PostgreSQL jsonb_array_length() Function

Summary: in this tutorial, you will learn how to use the PostgreSQL jsonb_array_length() function to get the number of elements in the top-level JSON array.

Introduction to the PostgreSQL jsonb_array_length() function

The jsonb_array_length() function returns the number of elements in the top-level JSON array.

Here’s the syntax of the jsonb_array_length() function:

jsonb_array_length(json_array)

In this syntax, you pass a JSON array with the type JSONB to the function. It’ll return the number of elements in the array.

If the array is empty, the jsonb_array_length() function returns zero. If the argument is not an array, the function will issue an error. In case the json_array is NULL, the jsonb_array_length() function will return NULL.

Note that the function will return the number of elements of the top-level array only. If the array contains nested arrays, the function will not count the elements in the nested arrays but consider the nested arrays as individual elements.

PostgreSQL jsonb_array_length() function examples

Let’s explore some examples of using the jsonb_array_length() function.

1) Basic PostgreSQL jsonb_array_length() function example

The following example uses the jsonb_array_length() function to get the number of elements in a JSON array:

SELECT jsonb_array_length('[1,2,3]');

Output:

jsonb_array_length
--------------------
                  3
(1 row)

The function returns 3 because the JSON array [1,2,3] contains three elements.

2) Using the jsonb_array_length() function with nested arrays

The following example uses the jsonb_array_length() function with an array that contains another array:

SELECT jsonb_array_length('[1,2,3, [4,5], 6]');

Output:

jsonb_array_length
--------------------
                  5
(1 row)

In this example, the function returns 5 because the top-level array contains 5 elements: 1, 2, 3, an array [4,5], and 6.

3) Using the jsonb_array_length() function with table data

First, create a table called person:

CREATE TABLE person (
    id SERIAL PRIMARY KEY,
    info JSONB
);

In this person table, the info column has the type JSONB that contains the person’s information including name, age, and pets.

Second, insert some rows into the person table:

INSERT INTO person (info)
VALUES
    ('{"name": "Alice", "age": 30, "pets": [{"type": "cat", "name": "Fluffy"}, {"type": "dog", "name": "Buddy"}]}'),
    ('{"name": "Bob", "age": 35, "pets": [{"type": "dog", "name": "Max"}]}'),
    ('{"name": "Charlie", "age": 40, "pets": [{"type": "rabbit", "name": "Snowball"}]}')
RETURNING *;

Third, retrieve the person names with their number of pets from the info column of the person table:

SELECT
  jsonb_path_query(info, '$.name') name,
  jsonb_array_length(
    jsonb_path_query(info, '$.pets')
  ) pet_count
FROM
  person;

Output:

name    | pet_count
-----------+-----------
 "Alice"   |         2
 "Bob"     |         1
 "Charlie" |         1
(3 rows)

In this example:

• The jsonb_path_query(info, ‘$.name’) returns the name of the person.
• The jsonb_path_query(info, ‘$.pets’) returns the pets array, and jsonb_array_length() returns the number of elements in the pets array.

Summary

• Use the jsonb_array_length() function to get the number of elements in the top-level JSON array.