How to Efficiently Index JSON Arrays in PostgreSQL for Faster Element Retrieval?

Efficiently index JSON arrays in PostgreSQL for faster element retrieval

Quickly identifying and retrieving elements within a JSON array in a PostgreSQL table can be challenging. For optimal performance, it's important to understand the indexing technology that's appropriate for your specific data and query requirements.

Using JSONB in ​​PostgreSQL 9.4

PostgreSQL 9.4 introduces the binary JSON data type jsonb, which greatly enhances indexing possibilities. Use jsonb to create GIN indexes directly on JSON arrays.

CREATE TABLE tracks (id serial, artists jsonb);
CREATE INDEX tracks_artists_gin_idx ON tracks USING gin (artists);

This enables queries that can use GIN indexes, for example:

SELECT * FROM tracks WHERE artists @> '["The Dirty Heads"]';

where @> represents the "contains" operator of jsonb.

Alternatively, you can use the jsonb_path_ops GIN operator class (non-default, usually smaller and faster):

CREATE INDEX tracks_artists_gin_idx ON tracks USING gin (artists jsonb_path_ops);

Optimize for unique name values

If your artists column only contains an array of uniquely named values, it is more efficient to store the values ​​as JSON text primitives, rather than objects. This eliminates the need for redundant keys.

CREATE TABLE tracks (id serial, artistnames jsonb);
INSERT INTO tracks VALUES (2, '["The Dirty Heads", "Louis Richards"]');
CREATE INDEX tracks_artistnames_gin_idx ON tracks USING gin (artistnames);

Query example:

SELECT * FROM tracks WHERE artistnames ? 'The Dirty Heads';
SELECT * FROM tracks WHERE artistnames @> '"The Dirty Heads"'::jsonb;

Handling json in PostgreSQL 9.3

For PostgreSQL versions prior to 9.4, invariant functions and GIN indexes can be used.

Create function:

CREATE OR REPLACE FUNCTION json2arr(_j json, _key text)
RETURNS text[] LANGUAGE sql IMMUTABLE AS
'SELECT ARRAY(SELECT elem->>_key FROM json_array_elements(_j) elem)';

Then create the function index:

CREATE INDEX tracks_artists_gin_idx ON tracks USING gin (json2arr(artists, 'name'));

Use a query that matches the index expression:

SELECT * FROM tracks WHERE '{"The Dirty Heads"}'::text[]

Always ensure that the functions used in the index are immutable to enable function indexing. Key operators...

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