How do I query using fields inside the new PostgreSQL JSON datatype?

222
votes

I am looking for some docs and/or examples for the new JSON functions in PostgreSQL 9.2.

Specifically, given a series of JSON records:

[
  {name: "Toby", occupation: "Software Engineer"},
  {name: "Zaphod", occupation: "Galactic President"}
]

How would I write the SQL to find a record by name?

In vanilla SQL:

SELECT * from json_data WHERE "name" = "Toby"

The official dev manual is quite sparse:

Update I

I've put together a gist detailing what is currently possible with PostgreSQL 9.2. Using some custom functions, it is possible to do things like:

SELECT id, json_string(data,'name') FROM things
WHERE json_string(data,'name') LIKE 'G%';

Update II

I've now moved my JSON functions into their own project:

PostSQL - a set of functions for transforming PostgreSQL and PL/v8 into a totally awesome JSON document store

3
sqljsonpostgresqlpostgresql-9.2postgresql-9.3
Just recently I found this blog post by Matt Schinckel, which explains in detail querying JSON in PostgreSQL schinckel.net/2014/05/25/querying-json-in-postgres – knowbody
@knowbody This post is actually about querying JSONB, which is quite distinct from JSON. My bad for not making that clearer in the post. – Matthew Schinckel

3 Answers

184
votes

Postgres 9.2

I quote Andrew Dunstan on the pgsql-hackers list:

At some stage there will possibly be some json-processing (as opposed to json-producing) functions, but not in 9.2.

Doesn't prevent him from providing an example implementation in PLV8 that should solve your problem. (Link is dead now, see modern PLV8 instead.)

Postgres 9.3

Offers an arsenal of new functions and operators to add "json-processing".

The answer to the original question in Postgres 9.3:

SELECT *
FROM   json_array_elements(
  '[{"name": "Toby", "occupation": "Software Engineer"},
    {"name": "Zaphod", "occupation": "Galactic President"} ]'
  ) AS elem
WHERE elem->>'name' = 'Toby';

Advanced example:

For bigger tables you may want to add an expression index to increase performance:

Postgres 9.4

Adds jsonb (b for "binary", values are stored as native Postgres types) and yet more functionality for both types. In addition to expression indexes mentioned above, jsonb also supports GIN, btree and hash indexes, GIN being the most potent of these.

The manual goes as far as suggesting:

In general, most applications should prefer to store JSON data as jsonb, unless there are quite specialized needs, such as legacy assumptions about ordering of object keys.

Bold emphasis mine.

Performance benefits from general improvements to GIN indexes.

Postgres 9.5

Complete jsonb functions and operators. Add more functions to manipulate jsonb in place and for display.

90
votes

With Postgres 9.3+, just use the -> operator. For example,

SELECT data->'images'->'thumbnail'->'url' AS thumb FROM instagram;

see http://clarkdave.net/2013/06/what-can-you-do-with-postgresql-and-json/ for some nice examples and a tutorial.

19
votes

With postgres 9.3 use -> for object access. 4 example

seed.rb

se = SmartElement.new
se.data = 
{
    params:
    [
        {
            type: 1,
            code: 1,
            value: 2012,
            description: 'year of producction'
        },
        {
            type: 1,
            code: 2,
            value: 30,
            description: 'length'
        }
    ]
}

se.save

rails c

SELECT data->'params'->0 as data FROM smart_elements;

returns

                                 data
----------------------------------------------------------------------
 {"type":1,"code":1,"value":2012,"description":"year of producction"}
(1 row)

You can continue nesting 

SELECT data->'params'->0->'type' as data FROM smart_elements;

return

 data
------
 1
(1 row)