PostgreSQL JSON Path

Summary: in this tutorial, you will learn about the PostgreSQL JSON path and how to use it to locate an element within a JSON document.

Introduction to PostgreSQL JSON Path

JSON path is a query language that allows you to locate specific values or elements within a JSON document.

JSON path offers a way to match elements within JSON data, similar to how XPATH is used for XML documents.

To construct JSON path expressions, you can use the combination of the following operators:

• $ – Represent the root element to query.
• .key – Use a dot (.) followed by a key name (.key) to access a field of a JSON object or (.*) to access all properties of a JSON object.
• [n] – Use square brackets ([]) to access an array element by its index (n), or [*] to access all array elements.
• @ – Represent the current node being processed by a filter predicate.
• [start: end] – Array slice operator.
• [?(expression)] – Filter expression that evaluates to a boolean value.

To extract specific elements from a JSON path jsonb_path_query() function:

jsonb_path_query(jsonb_data, json_path)

The function returns all JSON items for the jsonb_data based on a JSON path.

PostgreSQL JSON Path examples

Let’s take some examples of using JSON paths.

Setting up a sample table

First, create a table called person that includes a JSONB column:

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

Second, insert a new row into the person table:

INSERT INTO person (info)
VALUES
    ('{"name": "John", "age": 30, "city": "New York", "pets": [{"name": "Max", "species": "Dog"}, {"name": "Whiskers", "species": "Cat"}]}')
RETURNING *;

Output:

id |                                                                 info
----+--------------------------------------------------------------------------------------------------------------------------------------
  1 | {"age": 30, "city": "New York", "name": "John", "pets": [{"name": "Max", "species": "Dog"}, {"name": "Whiskers", "species": "Cat"}]}
(1 row)

The JSON data in the info column of the person table looks like the following:

{
  "name": "John",
  "age": 30,
  "city": "New York",
  "pets": [
    {"name": "Max", "species": "Dog"},
    {"name": "Whiskers", "species": "Cat"}
  ]
}

1) Extracting the name of the person

The following statement uses the jsonb_path_query() to extract the name of the person:

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

Output:

jsonb_path_query
------------------
 "John"
(1 row)

In this example, we use the $.name path to access the value of the name property of the top-level JSON object.

2) Extracting all values of a JSON object

The following example uses the jsonb_path_query() function to retrieve all values of the JSON object in the info column:

SELECT
  jsonb_path_query(info, '$.*')
FROM
  person;

Output:

jsonb_path_query
-----------------------------------------------------------------------------
 30
 "New York"
 "John"
 [{"name": "Max", "species": "Dog"}, {"name": "Whiskers", "species": "Cat"}]
(4 rows)

In this example, the $.* path locates the values of all properties of the top-level JSON object. Therefore, the jsonb_path_query returns the values of all properties of the JSON object.

3) Extracting array elements

The following example uses the jsonb_path_query() function to get the name of the first pet:

SELECT
  jsonb_path_query(info, '$.pets[0].name')
FROM
  person;

Output:

jsonb_path_query
------------------
 "Max"
(1 row)

In this example, we use the JSON path $.pets[0].name to locate the name of the first pet:

• $: represents the top-level JSON object.
• $.pets locates the values of the property with the name pets, which is a JSON array.
• $.pets[0] returns the first element of the $.pets array, which is a JSON object.
• $.pets[0].name returns the value of the property name of the $.pets[0] object.

The following example uses the JSON path $.pets[*].name to return all pet names of a person object:

SELECT
  jsonb_path_query(info, '$.pets[*].name')
FROM
  person;

Output:

jsonb_path_query
------------------
 "Max"
 "Whiskers"
(2 rows)

The wildcard * means all elements.

4) Filter JSON

The following example uses a filter expression to find the pet whose species is cat:

SELECT
  jsonb_path_query(
    info, '$.pets[*] ? (@.species == "Cat")'
  ) AS cat
FROM
  person;

Output:

cat
----------------------------------------
 {"name": "Whiskers", "species": "Cat"}

Here’s the break-down of the JSON path expression $.pets[*] ? (@.species == "Cat"):

• $.pets[*]: selects all elements (*) within the “pets” array. The $. denotes the root of the JSON document and pets[*] represents all array elements of the pets array.
• ? (@.species == "Cat"): filters the selected elements from the pets array. The ? is used to apply the filter condition (@.species == "Cat"), which checks if the value of the species key in each selected element is equal to Cat.

In short, the JSON path $.pets[*] ? (@.species == "Cat") matches all objects within the pets array where the value of the species key is Cat.

JSON path mode

PostgreSQL allows you to optionally specify a path mode at the beginning of the JSON path expression:

'mode json_path'

The mode can be lax or strict:

• In lax mode, the function returns an empty value (result set) if the JSON path expression has an error. For example, if you use the $.email path for the JSON document that doesn’t contain the email key, the function returns an empty result set.
• In strict mode, the function issues an error if the path expression contains an error.

The default is lax mode.

The following statement attempts to extract the email from the JSON data in the info column of the person table:

SELECT
  jsonb_path_query(info, '$.email')
FROM
  person;

Output:

jsonb_path_query
------------------
(0 rows)

It returns no row because the JSON object does not have the email key.

The following statement extracts the email key but uses the strict mode for the JSON path:

SELECT
  jsonb_path_query(info, 'strict $.email')
FROM
  person;

Output:

ERROR:  JSON object does not contain key "email"

SQL state: 2203A

The output shows that the function raises an error.

Summary

• Use JSON paths to locate specific values or elements within a JSON document.
• Use the jsonb_path_query() function to return all items within a JSON document that match a specified JSON path.