Postgresql unnest() with element number

Generating Subscripts: Your Guiding Compass

PostgreSQL's generate_subscripts() function turns on your navigation lights, providing ordinal numbers for each element in your array:

SELECT array_elements, subscript
-- It's like we're searching for pirate treasure here, matey.
FROM
  (SELECT unnest(array_elements) as array_elements FROM some_table) AS sub,
  generate_subscripts((SELECT array_agg(array_elements) FROM some_table), 1) AS subscript;


Like a map to a pirate's hidden treasure, generate_subscripts() uncovers the set of subscript numbers (indices) that mark the spot of each element's position in your array.

JOINing the Lateral and Window Functions Party

When simple unnest() seems like the party pooper during complex data structures or multiple arrays gatherings, it's time to invite some advanced SQL party crashers.

Advanced UNNEST with Lateral JOIN and Window Functions

This is when the LATERAL JOIN combined with WITH ORDINALITY become the life of the PostgreSQL fiesta:

SELECT items.id, a.elem, a.nr
FROM items
LEFT JOIN LATERAL unnest(items.array_column) WITH ORDINALITY AS a(elem, nr) ON true;
-- Everyone's invited to this SQL party! Even Lefty, our trusty LEFT JOIN.


Here, LATERAL works diligently, extracting each record, concurrently unnesting the array and maintaining the thoughful original invitation list order with indices.

Performance Tips: Handling Window Functions like a DJ

While window functions like ROW_NUMBER() can be the life of the party, remember to handle them responsibly. If your party doesn't need a specific dress code (aka PARTITION BY), just dress down, and simplify your query to:

SELECT id, array_elements, ROW_NUMBER() OVER ()
FROM some_table, unnest(array_column) as array_elements;
-- Who needs a fancy partition when a simple overcoat (= OVER()) is enough?


Doing so ensures each row in your result set gets a unique party badge starting from 1, without dividing your guests into unwanted groups.

Handling CSVs and Advanced Cases: PhD Level

Before you bravely plunge into unnesting, your comma-separated values might require a pre-transformation into arrays.

CSV -> Array: From Ugly Duckling to Beautiful Swan

If you're dealing with a CSV string, string_to_array() is your magical transformation spell:

SELECT string_to_array(csv_column, ',')
FROM csv_data;
-- Before: Ugly CSV Duckling. After: Beautiful Array Swan.


Once the transformation is complete, your new array is ready for the unnest techniques you've mastered.

Nested Queries & Correlated Subqueries: Inception Mode

Should your retrieval operation require an 'Inception' level of complexity, nested queries or correlated subqueries could be your dream within a dream, especially if WITH ORDINALITY doesn't support your PostgreSQL version.

Advanced Scenarios: Going Beyond the Books

De-pivoting Datasets: Introducing the Cirque du SQL

Deploy multiple UNNEST() operations with WITH ORDINALITY during versatile Cirque du SQL performances to de-pivot a wide dataset into a long and nimble format.

Enhancing Readability with CTEs: A Clear Crystal Ball

As your queries morph into complex beasties, Common Table Expressions (CTEs) become your magical crystal ball, enhancing their clarity and structure.

Defining Clear functions: A Well-scripted Formula

Engulf your unnest logic within boldly defined PostgreSQL functions. Using explict column aliases and qualified columns paves the way towards clear and self-documenting SQL.