postgresql - 索引已创建，但我得到了序列扫描

Question

PostgreSQL 9.5.7

我已经为用户名创建了一个索引。

现在我这样做了:

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

然后我得到:

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:为什么是Seq Scan？它应该是索引扫描。你能帮我踢一下吗？

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

有好的Postgres Wiki page about Index vs Seq scan usage对此进行解释。

在您的情况下，只是没有足够的数据来利用索引扫描的任何好处。