如何解决复杂的SQL查询优化
| 我正在尝试优化SQL查询。你能帮助我吗？ 基本上，每个用户都通过一个友谊表拥有朋友，并且每个用户都通过一个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。应该不是不可能，对吧？ :) 如您所见，查询的性能取决于用户有多少个朋友。现在，有150个朋友的用户将花费近7秒钟来执行。 更新：这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\"
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\"
@events = FeedEvent.paginate_by_sql(sql,:page => params[:page],:per_page => 30)
SQL> EXPLAIN ANALYZE (SELECT DISTINCT feed_events.id,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,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 (\'{1,521}\'::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 (\'{1,521}\'::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>
\"(SELECT
DISTINCT feed_events.id,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\"
解决方法
为什么使用IN列表？为什么不从所选用户开始？另外，我认为您不需要左外部联接：
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
SELECT
DISTINCT feed_events.id,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
SELECT feed_events.id,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
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
WHERE
SELECT
IN()
SELECT fe.id,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;
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