我正在尝试优化SQL查询。你能救我吗？
基本上每个用户都通过友谊表拥有朋友，每个用户通过user_feed_events表拥有许多feed_events。 我正在尝试列出给定用户的朋友的feed_events。不应该是不可能的，对吗？ ：）
正如您所看到的，查询的性能取决于用户拥有多少朋友。现在，拥有150个朋友的用户需要大约7秒钟才能执行。
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"
首先我请求rails给我用户朋友的用户ID列表，然后我在真实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"
然后我实际执行查询（将其分页并限制为30个结果）：
@events = FeedEvent.paginate_by_sql(sql, :page => params[:page], :per_page => 30)
更新＃2：这里是解析分析输出：
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>
UPDATE＃3 ：问题是我的rails应用程序我正在使用has_many_friends插件（https://github.com/swemoney/has_many_friends），这是照顾我的友谊。它的工作原理如下。我是user_id＃6并且我向user_id＃10询问友谊。当用户＃10接受我的友谊时，新的行被添加到表中user_id = 6和friend_id = 10.如果用户＃10问我友谊row是：user_id = 10和friend_id = 6.
这意味着为了找到friends_by_me我需要搜索“user_id = 6”，为了找到friends_for_me，我需要“friend_id = 6”。为了找到我的所有朋友，我需要搜索两列。这使得创建连接变得非常复杂！ 你会怎么处理这个？
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"
但目前还没有工作，我也不确定这是正确的做法！
谢谢， 奥古斯托
答案 0 :(得分：0)
为什么使用IN列表？为什么不从选定的用户开始？另外，我认为你不需要你的左外连接：
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
如果您想继续使用原始陈述并进行优化，请使用此：
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LEFT JOIN。
此外，请确保您在用于外键的列上创建了索引。
答案 1 :(得分：0)
好的，所以查询不是你的问题，你的数据库必须设置好，这样不会超过几微秒。首先，查询。它应该是这样的：
SELECT 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
user_friends ON user_friends.friend_id = user_feed_events.user_id
WHERE user_friends.user_id = ** The Id of the User in Question **
ORDER BY feed_events.created_at DESC
接下来，您需要确保您的Id列是主键，并且user_friends表中的（friend_id，user_id）等内容都有唯一索引。顺便说一句，我刚刚编写了这些名字，我试着猜猜你所说的是你存放友谊的桌子。
答案 2 :(得分：0)
select distinct fe.id, fe.event_type,
fe.type_id, fe.data, fe.created_at,
fe.updated_at, ufe.user_id
from friendships as f
inner join user_feed_events as ufe on f.friend_id = ufe.user_id
inner join feed_events as fe on ufe.user_id = fe.id
where f.user_id = 6 and f.accepted_at is not null
order by fe.created_at desc
不确定这里是否真的需要与众不同。查询返回指定用户的朋友的feed事件..我希望;）
编辑。 它发生的解决方案与Daniel Hilgarth提出的解决方案非常相似。
答案 3 :(得分：0)
在SELECT子句中使用子WHERE来构建IN()调用的Feed事件列表。像这样的东西（未经测试）：
SELECT fe.id,
fe.event_type,
fe.type_id,
fe.data,
fe.created_at,
fe.updated_at,
ufe.user_id
FROM feed_events AS fe, user_feed_events AS ufe
WHERE TRUE = TRUE
AND fe.id = ufe.feed_event_id
AND ufe.user_id = :user_id
AND fe.id IN((
SELECT ufe.feed_event_id
FROM user_feed_events AS ufe, user_friends AS uf
WHERE uf.friend_id = :user_id
))
ORDER BY feed_events.created_at DESC;
我很想知道EXPLAIN ANALYZE看起来是什么样的。