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.

<code class="language-sql">CREATE TABLE tracks (id serial, artists jsonb);
CREATE INDEX tracks_artists_gin_idx ON tracks USING gin (artists);</code>

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

<code class="language-sql">SELECT * FROM tracks WHERE artists @> '["The Dirty Heads"]';</code>

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

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

<code class="language-sql">CREATE INDEX tracks_artists_gin_idx ON tracks USING gin (artists jsonb_path_ops);</code>

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.

<code class="language-sql">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);</code>

Query example:

<code class="language-sql">SELECT * FROM tracks WHERE artistnames ? 'The Dirty Heads';
SELECT * FROM tracks WHERE artistnames @> '"The Dirty Heads"'::jsonb;</code>

Handling json in PostgreSQL 9.3

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

Create function:

<code class="language-sql">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)';</code>

Then create the function index:

<code class="language-sql">CREATE INDEX tracks_artists_gin_idx ON tracks USING gin (json2arr(artists, 'name'));</code>

Use a query that matches the index expression:

<code class="language-sql">SELECT * FROM tracks WHERE '{"The Dirty Heads"}'::text[]</code>

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

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