Why does this GIN index (jsonb) not used in my case - postgresql

Question

My code is here https://rextester.com/CBYD42261, I use '#>' with gin index to search, but postgresql using Seq Scan.
DROP TABLE IF EXISTS sponsor_projects;
CREATE TABLE sponsor_projects (
project_id BIGSERIAL PRIMARY KEY,
sponsor_id bigint NOT NULL,
status smallint NOT NULL DEFAULT 0,
name character varying(64) NOT NULL,
category character varying(10) NOT NULL,
purpose jsonb NOT NULL,
qrcode character varying(200) NOT NULL,
plaform character varying(10) NOT NULL,
budget double precision NOT NULL,
budget_used double precision NOT NULL,
sticker character varying(10) NOT NULL,
spread character varying(10) NOT NULL,
areas jsonb NOT NULL,
paused boolean DEFAULT false,
terminated boolean DEFAULT false,
start_time timestamp(0) without time zone NOT NULL,
end_time timestamp(0) without time zone,
shops jsonb NOT NULL,
created_by integer,
created_at timestamp(0) without time zone NOT NULL DEFAULT CURRENT_TIMESTAMP,
updated_at timestamp(0) without time zone NOT NULL DEFAULT CURRENT_TIMESTAMP,
deleted boolean DEFAULT false,
deleted_at timestamp(0) without time zone DEFAULT NULL::timestamp without time zone
);
-- Indices -------------------------------------------------------
CREATE INDEX idx_sponsor_projects_sponsor_id ON sponsor_projects(sponsor_id int8_ops);
CREATE INDEX idx_sponsor_projects ON sponsor_projects USING GIN (shops);
-- Insert Data ---------------------------------------------------
INSERT INTO "sponsor_projects"("sponsor_id","status","name","category","purpose","qrcode","plaform","budget","budget_used","sticker","spread","areas","paused","terminated","start_time","end_time","shops","created_by","created_at","updated_at","deleted","deleted_at")
VALUES
(1,0,E'京东广告',E'3C数码产品',E'["品牌宣传", "流量拉新"]',E'https://jd.com',E'不限',1000,0,E'标准桌贴',E'CPC',E'[{"id": "fc5d1d71-14b1-473d-9db2-c804b0d9ab6c", "path": [[116.68767, 39.877689], [116.712303, 39.868862], [116.704149, 39.8601], [116.6887, 39.865041]]}]',FALSE,FALSE,E'2019-08-20 06:51:26',NULL,E'[{"shop_id": 5}]',NULL,E'2019-08-21 14:55:01',E'2019-08-21 14:55:01',FALSE,NULL);
EXPLAIN ANALYZE
SELECT * FROM sponsor_projects t WHERE t.shops #> '[{"shop_id": 5}]';
It shows:
Seq Scan on sponsor_projects t (cost=0.00..11.00 rows=1 width=893) (actual time=0.013..0.016 rows=1 loops=1)
Filter: (shops #> '[{"shop_id": 1}]'::jsonb)
Rows Removed by Filter: 8
Planning Time: 0.076 ms
Execution Time: 0.033 ms
Using Seq Scan, not gin index.
What happend with my code?
Please someone can help me?

Answer 1

That is because the table contains only a single row, and a sequential scan is always faster with small tables like that.
To test if the index can be used, discourage the use of sequential scans in your database session:
SET enable_seqscan = off;
Then PostgreSQL will use the index if possible.
Make sure to reset the setting afterwards:
RESET enable_seqscan;