小编Ran*_*dle的帖子

问题：

我们有一个社交网站，会员可以在其中相互评价兼容性或匹配度。该user_match_ratings表包含超过 2.2 亿行（9 演出数据或近 20 演出索引）。针对此表的查询通常显示在 slow.log（阈值 > 2 秒）中，并且是系统中记录最频繁的慢查询：

Query_time: 3  Lock_time: 0  Rows_sent: 3  Rows_examined: 1051
"select rating, count(*) as tally from user_match_ratings where rated_user_id = 395357 group by rating;"

Query_time: 4  Lock_time: 0  Rows_sent: 3  Rows_examined: 1294
"select rating, count(*) as tally from user_match_ratings where rated_user_id = 4182969 group by rating;"

Query_time: 3  Lock_time: 0  Rows_sent: 3  Rows_examined: 446
"select rating, count(*) as tally from user_match_ratings where rated_user_id = 630148 group by rating;"

Query_time: …

名为“messages”的表中有大约 5000 万行。问题是，即使使用索引，特定查询也运行得非常慢。我的背景更多是 Mysql，所以我更像是 Postgres (9.5) 的新手。任何解决此表查询缓慢问题的建议或帮助将不胜感激。

该表的结构如下：

mydatabase=# \\d+ messages\n\n\n id              | integer                     | not null default nextval(\'messages_id_seq\'::regclass) | plain    |              | \n conversation_id | integer                     |                                                       | plain    |              | \n user_id         | integer                     |                                                       | plain    |              | \n content         | text                        |                                                       | extended |              | \n attached_photos | text                        |                                                       | extended |              | \n attached_video  | text                        |                                                       | extended |              | \n status          | character varying(255)      |                                                       | extended |              | …