索引已创建但我获得了Seq Scan

Question

PostgreSQL 9.5.7

我为用户名创建了一个索引。

现在我做了：

explain analyze select * from authentification_user where username='Pele';

我得到了：

QUERY PLAN                                                    
-----------------------------------------------------------------------------------------------------------------
 Seq Scan on authentification_user  (cost=0.00..1.25 rows=1 width=478) (actual time=0.017..0.019 rows=1 loops=1)
   Filter: ((username)::text = 'Pele'::text)
   Rows Removed by Filter: 19
 Planning time: 0.101 ms
 Execution time: 0.040 ms
(5 rows)

问题是：为什么它是Seq Scan？它应该是一个索引扫描。你能帮我一把吗？

simple_project=# \d authentification_user
                                  Table "public.authentification_user"
  Column  |          Type          |                             Modifiers                              
----------+------------------------+--------------------------------------------------------------------
 id       | integer                | not null default nextval('authentification_user_id_seq'::regclass)
 username | character varying(10)  | not null
 pw_hash  | character varying(100) | not null
 email    | character varying(100) | not null
Indexes:
    "authentification_user_pkey" PRIMARY KEY, btree (id)
    "authentification_user_username_e9ac7af7" btree (username)
    "authentification_user_username_e9ac7af7_like" btree (username varchar_pattern_ops)

simple_project=# 

Answer 1

很好Postgres Wiki page about Index vs Seq scan usage解释了这一点。

在您的情况下，根本没有足够的数据来获取索引扫描的任何好处。