Postgresql对索引列的查询非常慢

Question

我有50万行的表。名为u_sphinx的一列非常重要，可用值为1,2,3。现在所有行都有值3但是，当我检查新行（u_sphinx = 1）时，查询非常慢。可能有什么不对？也许索引被打破了？服务器：Debian，8GB 4x Intel（R）Xeon（R）CPU E3-1220 V2 @ 3.10GHz

表结构：

base=> \d u_user
Table "public.u_user"
         Column          |       Type        |                       Modifiers                       
 u_ip                    | character varying | 
 u_agent                 | text              | 
 u_agent_js              | text              | 
 u_resolution_id         | integer           | 
 u_os                    | character varying | 
 u_os_id                 | smallint          | 
 u_platform              | character varying | 
 u_language              | character varying | 
 u_language_id           | smallint          | 
 u_language_js           | character varying | 
 u_cookie                | smallint          | 
 u_java                  | smallint          | 
 u_color_depth           | integer           | 
 u_flash                 | character varying | 
 u_charset               | character varying | 
 u_doctype               | character varying | 
 u_compat_mode           | character varying | 
 u_sex                   | character varying | 
 u_age                   | character varying | 
 u_theme                 | character varying | 
 u_behave                | character varying | 
 u_targeting             | character varying | 
 u_resolution            | character varying | 
 u_user_hash             | bigint            | 
 u_tech_hash             | character varying | 
 u_last_target_data_time | integer           | 
 u_last_target_prof_time | integer           | 
 u_id                    | bigint            | not null default nextval('u_user_u_id_seq'::regclass)
 u_sphinx                | smallint          | not null default 1::smallint
Indexes:
    "u_user_u_id_pk" PRIMARY KEY, btree (u_id)
    "u_user_hash_index" btree (u_user_hash)
    "u_user_u_sphinx_ind" btree (u_sphinx)

慢查询：

base=> explain analyze SELECT u_id FROM u_user WHERE u_sphinx = 1 LIMIT 1;
                                                         QUERY PLAN                                                          
-----------------------------------------------------------------------------------------------------------------------------
 Limit  (cost=0.00..0.15 rows=1 width=8) (actual time=485146.252..485146.252 rows=0 loops=1)
   ->  Seq Scan on u_user  (cost=0.00..3023707.80 rows=19848860 width=8) (actual time=485146.249..485146.249 rows=0 loops=1)
         Filter: (u_sphinx = 1)
         Rows Removed by Filter: 23170476
 Total runtime: 485160.241 ms
(5 rows)

解决：

添加部分索引后

base=> explain analyze SELECT u_id FROM u_user WHERE u_sphinx = 1 LIMIT 1;
                                                              QUERY PLAN                                                              
--------------------------------------------------------------------------------------------------------------------------------------
 Limit  (cost=0.27..4.28 rows=1 width=8) (actual time=0.063..0.063 rows=0 loops=1)
   ->  Index Scan using u_user_u_sphinx_index_1 on u_user  (cost=0.27..4.28 rows=1 width=8) (actual time=0.061..0.061 rows=0 loops=1)
         Index Cond: (u_sphinx = 1)
 Total runtime: 0.106 ms

@Kouber Saparev的thx

Answer 1

尝试制作部分索引。

CREATE INDEX u_user_u_sphinx_idx ON u_user (u_sphinx) WHERE u_sphinx = 1;

Answer 2

您的查询计划看起来像数据库正在处理查询，好像1在数据库中如此常见，以至于为了识别相关行而不是添加它会更好地挖掘一两个磁盘页面翻阅索引并在随机磁盘页面中查找行的开销。

这可能表示您忘记运行以分析表格，以便规划人员具有适当的统计数据：

analyze u_user