BUG #13756: jsonb_path_ops gin index produce empty result on nested array

The following bug has been logged on the website:

Bug reference: 13756
Logged by: Daniel Cheng
Email address: sdiz(at)sdiz(dot)net
PostgreSQL version: 9.4.5
Operating system: Debian (testing)
Description:

Table with jsonb_path_ops produce wrong result when using the following
query.
Same query produce different result depends on which query plan is used.

db=> select version();
version

--------------------------------------------------------------------------------------------------------
PostgreSQL 9.4.5 on x86_64-unknown-linux-gnu, compiled by gcc (Debian
5.2.1-21) 5.2.1 20151003, 64-bit
(1 row)

db=>
db=> create table t ( j jsonb);
CREATE TABLE
db=> insert into t (j) values ('{"a":[ ["b",{"x":1}], ["b",{"x":2}]]}');
INSERT 0 1
db=> create index on t USING gin(j jsonb_path_ops) ;
CREATE INDEX
db=> select * from t where j @> '{"a":[[{"x":2}]]}'::jsonb;
j
-------------------------------------------
{"a": [["b", {"x": 1}], ["b", {"x": 2}]]}
(1 row)

db=> SET enable_seqscan = OFF;
SET
db=> select * from t where j @> '{"a":[[{"x":2}]]}'::jsonb;
j
---
(0 rows)
db=> explain analyse select * from t where j @> '{"a":[[{"x":2}]]}'::jsonb;
QUERY PLAN

-----------------------------------------------------------------------------------------------------------------
Bitmap Heap Scan on t (cost=8.00..12.01 rows=1 width=32) (actual
time=0.010..0.010 rows=0 loops=1)
Recheck Cond: (j @> '{"a": [[{"x": 2}]]}'::jsonb)
-> Bitmap Index Scan on t_j_idx1 (cost=0.00..8.00 rows=1 width=0)
(actual time=0.009..0.009 rows=0 loops=1)
Index Cond: (j @> '{"a": [[{"x": 2}]]}'::jsonb)
Planning time: 0.059 ms
Execution time: 0.032 ms
(6 rows)

db=> drop table t;
DROP TABLE

Regards,
Daniel