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记录一次性能优化
Hackershare 后台的一个 Dashboard 页面，由于很多统计类的查询，响应越来越慢，差不多要十几秒打开。主要是有两个 50w 左右的数据表，count 非常慢，还有一部分原因就是这台 2c4g 的服务器部署了很多程序，CPU 经常被其他服务占用。
大概的数据量：
hackershare=# \dt+
List of relations
Schema | Name | Type | Owner | Size | Description
--------+----------------------------+-------+--------+---------+-------------
public | bookmarks | table | deploy | 1259 MB |
public | clicks | table | deploy | 4360 kB |
public | comments | table | deploy | 134 MB |
public | follows | table | deploy | 48 kB |
public | likes | table | deploy | 88 kB |
public | rss_sources | table | deploy | 1280 kB |
public | tag_subscriptions | table | deploy | 88 kB |
public | taggings | table | deploy | 117 MB |
public | tags | table | deploy | 2200 kB |
public | users | table | deploy | 2848 kB |
第一招，使用 union all，把多条 count 合并成一条语句：
select 'bookmark' as key, count(*) as count from bookmarks
UNION ALL
select 'comment' as key, count(*) as count from comments
UNION ALL
select 'click' as key, count(*) as count from clicks
UNION ALL
others
返回结构大概这样：
key | count
----------+--------
click | 65103
comment | 421423
bookmark | 465078
比之前的有提升，但效果不大...
第二招，使用 explain
# usage
# FastCount.new(User.all).call
# => 826
# FastCount.new(User.where("id > 200")).call
# => 665
class FastCount
attr_reader :scope, :sql
def initialize(scope)
@scope = scope
@sql = scope.to_sql
end
def call
explain_sql = "explain (format json) #{sql}"
result = ApplicationRecord.connection.execute(explain_sql)[0]["QUERY PLAN"]
json = JSON.parse(result)
json[0]["Plan"]["Plan Rows"].to_i
end
end
看看效果：
explain (format json) select * from bookmarks;
QUERY PLAN
-------------------------------------
[ +
{ +
"Plan": { +
"Node Type": "Seq Scan", +
"Parallel Aware": false, +
"Relation Name": "bookmarks",+
"Alias": "bookmarks", +
"Startup Cost": 0.00, +
"Total Cost": 89629.30, +
"Plan Rows": 464730, +
"Plan Width": 1278 +
} +
} +
]
(1 row)
Time: 0.898 ms
不到 1 毫秒！！
另外，居然可以支持带过滤条件甚至带 JOIN 语句的 count，比如：
FastCount.new(User.where("id > 200")).call
- 适用场景：分页和 dashboard 之类不需要数据绝对准确，但对性能还有一些要求的场景。
- 缺点：并不能保证数据绝对准确，取决于你的 auto vacuum 设置，一般情况下如果你的表记录足够大，并且更新频繁，使用这种方案几乎误差范围都是很小的。
Full Code: https://github.com/hackershare/hackershare/pull/115/files
MySQL 有类似的，压测的时候用过，不过误差有点大，也没仔细找原因。
mysql> SELECT count(*) AS TOTALNUMBEROFTABLES
-> FROM INFORMATION_SCHEMA.TABLES
-> WHERE TABLE_SCHEMA = 'business';
Oracle 有个 Zone Map 的优化，会单独记录 count 之类的。
对于这种待条件的查询
FastCount.new(User.where("id > 200")).call
数据库的 explain 的估算值好像是根据直方图来估算的。如果直方图的粒度很粗，字段的值的分布也不均匀，估算值可能会有很大的误差。
hooopo
#13
2021年04月13日
对
xiaoronglv
回复
xiaoronglv
回复
一个小时的误差还真赶不上这种，有了新的过滤条件还要维护，在索引里的一般误差不是很大，这种方案的前提是表大并且更新频繁，auto vacuum 就会运行的频率高，数据也就准了
explain 的结果不准吧…… 本质上它是 PostgreSQL 为表存储的一套大概统计数据，用来决定 query plan 的。不过这脑洞确实大。
dsh0416
#24
2021年04月16日
之前用过一个 function 来把 explain 的估计 count 值拿出来。但还没想到这个 format json 的操作，这个可以拼接任意条件查询了，之前写法要是这么拼有注入风险。
ysllyfe
#26
2021年05月13日
表大一点，计划会复杂，即使拿里层的 Plan Rows 数据也取不出来
olive=> explain (format json) select count(*) from audits;
-[ RECORD 1 ]-------------------------------------------------------------
QUERY PLAN | [ +
| { +
| "Plan": { +
| "Node Type": "Aggregate", +
| "Strategy": "Plain", +
| "Partial Mode": "Finalize", +
| "Parallel Aware": false, +
| "Startup Cost": 1596821.82, +
| "Total Cost": 1596821.83, +
| "Plan Rows": 1, +
| "Plan Width": 8, +
| "Plans": [ +
| { +
| "Node Type": "Gather", +
| "Parent Relationship": "Outer", +
| "Parallel Aware": false, +
| "Startup Cost": 1596821.60, +
| "Total Cost": 1596821.81, +
| "Plan Rows": 2, +
| "Plan Width": 8, +
| "Workers Planned": 2, +
| "Single Copy": false, +
| "Plans": [ +
| { +
| "Node Type": "Aggregate", +
| "Strategy": "Plain", +
| "Partial Mode": "Partial", +
| "Parent Relationship": "Outer", +
| "Parallel Aware": false, +
| "Startup Cost": 1595821.60, +
| "Total Cost": 1595821.61, +
| "Plan Rows": 1, +
| "Plan Width": 8, +
| "Plans": [ +
| { +
| "Node Type": "Index Only Scan", +
| "Parent Relationship": "Outer", +
| "Parallel Aware": true, +
| "Scan Direction": "Forward", +
| "Index Name": "index_audits_on_created_at",+
| "Relation Name": "audits", +
| "Alias": "audits", +
| "Startup Cost": 0.56, +
| "Total Cost": 1548601.80, +
| "Plan Rows": 18887923, +
| "Plan Width": 0 +
| } +
| ] +
| } +
| ] +
| } +
| ] +
| } +
| } +
| ]
olive=> select id from audits order by id desc limit 1;
-[ RECORD 1 ]
id | 45752411
不知道是不是版本差异
psql (13.1, server 11.8)
ysllyfe
#28
2021年05月13日
对
hooopo
回复
hooopo
回复
QUERY PLAN
-----------------------------------------------
[ +
{ +
"Plan": { +
"Node Type": "Seq Scan", +
"Parallel Aware": false, +
"Relation Name": "nb_command_analyses",+
"Alias": "nb_command_analyses", +
"Startup Cost": 0.00, +
"Total Cost": 2686346.72, +
"Plan Rows": 71053872, +
"Plan Width": 222 +
} +
} +
]
(1 row)
OK 了~ 眼瞎，没有看仔细
带索引的和只有 filter 的条件都很快 
ysllyfe
#30
2021年05月13日
对
hooopo
回复
hooopo
回复
整个表我没有去算，而且可能没有 vacuum 了最近，我拿了单条的刚刚看了一下
explain (format json) select * from nb_command_analyses where created_at > '2021-01-01' and device_id = 'F34479F6F935';
QUERY PLAN
-------------------------------------------------------------------------------------
[ +
{ +
"Plan": { +
"Node Type": "Index Scan", +
"Parallel Aware": false, +
"Scan Direction": "Forward", +
"Index Name": "nba_edcm_idx", +
"Relation Name": "nb_command_analyses", +
"Alias": "nb_command_analyses", +
"Startup Cost": 0.57, +
"Total Cost": 1291320.82, +
"Plan Rows": 3669, +
"Plan Width": 222, +
"Index Cond": "((device_id)::text = 'F34479F6F935'::text)", +
"Filter": "(created_at > '2021-01-01 00:00:00'::timestamp without time zone)"+
} +
} +
]
(1 row)
explain (format json) select * from nb_command_analyses where created_at > '2021-02-01' and device_id = 'F34479F6F935';
QUERY PLAN
-------------------------------------------------------------------------------------
[ +
{ +
"Plan": { +
"Node Type": "Index Scan", +
"Parallel Aware": false, +
"Scan Direction": "Forward", +
"Index Name": "nba_edcm_idx", +
"Relation Name": "nb_command_analyses", +
"Alias": "nb_command_analyses", +
"Startup Cost": 0.57, +
"Total Cost": 1291320.82, +
"Plan Rows": 2938, +
"Plan Width": 222, +
"Index Cond": "((device_id)::text = 'F34479F6F935'::text)", +
"Filter": "(created_at > '2021-02-01 00:00:00'::timestamp without time zone)"+
} +
} +
]
(1 row)
explain (format json) select * from nb_command_analyses where created_at > '2021-03-01' and device_id = 'F34479F6F935';
QUERY PLAN
-------------------------------------------------------------------------------------
[ +
{ +
"Plan": { +
"Node Type": "Index Scan", +
"Parallel Aware": false, +
"Scan Direction": "Forward", +
"Index Name": "nba_edcm_idx", +
"Relation Name": "nb_command_analyses", +
"Alias": "nb_command_analyses", +
"Startup Cost": 0.57, +
"Total Cost": 1291320.82, +
"Plan Rows": 2416, +
"Plan Width": 222, +
"Index Cond": "((device_id)::text = 'F34479F6F935'::text)", +
"Filter": "(created_at > '2021-03-01 00:00:00'::timestamp without time zone)"+
} +
} +
]
(1 row)
explain (format json) select * from nb_command_analyses where created_at > '2021-04-01' and device_id = 'F34479F6F935';
QUERY PLAN
-------------------------------------------------------------------------------------
[ +
{ +
"Plan": { +
"Node Type": "Index Scan", +
"Parallel Aware": false, +
"Scan Direction": "Forward", +
"Index Name": "nba_edcm_idx", +
"Relation Name": "nb_command_analyses", +
"Alias": "nb_command_analyses", +
"Startup Cost": 0.57, +
"Total Cost": 1291320.82, +
"Plan Rows": 1401, +
"Plan Width": 222, +
"Index Cond": "((device_id)::text = 'F34479F6F935'::text)", +
"Filter": "(created_at > '2021-04-01 00:00:00'::timestamp without time zone)"+
} +
} +
]
(1 row)
explain (format json) select * from nb_command_analyses where created_at > '2021-05-01' and device_id = 'F34479F6F935';
QUERY PLAN
-------------------------------------------------------------------------------------
[ +
{ +
"Plan": { +
"Node Type": "Index Scan", +
"Parallel Aware": false, +
"Scan Direction": "Forward", +
"Index Name": "nba_edcm_idx", +
"Relation Name": "nb_command_analyses", +
"Alias": "nb_command_analyses", +
"Startup Cost": 0.57, +
"Total Cost": 1291320.82, +
"Plan Rows": 310, +
"Plan Width": 222, +
"Index Cond": "((device_id)::text = 'F34479F6F935'::text)", +
"Filter": "(created_at > '2021-05-01 00:00:00'::timestamp without time zone)"+
} +
} +
]
(1 row)
结果是 1.1 -> 3669
2.1 -> 2938
3.1 -> 2416
4.1 -> 1401
5.1 -> 310
实际count 下来 1.1 -> 3
select count(*) from nb_command_analyses where created_at > '2021-01-01' and device_id = 'F34479F6F935';
count
-------
3
(1 row)
这个表最后一条
select id from nb_command_analyses order by id desc limit 1;
id
----------
72204914
(1 row)
hooopo
#31
2021年05月13日
对
ysllyfe
回复
ysllyfe
回复
如果你的返回结果集已经很少了就没必要用这种方法了。你直接 count 也不会慢到哪里去。这种场景是查返回值会很大的那种 count 的
dsh0416
#33
2021年06月18日
# frozen_string_literal: true
# ApproximateCount count on large Postgres tables
# designed to be `extend` into inheritance classes of ActiveRecord::Base
# Example:
# User.approximate_count
# # (1.3ms) EXPLAIN (FORMAT JSON) SELECT "users".* FROM "users"
# # (0.8ms) SELECT COUNT(*) FROM "users"
# # => 0
#
# User.where('created_at > ?', 7.days.ago).approximate_count
# # (1.6ms) EXPLAIN (FORMAT JSON) SELECT "users".* FROM "users" WHERE (created_at > '2021-06-11 15:20:49.870311')
# # (0.8ms) SELECT COUNT(*) FROM "users" WHERE (created_at > '2021-06-11 15:20:49.870311')
# # => 0
module ApproximateCount
def approximate_count(threshold=10_000)
explain_sql = "EXPLAIN (FORMAT JSON) #{all.to_sql}"
result = self.connection.execute(explain_sql)[0]["QUERY PLAN"]
json = JSON.parse(result)
val = json[0]["Plan"]["Plan Rows"].to_i
return val if val > threshold
self.count
end
end
class ApplicationRecord < ActiveRecord::Base
extend ApproximateCount
self.abstract_class = true
end
施加了一点点魔法
