presto array

Daniel Parker

2 min read

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26-02-2025

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Presto, the distributed SQL query engine, offers a powerful and flexible data structure known as the array. Understanding how to effectively use Presto arrays can significantly enhance your data manipulation and analysis capabilities. This article provides a comprehensive guide to Presto arrays, covering their creation, manipulation, and application in various scenarios. We'll explore different functions and demonstrate practical examples to solidify your understanding.

Creating Presto Arrays

Presto arrays are created using square brackets [], enclosing a comma-separated list of elements. These elements can be of the same or different data types, although using consistent data types is generally recommended for better readability and efficiency.

Example:

SELECT ARRAY[1, 2, 3, 4, 5] AS my_array; -- Array of integers
SELECT ARRAY['apple', 'banana', 'cherry'] AS fruit_array; -- Array of strings
SELECT ARRAY[1, 'two', 3.14] AS mixed_array; -- Array with mixed data types (less ideal)

You can also create arrays from existing data using array constructor functions like array_agg(). This is particularly useful for aggregating data from multiple rows into a single array.

Example using array_agg():

SELECT order_id, array_agg(product_name) AS products
FROM order_items
GROUP BY order_id;

Manipulating Presto Arrays

Presto provides a rich set of functions for manipulating arrays. These functions allow you to perform various operations like adding, removing, accessing, and filtering elements. Let's explore some key functions:

Accessing Array Elements

The element_at(array, index) function retrieves a specific element from an array using its index (starting from 1).

Example:

SELECT element_at(ARRAY[10, 20, 30], 2) AS second_element; -- Returns 20

Array Length

The cardinality(array) function returns the number of elements in an array.

Example:

SELECT cardinality(ARRAY[1, 2, 3, 4, 5]) AS array_length; -- Returns 5

Array Concatenation

The array_concat(array1, array2) function concatenates two arrays.

Example:

SELECT array_concat(ARRAY[1, 2], ARRAY[3, 4]) AS concatenated_array; -- Returns [1, 2, 3, 4]

Filtering Arrays

The array_filter(array, function) function filters an array based on a given condition. The function should take a single element as input and return a boolean value.

Example:

SELECT array_filter(ARRAY[1, 2, 3, 4, 5], x -> x > 2) AS filtered_array; -- Returns [3, 4, 5]

Other Useful Array Functions

Presto offers numerous other array functions, including:

• array_distinct(array): Removes duplicate elements.
• array_sort(array): Sorts elements in ascending order.
• array_intersect(array1, array2): Returns the intersection of two arrays.
• array_union(array1, array2): Returns the union of two arrays.
• contains(array, element): Checks if an array contains a specific element.

Practical Applications of Presto Arrays

Presto arrays are invaluable in various data processing tasks:

• Aggregating related data: Combine multiple values associated with a single key into a single array. For example, aggregating product IDs associated with a customer.
• Handling lists and collections: Representing data naturally structured as lists or collections, like tags associated with a post.
• Working with semi-structured data: Processing JSON data where values are represented as arrays.
• Improving query efficiency: In some cases, using arrays can reduce the number of joins needed, leading to performance improvements.

Conclusion

Presto arrays provide a flexible and powerful mechanism for managing and manipulating collections of data within Presto queries. Mastering these array functions significantly expands your analytical capabilities and enables you to write more concise and efficient queries. By understanding their creation, manipulation, and practical applications, you can leverage the full potential of Presto's array functionality in your data analysis workflows. Remember to consult the official Presto documentation for a complete list of array functions and detailed explanations.