, or double value. This can be useful in scenarios where a column may contain different types of data.

Inserting Data into a UNION Type

To insert data into a UNION type column, you can use the CREATE_UNION function. Here's an example:

codeINSERT INTO employee VALUES (1, 'John Doe', 50000, CREATE_UNION(0, 'Senior Developer')); 

In this example, the CREATE_UNION function is used to create a UNION type value. The first argument to CREATE_UNION is the tag that represents the type of the value. The tag is a 0-based index into the list of types in the UNION type. So in this case, 0 corresponds to STRING , as STRING is the first type in the UNION type.

Retrieving Data from a UNION Type

To retrieve data from a UNION type column, you can use the get_ functions. Here's an example:

codeSELECT id, name, salary, get_string(additional_info) FROM employee; 

In this example, the get_string function is used to retrieve the STRING value from the additional_info column.

Limitations and Considerations

While UNION types can be helpful, they are not commonly used due to their complexity and the fact that they don't map well to common data analysis tasks. It's often better to structure your data in a way that avoids the need for UNION types. However, they can be beneficial in some scenarios, especially when dealing with complex, heterogeneous data sources.

Also, be aware that UNION types do not play well with many Hive features. For example, you can't use UNION types with the column statistics feature, and they're not supported in Hive's vectorized execution engine.

Conclusion

While UNION types are a less commonly used feature of Hive, understanding them can give you another tool in your data analysis toolkit. It's always a good idea to have an understanding of all the data types available to you, as you never know when they might come in handy. With careful consideration of their limitations, UNION types can provide flexible data representation in specific use cases.