为什么这个查询不使用我创建的索引？

Question

运行 postgres 9.5

SELECT events.id, events.start_time, events.host_id, events.title from events
JOIN accountsevents ON accountsevents.events_id = events.id
WHERE accountsevents.accounts_id = %(account_id)s OR events.host_id = %(account_id)s
GROUP BY events.id
ORDER BY start_time DESC

我有这个查询，postgres 说费用超过 100k。好像过分了这是我唯一没有使用我为每个 table.

创建的索引的查询

Indexes:
    "events_pkey" PRIMARY KEY, btree (id)
Foreign-key constraints:
    "events_host_id_fkey" FOREIGN KEY (host_id) REFERENCES accounts(id)
Referenced by:
    TABLE "accountsevents" CONSTRAINT "accountsevents_events_id_fkey" FOREIGN KEY (events_id) REFERENCES events(id)
    TABLE "eventsinterests" CONSTRAINT "eventsinterests_events_id_fkey" FOREIGN KEY (events_id) REFERENCES events(id)


Indexes:
    "accountsevents_pkey" PRIMARY KEY, btree (id, accounts_id, events_id)
Foreign-key constraints:
    "accountsevents_accounts_id_fkey" FOREIGN KEY (accounts_id) REFERENCES accounts(id)
    "accountsevents_events_id_fkey" FOREIGN KEY (events_id) REFERENCES events(id)

我觉得索引设置有误，或者我只是在查询中遗漏了一些内容。初始序列扫描正在杀死它。

添加了详细解释

"Sort  (cost=124388.27..124390.10 rows=732 width=40) (actual time=1533.902..1533.928 rows=470 loops=1)"
"  Output: events.id, events.start_time, events.host_id, events.title"
"  Sort Key: events.start_time DESC"
"  Sort Method: quicksort  Memory: 66kB"
"  ->  HashAggregate  (cost=124346.12..124353.44 rows=732 width=40) (actual time=1533.658..1533.759 rows=470 loops=1)"
"        Output: events.id, events.start_time, events.host_id, events.title"
"        Group Key: events.id"
"        ->  Hash Join  (cost=4912.30..124344.29 rows=732 width=40) (actual time=56.671..1532.831 rows=971 loops=1)"
"              Output: events.id, events.start_time, events.host_id, events.title"
"              Hash Cond: (accountsevents.events_id = events.id)"
"              Join Filter: ((accountsevents.accounts_id = 1) OR (events.host_id = 1))"
"              Rows Removed by Join Filter: 2761882"
"              ->  Seq Scan on public.accountsevents  (cost=0.00..45228.52 rows=2762852 width=8) (actual time=0.005..466.094 rows=2762853 loops=1)"
"                    Output: accountsevents.events_id, accountsevents.accounts_id"
"              ->  Hash  (cost=2795.91..2795.91 rows=104191 width=40) (actual time=53.579..53.579 rows=104181 loops=1)"
"                    Output: events.id, events.start_time, events.host_id, events.title"
"                    Buckets: 65536  Batches: 4  Memory Usage: 2462kB"
"                    ->  Seq Scan on public.events  (cost=0.00..2795.91 rows=104191 width=40) (actual time=0.004..26.171 rows=104181 loops=1)"
"                          Output: events.id, events.start_time, events.host_id, events.title"
"Planning time: 0.201 ms"
"Execution time: 1534.024 ms"

Answer 1

没有索引可以帮助您完成此查询。

问题是您在 WHERE 条件中有一个 OR，因此无法在表连接之前应用过滤器 ，这是索引可以帮助您的地方。尝试用 AND 替换 OR，看看 PostgreSQL 如何做得更好。

这样 PostgreSQL 必须计算整个连接并且只能在之后过滤掉行 – 请参阅连接过滤器删除的大量行。

唯一可以使用索引的是嵌套循环连接，这会更加昂贵。所以我不认为这个查询有更好的计划。

你可以看到PostgreSQL对行数的预估非常好，这通常是PostgreSQL确实做对了的标志。

也许您可以使用
这样的查询做得更好
SELECT * FROM (SELECT ... FROM events JOIN accountsevents ON ... WHERE accountsevents.accounts_id = 1 UNION SELECT ... FROM events JOIN accountsevents ON ... WHERE events.host_id = 1) sub GROUP BY ... ORDER BY ...

但我不会打赌。
（注意：此查询在语义上略有不同，但在您的情况下可能无关紧要。）

为什么这个查询不使用我创建的索引？

Why doesn't this query use the indexes i've created?

sql

database

postgresql

postgresql-9.5