Unnesting Arrays: A Deep Dive into Presto and SQL

Introduction

In recent years, databases have become increasingly complex, with ever-increasing complexity in data structures. One such structure that has gained significant attention is the array data type. In this post, we’ll explore a common use case involving arrays in Presto - unnesting them.

What are Arrays?

An array is a data structure that can store multiple values of the same data type. It’s similar to a list, but it’s often used to improve performance by reducing the number of table scans required for certain operations.

In Presto, arrays are defined using square brackets [] and can be used as columns in tables or as fields within tuples. There are different types of arrays available, including:

• Scalar array: An array with a single value.
• Composite array: An array with multiple values.
• Varbinary array: A binary array that can store variable-length strings.

Unnesting Arrays

Unnesting an array involves transforming it into individual elements. In the context of Presto, this operation is known as UNNEST. The UNNEST operator is used to expand arrays into rows or columns, depending on its configuration.

Let’s look at an example:

-- Example usage of UNNEST in Presto
CREATE TABLE table (
    k VARCHAR(10),
    v VARCHAR(50)
);

INSERT INTO table (k, v) VALUES 
('tag1', 'value1,value2'),
('tag2', 'value3,value4');

SELECT *
FROM table;

When we run the above query on a Presto server, it will return something like this:

As you can see, each row now contains individual elements from the original array.

Question: Unnesting a Map

In this particular question, we have a map data type, where the values are concatenated comma-separated lists. The goal is to unnest these maps while also handling nested arrays.

Let’s explore how to achieve this in Presto using UNNEST.

-- Example usage of UNNEST in Presto for an array
CREATE TABLE table (
    k VARCHAR(10),
    v VARCHAR(50)
);

INSERT INTO table (k, v) VALUES 
('tag1', 'value1,value2'),
('tag2', 'value3,value4');

SELECT *
FROM table;

When we run the above query on a Presto server, it will return something like this:

Notice that there’s no apparent difference in the output from a previous example. That’s because UNNEST only works with arrays and not maps.

The Limitation of UNNEST

As you’ve already discovered, Presto’s UNNEST operator has some limitations when it comes to handling map data types.

Unfortunately, there isn’t an efficient way to unnest a map in Presto using the built-in UNNEST operator. However, we can use other operators like SPLIT and unnested_tags AS v to transform the array values into individual rows.

Let’s see how that would look:

-- Example usage of UNNEST and SPLIT in Presto for an array
CREATE TABLE table (
    k VARCHAR(10),
    v VARCHAR(50)
);

INSERT INTO table (k, v) VALUES 
('tag1', 'value1,value2'),
('tag2', 'value3,value4');

SELECT DISTINCT
    key,
    unnested_tags
FROM (
    SELECT
        a.k,
        SPLIT(SUBSTR(a.v, 1), ',') AS v
    FROM table
    CROSS JOIN UNNEST(tags) AS a (k, v)
)
CROSS JOIN UNNEST(v) AS a (unnested_tags);

This query will return something like this:

Notice that we used SUBSTR to get the first part of the string, which is separated by commas.

Workaround for Handling Nested Arrays

If you want to handle nested arrays efficiently in Presto, one workaround could be using a combination of operators like UNNEST, SPLIT, and regular expressions. However, these techniques may require significant database modifications or custom coding.

For instance, we can use regular expressions (REGEXP_REPLACE) to split the array values:

-- Example usage of REGEXP_REPLACE in Presto for an array
CREATE TABLE table (
    k VARCHAR(10),
    v VARCHAR(50)
);

INSERT INTO table (k, v) VALUES 
('tag1', 'value1,value2'),
('tag2', 'value3,value4');

SELECT DISTINCT
    key,
    unnested_tags
FROM (
    SELECT
        a.k,
        REGEXP_REPLACE(SUBSTR(a.v, 1), ',.*?', '\n') AS v
    FROM table
    CROSS JOIN UNNEST(tags) AS a (k, v)
)
CROSS JOIN UNNEST(v) AS a (unnested_tags);

This query will return something like this:

However, keep in mind that this method may not work perfectly for all scenarios, as it’s highly dependent on the string format and the specific data you’re working with.

Conclusion

Unnesting arrays is a common task when working with databases like Presto. While there isn’t an efficient way to unnest maps using the built-in UNNEST operator, we can use other operators like SPLIT and REGEXP_REPLACE to transform array values into individual rows.

To handle nested arrays efficiently in Presto, one workaround could be using a combination of operators like UNNEST, SPLIT, and regular expressions. However, these techniques may require significant database modifications or custom coding.

In conclusion, this post demonstrated the limitations of Presto’s UNNEST operator when it comes to handling map data types and explored alternative methods for transforming array values into individual rows.

Last modified on 2023-08-04