从联接的列优化ORDER BY

我有一个显示器，其中列出了一个城市的每个用户，并通过最新的消息第一排序表的最后一条消息：从联接的列优化ORDER BY

Users 
------ 
Caleb - Hey what's up? 
------ 
Bill - Is there anything up tonight? 
------ 
Jon - What's up man? 
------

任何低于和/或优化这个查询帮助帮助确定添加索引的位置会很有帮助。

我可能会非规范化并将last_message_created_at存储在users上，但希望避免回填。

用户表：

    Table "public.users" 
     Column  |   Type    | Modifiers 
-------------------+-----------------------------+----------- 
id    | integer      | not null 
role    | user_role     | not null 
last_message_id | integer      | 
city_id   | integer      | 

Indexes: 
    "users_pkey" PRIMARY KEY, btree (id) 
    "ix_users_city_id" btree (city_id) 
    "ix_users_last_message_id" btree (last_message_id) 
Foreign-key constraints: 
    "messages_last_message_id_fkey" FOREIGN KEY (last_message_id) REFERENCES messages(id) 
    "users_city_id_fkey" FOREIGN KEY (city_id) REFERENCES cities(id) 
Referenced by: 
    TABLE "messages" CONSTRAINT "messages_from_user_id_fkey" FOREIGN KEY (from_user_id) REFERENCES users(id) 
    TABLE "messages" CONSTRAINT "messages_to_user_id_fkey" FOREIGN KEY (to_user_id) REFERENCES users(id)

信息表：

         Table "public.messages" 
     Column  |   Type    |      Modifiers 
------------------+-----------------------------+------------------------------------------------------- 
id    | integer      | not null default nextval('messages_id_seq'::regclass) 
content   | character varying   | 
from_user_id  | integer      | 
to_user_id  | integer      | 
created_at  | timestamp without time zone | 

Indexes: 
    "messages_pkey" PRIMARY KEY, btree (id) 
    "ix_messages_from_user_id" btree (from_user_id) 
    "ix_messages_to_user_id" btree (to_user_id) 
Foreign-key constraints: 
    "messages_from_user_id_fkey" FOREIGN KEY (from_user_id) REFERENCES users(id) 
    "messages_to_user_id_fkey" FOREIGN KEY (to_user_id) REFERENCES users(id) 
Referenced by: 
    TABLE "users" CONSTRAINT "messages_last_message_id_fkey" FOREIGN KEY (last_message_id) REFERENCES messages(id)

查询和计划：

EXPLAIN ANALYZE 
SELECT users.city_id AS users_city_id, users.id AS users_id, users.last_message_id AS users_last_message_id 
FROM users 
JOIN messages ON messages.id = users.last_message_id 
WHERE users.city_id = 1 ORDER BY users.last_message_id DESC; 

QUERY PLAN 
--------------------------------------------------------------------------------------------------------------------------- 
Sort (cost=951606.67..951811.07 rows=81760 width=12) (actual time=1934.501..1998.216 rows=79454 loops=1) 
    Sort Key: users.last_message_id 
    Sort Method: quicksort Memory: 6797kB 
    -> Nested Loop (cost=1575.71..944935.41 rows=81760 width=12) (actual time=26.784..1817.478 rows=79454 loops=1) 
     -> Bitmap Heap Scan on users (cost=1575.71..33209.21 rows=84040 width=12) (actual time=26.656..393.749 rows=85348 loops=1) 
       Recheck Cond: (city_id = 1) 
       -> Bitmap Index Scan on ix_users_city_id (cost=0.00..1554.70 rows=84040 width=0) (actual time=20.679..20.679 rows=85348 loops=1) 
        Index Cond: (city_id = 1) 
     -> Index Only Scan using messages_pkey on messages (cost=0.00..10.84 rows=1 width=4) (actual time=0.012..0.013 rows=1 loops=85348) 
       Index Cond: (id = users.last_message_id) 
       Heap Fetches: 79454 
Total runtime: 2058.134 ms 
(12 rows)

来源

2014-09-27 Mingle

你永远不在查询中使用messages表除排除没有消息的用户以外的任何其他目的，因此这可能会更快：

select 
    u.city_id as users_city_id, 
    u.id as users_id, 
    u.last_message_id as users_last_message_id 
from users u 
where 
    u.city_id = 1 
    and exists (
     select 1 
     from messages 
     where id = u.last_message_id 
    ) 
order by u.last_message_id desc

来源

2014-09-28 00:17:32

从联接的列优化ORDER BY

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