我正在尝试优化一个SQL查询。你能帮助我吗？
基本上，每个用户通过一个友谊表拥有朋友，每个用户通过一个用户的feed事件表拥有许多feed事件。
我试图列出给定用户朋友的feed_事件。不应该是不可能的，对吧？:)
正如您所看到的，查询的性能取决于用户有多少朋友。现在，一个拥有150个好友的用户几乎需要7秒来执行。
更新：以下是我的友谊表的构建方式：
create_table "friendships", :force => true do |t|
t.integer "user_id", :null => false
t.integer "friend_id", :null => false
t.datetime "created_at"
t.datetime "accepted_at"
end
add_index "friendships", ["friend_id"], :name => "index_friendships_on_friend_id"
add_index "friendships", ["user_id"], :name => "index_friendships_on_user_id"
首先，我让rails给我用户朋友的用户id列表，然后在真正的查询中使用这个字符串。
friends_id = current_user.friends.collect {|f| f.id}.join(",")
sql = "
SELECT
DISTINCT feed_events.id,
feed_events.event_type,
feed_events.type_id,
feed_events.data,
feed_events.created_at,
feed_events.updated_at,
user_feed_events.user_id
FROM feed_events
LEFT JOIN user_feed_events
ON feed_events.id = user_feed_events.feed_event_id
WHERE user_feed_events.user_id IN (#{friends_id})
ORDER BY feed_events.created_at DESC"
然后我准确地执行查询（分页并限制为30个结果）：
@events = FeedEvent.paginate_by_sql(sql, :page => params[:page], :per_page => 30)
更新#2：以下是解释分析输出：
SQL> EXPLAIN ANALYZE (SELECT DISTINCT feed_events.id, feed_events.event_type, feed_events.type_id, feed_events.data, feed_events.created_at, feed_events.updated_at, user_feed_events.user_id FROM user_feed_events INNER JOIN feed_events ON feed_events.id = user_feed_events.feed_event_id WHERE user_feed_events.user_id IN (1,7,9,8,14,15,20,35,40,39,41,42,57,84,98,109,121,74,129,64,137,77,172,182,206,201,284,31,94,232,311,168,30,114,50,174,419,403,438,464,423,513,351,349,385,622,751,359,809,838,844,962,831,786,896,1001,992,998,990,256,67,623,957,1226,1060,1009,1490,132,1467,1672,619,1459,1466,993,1599,1365,607,1381,1714,1154,2032,2230,2240,2354,598,2345,1804,634,1900,2652,1975,2164,1759,3288,1004,3487,3507,3542,3566,514,3787,3137,3803,3090,4012,855,17,2026,1463,335,1000,935,5,12,10,13,19,18,16,22,34,27,29,59,126,90,46,23,63,291,134,229,107,439,521) ORDER BY feed_events.created_at DESC)
+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| QUERY PLAN |
+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Unique (cost=6090.87..6162.93 rows=18014 width=389) (actual time=1641.210..1733.010 rows=29691 loops=1) |
| -> Sort (cost=6090.87..6099.88 rows=18014 width=389) (actual time=1641.206..1670.882 rows=29694 loops=1) |
| Sort Key: feed_events.created_at, feed_events.id, feed_events.event_type, feed_events.type_id, feed_events.data, feed_events.updated_at, user_feed_events.user_id |
| Sort Method: quicksort Memory: 17755kB |
| -> Hash Join (cost=3931.63..5836.21 rows=18014 width=389) (actual time=258.541..361.345 rows=29694 loops=1) |
| Hash Cond: (user_feed_events.feed_event_id = feed_events.id) |
| -> Bitmap Heap Scan on user_feed_events (cost=926.64..2745.66 rows=18014 width=8) (actual time=6.930..42.367 rows=29694 loops=1) |
| Recheck Cond: (user_id = ANY ('{1,7,9,8,14,15,20,35,40,39,41,42,57,84,98,109,121,74,129,64,137,77,172,182,206,201,284,31,94,232,311,168,30,114,50,174,419,403,438,464,423,513,351,349,385,622,751,359,809,838,844,962,831,786,896,1001,992,998,990,256,67,623,957,1226,1060,1009,1490,132,1467,1672,619,1459,1466,993,1599,1365,607,1381,1714,1154,2032,2230,2240,2354,598,2345,1804,634,1900,2652,1975,2164,1759,3288,1004,3487,3507,3542,3566,514,3787,3137,3803,3090,4012,855,17,2026,1463,335,1000,935,5,12,10,13,19,18,16,22,34,27,29,59,126,90,46,23,63,291,134,229,107,439,521}'::integer[])) |
| -> Bitmap Index Scan on index_user_feed_events_on_user_id (cost=0.00..925.74 rows=18014 width=0) (actual time=6.836..6.836 rows=29694 loops=1) |
| Index Cond: (user_id = ANY ('{1,7,9,8,14,15,20,35,40,39,41,42,57,84,98,109,121,74,129,64,137,77,172,182,206,201,284,31,94,232,311,168,30,114,50,174,419,403,438,464,423,513,351,349,385,622,751,359,809,838,844,962,831,786,896,1001,992,998,990,256,67,623,957,1226,1060,1009,1490,132,1467,1672,619,1459,1466,993,1599,1365,607,1381,1714,1154,2032,2230,2240,2354,598,2345,1804,634,1900,2652,1975,2164,1759,3288,1004,3487,3507,3542,3566,514,3787,3137,3803,3090,4012,855,17,2026,1463,335,1000,935,5,12,10,13,19,18,16,22,34,27,29,59,126,90,46,23,63,291,134,229,107,439,521}'::integer[])) |
| -> Hash (cost=2848.84..2848.84 rows=44614 width=385) (actual time=251.490..251.490 rows=44663 loops=1) |
| -> Seq Scan on feed_events (cost=0.00..2848.84 rows=44614 width=385) (actual time=0.035..77.044 rows=44663 loops=1) |
| Total runtime: 1780.200 ms |
+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
SQL>
更新#3：问题是，对于我的rails应用程序，我正在使用has戋u many戋u friends插件（https://github.com/swemoney/has戋u many戋u friends），它负责我的友谊。它是这样工作的。我是用户id 6，我向用户id 10请求友谊。当用户#10接受我的友谊时，将向表中添加一个新的行，其中用户#id=6，朋友#id=10。如果用户#10向我请求友谊，则行为：用户#id=10和朋友#id=6。
这意味着，为了逐个查找朋友，我需要搜索“用户id=6”，为了查找朋友，我需要“朋友id=6”。为了找到我所有的朋友，我需要搜索这两个列。这使得创建连接变得非常复杂！
你会怎么处理？
我能想到的唯一选择是：
"(SELECT
DISTINCT feed_events.id,
feed_events.event_type,
feed_events.type_id,
feed_events.data,
feed_events.created_at,
feed_events.updated_at,
user_feed_events.user_id
FROM feed_events
INNER JOIN user_feed_events
ON feed_events.id = user_feed_events.feed_event_id
INNER JOIN friendships
ON user_feed_events.user_id = friendships.user_id
WHERE friendships.user_id = 6
AND friendships.accepted_at IS NOT NULL)
UNION DISTINCT
(SELECT
DISTINCT additional_feed_events.id,
additional_feed_events.event_type,
additional_feed_events.type_id,
additional_feed_events.data,
additional_feed_events.created_at,
additional_feed_events.updated_at,
user_feed_events.user_id
FROM feed_events AS additional_feed_events
INNER JOIN user_feed_events
ON additional_feed_events.id = user_feed_events.feed_event_id
INNER JOIN friendships
ON user_feed_events.user_id = friendships.friend_id
WHERE friendships.friend_id = 6
AND friendships.accepted_at IS NOT NULL)
ORDER BY feed_events.created_at DESC"
但是现在不行，我也不确定是不是正确的方法！
谢谢，
奥古斯都
最佳答案
你为什么用收信人名单？为什么不从选定的用户开始？另外，我认为不需要您的左外连接：
SELECT
DISTINCT feed_events.id,
feed_events.event_type,
feed_events.type_id,
feed_events.data,
feed_events.created_at,
feed_events.updated_at,
user_feed_events.user_id
FROM
(
select friend_id from friendship where user_id = YOURUSER
UNION
select user_id as friend_id from friendship where friend_id = YOURUSER
) friendship
inner join user_feed_events
on friendship.friend_id = user_feed_events.user_id
inner join feed_events
on user_feed_events.feed_event_id = feed_events.id
ORDER BY feed_events.created_at DESC
如果您想保留原来的语句并对其进行优化，请使用以下命令：
SELECT
DISTINCT feed_events.id,
feed_events.event_type,
feed_events.type_id,
feed_events.data,
feed_events.created_at,
feed_events.updated_at,
user_feed_events.user_id
FROM user_feed_events
INNER JOIN feed_events
ON feed_events.id = user_feed_events.feed_event_id
WHERE user_feed_events.user_id IN (#{friends_id})
ORDER BY feed_events.created_at DESC
这将删除不必要的左连接。
此外，请确保在用于外键的列上创建了索引。