A question on GIN indexes and arrays

[Wells Oliver <wells.oliver@xxxxxxxxx>]

Hey guys, I hope you can help. I am trying to use a GIN Index on an array column and, weirdly, on the exact same server (9.6.3) it uses the index on one database, and not on the other. Here's the test query I am using:

CREATE TEMPORARY TABLE objs (obj_id integer PRIMARY KEY);
CREATE TEMPORARY TABLE sets (obj_id integer[], somecount smallint);

INSERT INTO objs SELECT generate_series(0,1000000);
INSERT INTO sets SELECT ARRAY[p1.obj_id, p2.obj_id,p3.obj_id], generate_series(0,100)
FROM objs as p1
CROSS JOIN objs AS p2
CROSS JOIN objs AS p3
WHERE p2.obj_id = p1.obj_id + 1 AND p3.obj_id = p2.obj_id + 1;


CREATE INDEX ON sets USING GIN(obj_id);
SET enable_seqscan = off;
-- SELECT * FROM sets WHERE obj_id @> ARRAY[2,3]::integer[];
EXPLAIN ANALYZE SELECT * FROM sets WHERE obj_id @> ARRAY[2,3]::integer[];

On one database (same server!) I get the correct output:

 Bitmap Heap Scan on sets  (cost=1026.14..2086.17 rows=276 width=34) (actual time=6.991..7.028 rows=202 loops=1)
   Recheck Cond: (obj_id @> '{2,3}'::integer[])
   Heap Blocks: exact=3
   ->  Bitmap Index Scan on sets_obj_id_idx  (cost=0.00..1026.07 rows=276 width=0) (actual time=6.974..6.974 rows=202 loops=1)
         Index Cond: (obj_id @> '{2,3}'::integer[])
 Planning time: 11.076 ms
 Execution time: 7.084 ms

But on another DB (same! server!)

 Seq Scan on sets  (cost=10000000000.00..10002104164.70 rows=101000 width=34) (actual time=0.531..30015.853 rows=202 loops=1)

   Filter: (obj_id @> '{2,3}'::integer[])

   Rows Removed by Filter: 100999697

 Planning time: 0.206 ms

 Execution time: 30015.883 ms

Why is this happening and what can I do to get my GIN indexes working? Thanks!

--

Wells Oliver
wells.oliver@xxxxxxxxx