MLab is being acquired by MongoDB(blog.mlab.com)

  -- create a table with data
  CREATE TABLE arrs AS SELECT format('{ "arr": [{ "a": [{ "b": %s, "c": true }] }] }', g.i)::jsonb AS data FROM generate_series(1,1000) g(i);
  -- index, this one only supports "rooted" paths, but you can create one that allows searches not starting from the root too
  CREATE INDEX idx ON arrs USING gin (data jsonb_path_ops);

  -- search
  postgres[22708][1]=# SELECT data->'arr' FROM arrs WHERE data @> '{"arr": [{"a":[{"b":5}]}]}';
  ┌────────────────────────────────┐
  │            ?column?            │
  ├────────────────────────────────┤
  │ [{"a": [{"b": 5, "c": true}]}] │
  └────────────────────────────────┘
  (1 row)

  -- show index usage
  postgres[22708][1]=# EXPLAIN ANALYZE SELECT data->'arr' FROM arrs WHERE data @> '{"arr": [{"a":[{"b":5}]}]}';
  ┌─────────────────────────────────────────────────────────────────────────────────────────────────────────────┐
  │                                                 QUERY PLAN                                                  │
  ├─────────────────────────────────────────────────────────────────────────────────────────────────────────────┤
  │ Bitmap Heap Scan on arrs  (cost=20.01..24.02 rows=1 width=32) (actual time=0.131..0.132 rows=1 loops=1)     │
  │   Recheck Cond: (data @> '{"arr": [{"a": [{"b": 5}]}]}'::jsonb)                                             │
  │   Heap Blocks: exact=1                                                                                      │
  │   ->  Bitmap Index Scan on idx  (cost=0.00..20.01 rows=1 width=0) (actual time=0.107..0.108 rows=1 loops=1) │
  │         Index Cond: (data @> '{"arr": [{"a": [{"b": 5}]}]}'::jsonb)                                         │
  │ Planning Time: 0.107 ms                                                                                     │
  │ Execution Time: 0.186 ms                                                                                    │
  └─────────────────────────────────────────────────────────────────────────────────────────────────────────────┘
  (7 rows)